Ignore:
Timestamp:
Aug 22, 2008, 4:56:21 AM (11 years ago)
Author:
julian.reschke@…
Message:

Update orig RFCs, also add 2145 and 5234

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1 edited

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  • draft-ietf-httpbis/orig/rfc2617.html

    r227 r311  
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    12<!DOCTYPE html
    23  PUBLIC "-//W3C//DTD HTML 4.01//EN">
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    4    <head profile="http://www.w3.org/2006/03/hcard">
    5       <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    6       <title>HTTP Authentication: Basic and Digest Access Authentication</title><style type="text/css" title="Xml2Rfc (sans serif)">
     4<html lang="en"><head profile="http://www.w3.org/2006/03/hcard">
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     6   <title>HTTP Authentication: Basic and Digest Access Authentication</title><style type="text/css" title="Xml2Rfc (sans serif)">
    77a {
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    331331    }
    332332}
    333 </style><link rel="Contents" href="#rfc.toc">
    334       <link rel="Author" href="#rfc.authors">
    335       <link rel="Copyright" href="#rfc.copyright">
    336       <link rel="Index" href="#rfc.index">
    337       <link rel="Chapter" title="1 Access Authentication" href="#rfc.section.1">
    338       <link rel="Chapter" title="2 Basic Authentication Scheme" href="#rfc.section.2">
    339       <link rel="Chapter" title="3 Digest Access Authentication Scheme" href="#rfc.section.3">
    340       <link rel="Chapter" title="4 Security Considerations" href="#rfc.section.4">
    341       <link rel="Chapter" title="5 Sample implementation" href="#rfc.section.5">
    342       <link rel="Chapter" title="6 Acknowledgments" href="#rfc.section.6">
    343       <link rel="Chapter" href="#rfc.section.7" title="7 References">
    344       <link rel="Alternate" title="Authorative ASCII version" href="http://www.ietf.org/rfc/rfc2617.txt">
    345       <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.362, 2008-02-29 17:10:19, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
    346       <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
    347       <meta name="DC.Creator" content="Franks, J.">
    348       <meta name="DC.Creator" content="Hallam-Baker, P.M.">
    349       <meta name="DC.Creator" content="Hostetler, J.L.">
    350       <meta name="DC.Creator" content="Lawrence, S.D.">
    351       <meta name="DC.Creator" content="Leach, P.J.">
    352       <meta name="DC.Creator" content="Luotonen, A.">
    353       <meta name="DC.Creator" content="Stewart, L.">
    354       <meta name="DC.Identifier" content="urn:ietf:rfc:2617">
    355       <meta name="DC.Date.Issued" scheme="ISO8601" content="1999-06">
    356       <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2069">
    357       <meta name="DC.Description.Abstract" content="&#34;HTTP/1.0&#34;, includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication (unless used in conjunction with some external secure system such as SSL ), as the user name and password are passed over the network as cleartext. This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme and a scheme based on cryptographic hashes, referred to as &#34;Digest Access Authentication&#34;. It is therefore also intended to serve as a replacement for RFC 2069 . Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication; other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended. Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use.">
    358    </head>
    359    <body>
    360       <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
    361          <tr>
    362             <td class="header left">Network Working Group</td>
    363             <td class="header right">J. Franks</td>
    364          </tr>
    365          <tr>
    366             <td class="header left">Request for Comments: 2617</td>
    367             <td class="header right">Northwestern University, Department of Mathematics</td>
    368          </tr>
    369          <tr>
    370             <td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2069">2069</a></td>
    371             <td class="header right">P.M. Hallam-Baker</td>
    372          </tr>
    373          <tr>
    374             <td class="header left">Category: Standards Track</td>
    375             <td class="header right">Verisign Inc.</td>
    376          </tr>
    377          <tr>
    378             <td class="header left"></td>
    379             <td class="header right">J.L. Hostetler</td>
    380          </tr>
    381          <tr>
    382             <td class="header left"></td>
    383             <td class="header right">AbiSource, Inc.</td>
    384          </tr>
    385          <tr>
    386             <td class="header left"></td>
    387             <td class="header right">S.D. Lawrence</td>
    388          </tr>
    389          <tr>
    390             <td class="header left"></td>
    391             <td class="header right">Agranat Systems, Inc.</td>
    392          </tr>
    393          <tr>
    394             <td class="header left"></td>
    395             <td class="header right">P.J. Leach</td>
    396          </tr>
    397          <tr>
    398             <td class="header left"></td>
    399             <td class="header right">Microsoft Corporation</td>
    400          </tr>
    401          <tr>
    402             <td class="header left"></td>
    403             <td class="header right">A. Luotonen</td>
    404          </tr>
    405          <tr>
    406             <td class="header left"></td>
    407             <td class="header right">Netscape Communications Corporation</td>
    408          </tr>
    409          <tr>
    410             <td class="header left"></td>
    411             <td class="header right">L. Stewart</td>
    412          </tr>
    413          <tr>
    414             <td class="header left"></td>
    415             <td class="header right">Open Market, Inc.</td>
    416          </tr>
    417          <tr>
    418             <td class="header left"></td>
    419             <td class="header right">June 1999</td>
    420          </tr>
    421       </table>
    422       <p class="title">HTTP Authentication: Basic and Digest Access Authentication</p>
    423       <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
    424       <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
    425          for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
    426          state and status of this protocol. Distribution of this memo is unlimited.
    427       </p>
    428       <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    429       <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
    430       <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    431       <p>"HTTP/1.0", includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure
    432          method of user authentication (unless used in conjunction with some external secure system such as SSL <a href="#RFC2246" id="rfc.xref.RFC2246.1"><cite title="The TLS Protocol Version 1.0">[5]</cite></a>), as the user name and password are passed over the network as cleartext.
    433       </p>
    434       <p>This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme
    435          and a scheme based on cryptographic hashes, referred to as "Digest Access Authentication". It is therefore also intended to
    436          serve as a replacement for RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.1"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>. Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication;
    437          other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended.
    438       </p>
    439       <p>Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password);
    440          unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness.
    441          As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself
    442          but in policies and procedures surrounding its use.
    443       </p>
    444       <hr class="noprint">
    445       <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
    446       <ul class="toc">
    447          <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#access.authentication">Access Authentication</a><ul class="toc">
    448                <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Reliance on the HTTP/1.1 Specification</a></li>
    449                <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#access.authentication.framework">Access Authentication Framework</a></li>
    450             </ul>
    451          </li>
    452          <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Basic Authentication Scheme</a></li>
    453          <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Digest Access Authentication Scheme</a><ul class="toc">
    454                <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Introduction</a><ul class="toc">
    455                      <li class="tocline1">3.1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.1">Purpose</a></li>
    456                      <li class="tocline1">3.1.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.2">Overall Operation</a></li>
    457                      <li class="tocline1">3.1.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.3">Representation of digest values</a></li>
    458                      <li class="tocline1">3.1.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.4">Limitations</a></li>
    459                   </ul>
    460                </li>
    461                <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a><ul class="toc">
    462                      <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></li>
    463                      <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a><ul class="toc">
    464                            <li class="tocline1">3.2.2.1&nbsp;&nbsp;&nbsp;<a href="#request-digest">Request-Digest</a></li>
    465                            <li class="tocline1">3.2.2.2&nbsp;&nbsp;&nbsp;<a href="#A1">A1</a></li>
    466                            <li class="tocline1">3.2.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.3">A2</a></li>
    467                            <li class="tocline1">3.2.2.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.4">Directive values and quoted-string</a></li>
    468                            <li class="tocline1">3.2.2.5&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.5">Various considerations</a></li>
    469                         </ul>
    470                      </li>
    471                      <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.3">The Authentication-Info Header</a></li>
    472                   </ul>
    473                </li>
    474                <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#digest.operation">Digest Operation</a></li>
    475                <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Security Protocol Negotiation</a></li>
    476                <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers.example">Example</a></li>
    477                <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></li>
    478             </ul>
    479          </li>
    480          <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc">
    481                <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Authentication of Clients using Basic Authentication</a></li>
    482                <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">Authentication of Clients using Digest Authentication</a></li>
    483                <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Limited Use Nonce Values</a></li>
    484                <li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.4">Comparison of Digest with Basic Authentication</a></li>
    485                <li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.5">Replay Attacks</a></li>
    486                <li class="tocline1">4.6&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.6">Weakness Created by Multiple Authentication Schemes</a></li>
    487                <li class="tocline1">4.7&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.7">Online dictionary attacks</a></li>
    488                <li class="tocline1">4.8&nbsp;&nbsp;&nbsp;<a href="#man.in.the.middle">Man in the Middle</a></li>
    489                <li class="tocline1">4.9&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.9">Chosen plaintext attacks</a></li>
    490                <li class="tocline1">4.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.10">Precomputed dictionary attacks</a></li>
    491                <li class="tocline1">4.11&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.11">Batch brute force attacks</a></li>
    492                <li class="tocline1">4.12&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.12">Spoofing by Counterfeit Servers</a></li>
    493                <li class="tocline1">4.13&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.13">Storing passwords</a></li>
    494                <li class="tocline1">4.14&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.14">Summary</a></li>
    495             </ul>
    496          </li>
    497          <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Sample implementation</a></li>
    498          <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Acknowledgments</a></li>
    499          <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
    500          <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
    501          <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
    502          <li class="tocline0"><a href="#rfc.index">Index</a></li>
    503       </ul>
    504       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;<a id="access.authentication" href="#access.authentication">Access Authentication</a></h1>
    505       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Reliance on the HTTP/1.1 Specification
    506       </h2>
    507       <p id="rfc.section.1.1.p.1">This specification is a companion to the HTTP/1.1 specification <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. It uses the augmented BNF section <a href="http://tools.ietf.org/html/rfc2616#section-2.1" id="rfc.xref.RFC2616.2">2.1</a> of that document, and relies on both the non-terminals defined in that document and other aspects of the HTTP/1.1 specification.
    508       </p>
    509       <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="access.authentication.framework" href="#access.authentication.framework">Access Authentication Framework</a></h2>
    510       <p id="rfc.section.1.2.p.1">HTTP provides a simple challenge-response authentication mechanism that <em class="bcp14">MAY</em> be used by a server to challenge a client request and by a client to provide authentication information. It uses an extensible,
    511          case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs
    512          which carry the parameters necessary for achieving authentication via that scheme.
    513       </p>
    514       <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.a.1"></span>      auth-scheme    = token
     333</style><link rel="Contents" href="#rfc.toc"><link rel="Author" href="#rfc.authors"><link rel="Copyright" href="#rfc.copyright"><link rel="Index" href="#rfc.index"><link rel="Chapter" title="1 Access Authentication" href="#rfc.section.1"><link rel="Chapter" title="2 Basic Authentication Scheme" href="#rfc.section.2"><link rel="Chapter" title="3 Digest Access Authentication Scheme" href="#rfc.section.3"><link rel="Chapter" title="4 Security Considerations" href="#rfc.section.4"><link rel="Chapter" title="5 Sample implementation" href="#rfc.section.5"><link rel="Chapter" title="6 Acknowledgments" href="#rfc.section.6"><link rel="Chapter" href="#rfc.section.7" title="7 References"><link rel="Alternate" title="Authorative ASCII version" href="http://www.ietf.org/rfc/rfc2617.txt"><meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.389, 2008-08-20 14:21:35, XSLT vendor: SAXON 6.5.5 from Michael Kay http://saxon.sf.net/"><link rel="schema.DC" href="http://purl.org/dc/elements/1.1/"><meta name="DC.Creator" content="Franks, J."><meta name="DC.Creator" content="Hallam-Baker, P.M."><meta name="DC.Creator" content="Hostetler, J.L."><meta name="DC.Creator" content="Lawrence, S.D."><meta name="DC.Creator" content="Leach, P.J."><meta name="DC.Creator" content="Luotonen, A."><meta name="DC.Creator" content="Stewart, L."><meta name="DC.Identifier" content="urn:ietf:rfc:2617"><meta name="DC.Date.Issued" scheme="ISO8601" content="1999-06"><meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2069"><meta name="DC.Description.Abstract" content="&#34;HTTP/1.0&#34;, includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication (unless used in conjunction with some external secure system such as SSL ), as the user name and password are passed over the network as cleartext. This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme and a scheme based on cryptographic hashes, referred to as &#34;Digest Access Authentication&#34;. It is therefore also intended to serve as a replacement for RFC 2069 . Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication; other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended. Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use."></head><body><table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1"><tr><td class="header left">Network Working Group</td><td class="header right">J. Franks</td></tr><tr><td class="header left">Request for Comments: 2617</td><td class="header right">Northwestern University, Department of Mathematics</td></tr><tr><td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2069">2069</a></td><td class="header right">P.M. Hallam-Baker</td></tr><tr><td class="header left">Category: Standards Track</td><td class="header right">Verisign Inc.</td></tr><tr><td class="header left"></td><td class="header right">J.L. Hostetler</td></tr><tr><td class="header left"></td><td class="header right">AbiSource, Inc.</td></tr><tr><td class="header left"></td><td class="header right">S.D. Lawrence</td></tr><tr><td class="header left"></td><td class="header right">Agranat Systems, Inc.</td></tr><tr><td class="header left"></td><td class="header right">P.J. Leach</td></tr><tr><td class="header left"></td><td class="header right">Microsoft Corporation</td></tr><tr><td class="header left"></td><td class="header right">A. Luotonen</td></tr><tr><td class="header left"></td><td class="header right">Netscape Communications Corporation</td></tr><tr><td class="header left"></td><td class="header right">L. Stewart</td></tr><tr><td class="header left"></td><td class="header right">Open Market, Inc.</td></tr><tr><td class="header left"></td><td class="header right">June 1999</td></tr></table><p class="title">HTTP Authentication: Basic and Digest Access Authentication</p><h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1><p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the &#8220;Internet Official Protocol Standards&#8221; (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.</p><h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1><p>Copyright &copy; The Internet Society (1999). All Rights Reserved.</p><h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> <p>"HTTP/1.0", includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication (unless used in conjunction with some external secure system such as SSL <a href="#RFC2246" id="rfc.xref.RFC2246.1"><cite title="The TLS Protocol Version 1.0">[5]</cite></a>), as the user name and password are passed over the network as cleartext.</p><p>This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme and a scheme based on cryptographic hashes, referred to as "Digest Access Authentication". It is therefore also intended to serve as a replacement for RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.1"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>. Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication; other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended.</p><p>Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use.</p> <hr class="noprint"><h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1><ul class="toc"><li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Access Authentication</a><ul class="toc"><li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Reliance on the HTTP/1.1 Specification</a></li><li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#access.authentication.framework">Access Authentication Framework</a></li></ul></li><li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Basic Authentication Scheme</a></li><li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Digest Access Authentication Scheme</a><ul class="toc"><li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Introduction</a><ul class="toc"><li class="tocline1">3.1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.1">Purpose</a></li><li class="tocline1">3.1.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.2">Overall Operation</a></li><li class="tocline1">3.1.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.3">Representation of digest values</a></li><li class="tocline1">3.1.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.4">Limitations</a></li></ul></li><li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a><ul class="toc"><li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></li><li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a><ul class="toc"><li class="tocline1">3.2.2.1&nbsp;&nbsp;&nbsp;<a href="#request-digest">Request-Digest</a></li><li class="tocline1">3.2.2.2&nbsp;&nbsp;&nbsp;<a href="#A1">A1</a></li><li class="tocline1">3.2.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.3">A2</a></li><li class="tocline1">3.2.2.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.4">Directive values and quoted-string</a></li><li class="tocline1">3.2.2.5&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.5">Various considerations</a></li></ul></li><li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.3">The Authentication-Info Header</a></li></ul></li><li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#digest.operation">Digest Operation</a></li><li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Security Protocol Negotiation</a></li><li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers.example">Example</a></li><li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></li></ul></li><li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc"><li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Authentication of Clients using Basic Authentication</a></li><li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">Authentication of Clients using Digest Authentication</a></li><li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Limited Use Nonce Values</a></li><li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.4">Comparison of Digest with Basic Authentication</a></li><li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#replay.attacks">Replay Attacks</a></li><li class="tocline1">4.6&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.6">Weakness Created by Multiple Authentication Schemes</a></li><li class="tocline1">4.7&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.7">Online dictionary attacks</a></li><li class="tocline1">4.8&nbsp;&nbsp;&nbsp;<a href="#man.in.the.middle">Man in the Middle</a></li><li class="tocline1">4.9&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.9">Chosen plaintext attacks</a></li><li class="tocline1">4.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.10">Precomputed dictionary attacks</a></li><li class="tocline1">4.11&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.11">Batch brute force attacks</a></li><li class="tocline1">4.12&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.12">Spoofing by Counterfeit Servers</a></li><li class="tocline1">4.13&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.13">Storing passwords</a></li><li class="tocline1">4.14&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.14">Summary</a></li></ul></li><li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Sample implementation</a></li><li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Acknowledgments</a></li><li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li><li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li><li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li><li class="tocline0"><a href="#rfc.index">Index</a></li></ul><h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Access Authentication</h1><h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Reliance on the HTTP/1.1 Specification</h2><p id="rfc.section.1.1.p.1">This specification is a companion to the HTTP/1.1 specification <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. It uses the augmented BNF section <a href="http://tools.ietf.org/html/rfc2616#section-2.1" title="Augmented BNF" id="rfc.xref.RFC2616.2">2.1</a> of that document, and relies on both the non-terminals defined in that document and other aspects of the HTTP/1.1 specification.</p><h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a id="access.authentication.framework" href="#access.authentication.framework">Access Authentication Framework</a></h2><p id="rfc.section.1.2.p.1">HTTP provides a simple challenge-response authentication mechanism that <em class="bcp14">MAY</em> be used by a server to challenge a client request and by a client to provide authentication information. It uses an extensible, case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs which carry the parameters necessary for achieving authentication via that scheme.</p><div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.a.1"></span>      auth-scheme    = token
    515334<span id="rfc.iref.a.2"></span>      auth-param     = token "=" ( token | quoted-string )
    516 </pre><p id="rfc.section.1.2.p.3">The 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent. This response <em class="bcp14">MUST</em> include a WWW-Authenticate header field containing at least one challenge applicable to the requested resource. The 407 (Proxy
    517          Authentication Required) response message is used by a proxy to challenge the authorization of a client and <em class="bcp14">MUST</em> include a Proxy-Authenticate header field containing at least one challenge applicable to the proxy for the requested resource.
    518       </p>
    519       <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.c.1"></span>      challenge   = auth-scheme 1*SP 1#auth-param
    520 </pre><p id="rfc.section.1.2.p.5">Note: User agents will need to take special care in parsing the WWW-Authenticate or Proxy-Authenticate header field value
    521          if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents
    522          of a challenge may itself contain a comma-separated list of authentication parameters.
    523       </p>
    524       <p id="rfc.section.1.2.p.6">The authentication parameter realm is defined for all authentication schemes:</p>
    525       <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.r.1"></span>      realm       = "realm" "=" realm-value
     335</pre><p id="rfc.section.1.2.p.3">The 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent. This response <em class="bcp14">MUST</em> include a WWW-Authenticate header field containing at least one challenge applicable to the requested resource. The 407 (Proxy Authentication Required) response message is used by a proxy to challenge the authorization of a client and <em class="bcp14">MUST</em> include a Proxy-Authenticate header field containing at least one challenge applicable to the proxy for the requested resource.</p><div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.c.1"></span>      challenge   = auth-scheme 1*SP 1#auth-param
     336</pre><p id="rfc.section.1.2.p.5">Note: User agents will need to take special care in parsing the WWW-Authenticate or Proxy-Authenticate header field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters.</p><p id="rfc.section.1.2.p.6">The authentication parameter realm is defined for all authentication schemes:</p><div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.r.1"></span>      realm       = "realm" "=" realm-value
    526337<span id="rfc.iref.r.2"></span>      realm-value = quoted-string
    527 </pre><p id="rfc.section.1.2.p.8">The realm directive (case-insensitive) is required for all authentication schemes that issue a challenge. The realm value
    528          (case-sensitive), in combination with the canonical root URL (the absoluteURI for the server whose abs_path is empty; see
    529          section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" id="rfc.xref.RFC2616.3">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.4"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be
    530          partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm
    531          value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication
    532          scheme. Note that there may be multiple challenges with the same auth-scheme but different realms.
    533       </p>
    534       <p id="rfc.section.1.2.p.9">A user agent that wishes to authenticate itself with an origin server--usually, but not necessarily, after receiving a 401
    535          (Unauthorized)--MAY do so by including an Authorization header field with the request. A client that wishes to authenticate
    536          itself with a proxy--usually, but not necessarily, after receiving a 407 (Proxy Authentication Required)--MAY do so by including
    537          a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization field
    538          value consist of credentials containing the authentication information of the client for the realm of the resource being requested.
    539          The user agent <em class="bcp14">MUST</em> choose to use one of the challenges with the strongest auth-scheme it understands and request credentials from the user based
    540          upon that challenge.
    541       </p>
    542       <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.c.2"></span>   credentials = auth-scheme #auth-param
    543 </pre><p id="rfc.section.1.2.p.11"> </p>
    544       <dl class="empty">
    545          <dd>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
    546             only include Basic if it is minimally acceptable.
    547          </dd>
    548       </dl>
    549       <p id="rfc.section.1.2.p.12">The protection space determines the domain over which credentials can be automatically applied. If a prior request has been
    550          authorized, the same credentials <em class="bcp14">MAY</em> be reused for all other requests within that protection space for a period of time determined by the authentication scheme,
    551          parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot
    552          extend outside the scope of its server.
    553       </p>
    554       <p id="rfc.section.1.2.p.13">If the origin server does not wish to accept the credentials sent with a request, it <em class="bcp14">SHOULD</em> return a 401 (Unauthorized) response. The response <em class="bcp14">MUST</em> include a WWW-Authenticate header field containing at least one (possibly new) challenge applicable to the requested resource.
    555          If a proxy does not accept the credentials sent with a request, it <em class="bcp14">SHOULD</em> return a 407 (Proxy Authentication Required). The response <em class="bcp14">MUST</em> include a Proxy-Authenticate header field containing a (possibly new) challenge applicable to the proxy for the requested
    556          resource.
    557       </p>
    558       <p id="rfc.section.1.2.p.14">The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional
    559          mechanisms <em class="bcp14">MAY</em> be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying
    560          authentication information. However, these additional mechanisms are not defined by this specification.
    561       </p>
    562       <p id="rfc.section.1.2.p.15">Proxies <em class="bcp14">MUST</em> be completely transparent regarding user agent authentication by origin servers. That is, they must forward the WWW-Authenticate
    563          and Authorization headers untouched, and follow the rules found in section <a href="http://tools.ietf.org/html/rfc2616#section-14.8" id="rfc.xref.RFC2616.5">14.8</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.6"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. Both the Proxy-Authenticate and the Proxy-Authorization header fields are hop-by-hop headers (see section <a href="http://tools.ietf.org/html/rfc2616#section-13.5.1" id="rfc.xref.RFC2616.7">13.5.1</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.8"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>).
    564       </p>
    565       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;Basic Authentication Scheme
    566       </h1>
    567       <p id="rfc.section.2.p.1">The "basic" authentication scheme is based on the model that the client must authenticate itself with a user-ID and a password
    568          for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms
    569          on that server. The server will service the request only if it can validate the user-ID and password for the protection space
    570          of the Request-URI. There are no optional authentication parameters.
    571       </p>
    572       <p id="rfc.section.2.p.2">For Basic, the framework above is utilized as follows:</p>
    573       <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.c.3"></span>      challenge   = "Basic" realm
     338</pre><p id="rfc.section.1.2.p.8">The realm directive (case-insensitive) is required for all authentication schemes that issue a challenge. The realm value (case-sensitive), in combination with the canonical root URL (the absoluteURI for the server whose abs_path is empty; see section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" title="Request-URI" id="rfc.xref.RFC2616.3">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.4"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication scheme. Note that there may be multiple challenges with the same auth-scheme but different realms.</p><p id="rfc.section.1.2.p.9">A user agent that wishes to authenticate itself with an origin server--usually, but not necessarily, after receiving a 401 (Unauthorized)--MAY do so by including an Authorization header field with the request. A client that wishes to authenticate itself with a proxy--usually, but not necessarily, after receiving a 407 (Proxy Authentication Required)--MAY do so by including a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization field value consist of credentials containing the authentication information of the client for the realm of the resource being requested. The user agent <em class="bcp14">MUST</em> choose to use one of the challenges with the strongest auth-scheme it understands and request credentials from the user based upon that challenge.</p><div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.c.2"></span>   credentials = auth-scheme #auth-param
     339</pre><p id="rfc.section.1.2.p.11"> </p><dl class="empty"><dd>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should only include Basic if it is minimally acceptable.</dd></dl><p id="rfc.section.1.2.p.12">The protection space determines the domain over which credentials can be automatically applied. If a prior request has been authorized, the same credentials <em class="bcp14">MAY</em> be reused for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot extend outside the scope of its server.</p><p id="rfc.section.1.2.p.13">If the origin server does not wish to accept the credentials sent with a request, it <em class="bcp14">SHOULD</em> return a 401 (Unauthorized) response. The response <em class="bcp14">MUST</em> include a WWW-Authenticate header field containing at least one (possibly new) challenge applicable to the requested resource. If a proxy does not accept the credentials sent with a request, it <em class="bcp14">SHOULD</em> return a 407 (Proxy Authentication Required). The response <em class="bcp14">MUST</em> include a Proxy-Authenticate header field containing a (possibly new) challenge applicable to the proxy for the requested resource.</p><p id="rfc.section.1.2.p.14">The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms <em class="bcp14">MAY</em> be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, these additional mechanisms are not defined by this specification.</p><p id="rfc.section.1.2.p.15">Proxies <em class="bcp14">MUST</em> be completely transparent regarding user agent authentication by origin servers. That is, they must forward the WWW-Authenticate and Authorization headers untouched, and follow the rules found in section <a href="http://tools.ietf.org/html/rfc2616#section-14.8" title="Authorization" id="rfc.xref.RFC2616.5">14.8</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.6"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. Both the Proxy-Authenticate and the Proxy-Authorization header fields are hop-by-hop headers (see section <a href="http://tools.ietf.org/html/rfc2616#section-13.5.1" title="End-to-end and Hop-by-hop Headers" id="rfc.xref.RFC2616.7">13.5.1</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.8"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>).</p><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;Basic Authentication Scheme</h1><p id="rfc.section.2.p.1">The "basic" authentication scheme is based on the model that the client must authenticate itself with a user-ID and a password for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms on that server. The server will service the request only if it can validate the user-ID and password for the protection space of the Request-URI. There are no optional authentication parameters.</p><p id="rfc.section.2.p.2">For Basic, the framework above is utilized as follows:</p><div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.c.3"></span>      challenge   = "Basic" realm
    574340<span id="rfc.iref.c.4"></span>      credentials = "Basic" basic-credentials
    575 </pre><p id="rfc.section.2.p.4">Upon receipt of an unauthorized request for a URI within the protection space, the origin server <em class="bcp14">MAY</em> respond with a challenge like the following:
    576       </p>
    577       <div id="rfc.figure.u.6"></div><pre class="text">      WWW-Authenticate: Basic realm="WallyWorld"
    578 </pre><p id="rfc.section.2.p.6">where "WallyWorld" is the string assigned by the server to identify the protection space of the Request-URI. A proxy may respond
    579          with the same challenge using the Proxy-Authenticate header field.
    580       </p>
    581       <p id="rfc.section.2.p.7">To receive authorization, the client sends the userid and password, separated by a single colon (":") character, within a
    582          base64 <a href="#RFC2396" id="rfc.xref.RFC2396.1"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[7]</cite></a> encoded string in the credentials.
    583       </p>
    584       <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.b.1"></span>      basic-credentials = base64-user-pass
     341</pre><p id="rfc.section.2.p.4">Upon receipt of an unauthorized request for a URI within the protection space, the origin server <em class="bcp14">MAY</em> respond with a challenge like the following:</p><div id="rfc.figure.u.6"></div><pre class="text">      WWW-Authenticate: Basic realm="WallyWorld"
     342</pre><p id="rfc.section.2.p.6">where "WallyWorld" is the string assigned by the server to identify the protection space of the Request-URI. A proxy may respond with the same challenge using the Proxy-Authenticate header field.</p><p id="rfc.section.2.p.7">To receive authorization, the client sends the userid and password, separated by a single colon (":") character, within a base64 <a href="#RFC2396" id="rfc.xref.RFC2396.1"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[7]</cite></a> encoded string in the credentials.</p><div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.b.1"></span>      basic-credentials = base64-user-pass
    585343<span id="rfc.iref.b.2"></span>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
    586344                       except not limited to 76 char/line&gt;
     
    588346<span id="rfc.iref.u.2"></span>      userid      = *&lt;TEXT excluding ":"&gt;
    589347<span id="rfc.iref.p.1"></span>      password    = *TEXT
    590 </pre><p id="rfc.section.2.p.9">Userids might be case sensitive.</p>
    591       <p id="rfc.section.2.p.10">If the user agent wishes to send the userid "Aladdin" and password "open sesame", it would use the following header field:</p>
    592       <div id="rfc.figure.u.8"></div><pre class="text">      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
    593 </pre><p id="rfc.section.2.p.12">A client <em class="bcp14">SHOULD</em> assume that all paths at or deeper than the depth of the last symbolic element in the path field of the Request-URI also are
    594          within the protection space specified by the Basic realm value of the current challenge. A client <em class="bcp14">MAY</em> preemptively send the corresponding Authorization header with requests for resources in that space without receipt of another
    595          challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the
    596          Proxy-Authorization header field without receiving another challenge from the proxy server. See <a href="#security.considerations" title="Security Considerations">Section&nbsp;4</a> for security considerations associated with Basic authentication.
    597       </p>
    598       <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Digest Access Authentication Scheme
    599       </h1>
    600       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Introduction
    601       </h2>
    602       <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;Purpose
    603       </h3>
    604       <p id="rfc.section.3.1.1.p.1">The protocol referred to as "HTTP/1.0" includes the specification for a Basic Access Authentication scheme<a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a>. That scheme is not considered to be a secure method of user authentication, as the user name and password are passed over
    605          the network in an unencrypted form. This section provides the specification for a scheme that does not send the password in
    606          cleartext, referred to as "Digest Access Authentication".
    607       </p>
    608       <p id="rfc.section.3.1.1.p.2">The Digest Access Authentication scheme is not intended to be a complete answer to the need for security in the World Wide
    609          Web. This scheme provides no encryption of message content. The intent is simply to create an access authentication method
    610          that avoids the most serious flaws of Basic authentication.
    611       </p>
    612       <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;Overall Operation
    613       </h3>
    614       <p id="rfc.section.3.1.2.p.1">Like Basic Access Authentication, the Digest scheme is based on a simple challenge-response paradigm. The Digest scheme challenges
    615          using a nonce value. A valid response contains a checksum (by default, the MD5 checksum) of the username, the password, the
    616          given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with
    617          the Basic scheme, the username and password must be prearranged in some fashion not addressed by this document.
    618       </p>
    619       <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;Representation of digest values
    620       </h3>
    621       <p id="rfc.section.3.1.3.p.1">An optional header allows the server to specify the algorithm used to create the checksum or digest. By default the MD5 algorithm
    622          is used and that is the only algorithm described in this document.
    623       </p>
    624       <p id="rfc.section.3.1.3.p.2">For the purposes of this document, an MD5 digest of 128 bits is represented as 32 ASCII printable characters. The bits in
    625          the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation
    626          as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That
    627          is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'.
    628       </p>
    629       <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;Limitations
    630       </h3>
    631       <p id="rfc.section.3.1.4.p.1">The Digest authentication scheme described in this document suffers from many known limitations. It is intended as a replacement
    632          for Basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same
    633          problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between
    634          user and server to establish the user's password.
    635       </p>
    636       <p id="rfc.section.3.1.4.p.2">Users and implementors should be aware that this protocol is not as secure as Kerberos, and not as secure as any client-side
    637          private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better
    638          than Basic authentication.
    639       </p>
    640       <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="specification.of.digest.headers" href="#specification.of.digest.headers">Specification of Digest Headers</a></h2>
    641       <p id="rfc.section.3.2.p.1">The Digest Access Authentication scheme is conceptually similar to the Basic scheme. The formats of the modified WWW-Authenticate
    642          header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.
    643       </p>
    644       <div id="rfc.iref.h.1"></div>
    645       <div id="rfc.iref.w.1"></div>
    646       <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="the.www-authenticate.response.header" href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></h3>
    647       <p id="rfc.section.3.2.1.p.1">If a server receives a request for an access-protected object, and an acceptable Authorization header is not sent, the server
    648          responds with a "401 Unauthorized" status code, and a WWW-Authenticate header as per the framework defined above, which for
    649          the digest scheme is utilized as follows:
    650       </p>
    651       <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.c.5"></span>      challenge        =  "Digest" digest-challenge
     348</pre><p id="rfc.section.2.p.9">Userids might be case sensitive.</p><p id="rfc.section.2.p.10">If the user agent wishes to send the userid "Aladdin" and password "open sesame", it would use the following header field:</p><div id="rfc.figure.u.8"></div><pre class="text">      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
     349</pre><p id="rfc.section.2.p.12">A client <em class="bcp14">SHOULD</em> assume that all paths at or deeper than the depth of the last symbolic element in the path field of the Request-URI also are within the protection space specified by the Basic realm value of the current challenge. A client <em class="bcp14">MAY</em> preemptively send the corresponding Authorization header with requests for resources in that space without receipt of another challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the Proxy-Authorization header field without receiving another challenge from the proxy server. See <a href="#security.considerations" title="Security Considerations">Section&nbsp;4</a> for security considerations associated with Basic authentication.</p><h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Digest Access Authentication Scheme</h1><h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Introduction</h2><h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;Purpose</h3><p id="rfc.section.3.1.1.p.1">The protocol referred to as "HTTP/1.0" includes the specification for a Basic Access Authentication scheme<a href="#RFC1945" id="rfc.xref.RFC1945.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.0">[1]</cite></a>. That scheme is not considered to be a secure method of user authentication, as the user name and password are passed over the network in an unencrypted form. This section provides the specification for a scheme that does not send the password in cleartext, referred to as "Digest Access Authentication".</p><p id="rfc.section.3.1.1.p.2">The Digest Access Authentication scheme is not intended to be a complete answer to the need for security in the World Wide Web. This scheme provides no encryption of message content. The intent is simply to create an access authentication method that avoids the most serious flaws of Basic authentication.</p><h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;Overall Operation</h3><p id="rfc.section.3.1.2.p.1">Like Basic Access Authentication, the Digest scheme is based on a simple challenge-response paradigm. The Digest scheme challenges using a nonce value. A valid response contains a checksum (by default, the MD5 checksum) of the username, the password, the given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with the Basic scheme, the username and password must be prearranged in some fashion not addressed by this document.</p><h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;Representation of digest values</h3><p id="rfc.section.3.1.3.p.1">An optional header allows the server to specify the algorithm used to create the checksum or digest. By default the MD5 algorithm is used and that is the only algorithm described in this document.</p><p id="rfc.section.3.1.3.p.2">For the purposes of this document, an MD5 digest of 128 bits is represented as 32 ASCII printable characters. The bits in the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'.</p><h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;Limitations</h3><p id="rfc.section.3.1.4.p.1">The Digest authentication scheme described in this document suffers from many known limitations. It is intended as a replacement for Basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between user and server to establish the user's password.</p><p id="rfc.section.3.1.4.p.2">Users and implementors should be aware that this protocol is not as secure as Kerberos, and not as secure as any client-side private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better than Basic authentication.</p><h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="specification.of.digest.headers" href="#specification.of.digest.headers">Specification of Digest Headers</a></h2><p id="rfc.section.3.2.p.1">The Digest Access Authentication scheme is conceptually similar to the Basic scheme. The formats of the modified WWW-Authenticate header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.</p><div id="rfc.iref.h.1"></div><div id="rfc.iref.w.1"></div><h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a id="the.www-authenticate.response.header" href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></h3><p id="rfc.section.3.2.1.p.1">If a server receives a request for an access-protected object, and an acceptable Authorization header is not sent, the server responds with a "401 Unauthorized" status code, and a WWW-Authenticate header as per the framework defined above, which for the digest scheme is utilized as follows:</p><div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.c.5"></span>      challenge        =  "Digest" digest-challenge
    652350
    653351<span id="rfc.iref.d.1"></span>      digest-challenge  = 1#( realm | [ domain ] | nonce |
     
    666364<span id="rfc.iref.q.1"></span>      qop-options       = "qop" "=" &lt;"&gt; 1#qop-value &lt;"&gt;
    667365<span id="rfc.iref.q.2"></span>      qop-value         = "auth" | "auth-int" | token
    668 </pre><p id="rfc.section.3.2.1.p.3">The meanings of the values of the directives used above are as follows:</p>
    669       <p id="rfc.section.3.2.1.p.4">realm </p>
    670       <dl class="empty">
    671          <dd>A string to be displayed to users so they know which username and password to use. This string should contain at least the
    672             name of the host performing the authentication and might additionally indicate the collection of users who might have access.
    673             An example might be "registered_users@gotham.news.com".
    674          </dd>
    675       </dl>
    676       <p id="rfc.section.3.2.1.p.5">domain </p>
    677       <dl class="empty">
    678          <dd>A quoted, space-separated list of URIs, as specified in RFC XURI <a href="#RFC2396" id="rfc.xref.RFC2396.2"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[7]</cite></a>, that define the protection space. If a URI is an abs_path, it is relative to the canonical root URL (see <a href="#access.authentication.framework" title="Access Authentication Framework">Section&nbsp;1.2</a> above) of the server being accessed. An absoluteURI in this list may refer to a different server than the one being accessed.
    679             The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI
    680             that has a URI in this list as a prefix (after both have been made absolute) may be assumed to be in the same protection space.
    681             If this directive is omitted or its value is empty, the client should assume that the protection space consists of all URIs
    682             on the responding server. This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is
    683             always the entire proxy; if present it should be ignored.
    684          </dd>
    685       </dl>
    686       <p id="rfc.section.3.2.1.p.6">nonce </p>
    687       <dl class="empty">
    688          <dd>A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that
    689             this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string,
    690             the double-quote character is not allowed.
    691          </dd>
    692          <dd>The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce
    693             might, for example, be constructed as the base 64 encoding of
    694          </dd>
    695          <dd>
    696             <div id="rfc.figure.u.10"></div><pre class="text">         time-stamp H(time-stamp ":" ETag ":" private-key)
    697   </pre></dd>
    698          <dd>where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated
    699             with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate
    700             the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from
    701             that header or if the time-stamp value is not recent enough. In this way the server can limit the time of the nonce's validity.
    702             The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address
    703             of the client in the nonce would appear to offer the server the ability to limit the reuse of the nonce to the same client
    704             that originally got it. However, that would break proxy farms, where requests from a single user often go through different
    705             proxies in the farm. Also, IP address spoofing is not that hard.)
    706          </dd>
    707          <dd>An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against
    708             a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp
    709             for GET requests. For more details on the issues involved see <a href="#security.considerations" title="Security Considerations">Section&nbsp;4</a> of this document.
    710          </dd>
    711          <dd>The nonce is opaque to the client.</dd>
    712       </dl>
    713       <p id="rfc.section.3.2.1.p.7">opaque </p>
    714       <dl class="empty">
    715          <dd>A string of data, specified by the server, which should be returned by the client unchanged in the Authorization header of
    716             subsequent requests with URIs in the same protection space. It is recommended that this string be base64 or hexadecimal data.
    717          </dd>
    718       </dl>
    719       <p id="rfc.section.3.2.1.p.8">stale </p>
    720       <dl class="empty">
    721          <dd>A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE
    722             (case-insensitive), the client may wish to simply retry the request with a new encrypted response, without reprompting the
    723             user for a new username and password. The server should only set stale to TRUE if it receives a request for which the nonce
    724             is invalid but with a valid digest for that nonce (indicating that the client knows the correct username/password). If stale
    725             is FALSE, or anything other than TRUE, or the stale directive is not present, the username and/or password are invalid, and
    726             new values must be obtained.
    727          </dd>
    728       </dl>
    729       <p id="rfc.section.3.2.1.p.9">algorithm </p>
    730       <dl class="empty">
    731          <dd>A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present it is assumed to
    732             be "MD5". If the algorithm is not understood, the challenge should be ignored (and a different one used, if there is more
    733             than one).
    734          </dd>
    735          <dd>In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted
    736             by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data).
    737             The notation unq(X) means the value of the quoted-string X without the surrounding quotes.
    738          </dd>
    739          <dd>For the "MD5" and "MD5-sess" algorithms</dd>
    740          <dd>
    741             <div id="rfc.figure.u.11"></div><pre class="text">         H(data) = MD5(data)
    742     </pre></dd>
    743          <dd>and</dd>
    744          <dd>
    745             <div id="rfc.figure.u.12"></div><pre class="text">         KD(secret, data) = H(concat(secret, ":", data))
    746     </pre></dd>
    747          <dd>i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The "MD5-sess" algorithm is
    748             intended to allow efficient 3rd party authentication servers; for the difference in usage, see the description in <a href="#A1" title="A1">Section&nbsp;3.2.2.2</a>.
    749          </dd>
    750       </dl>
    751       <p id="rfc.section.3.2.1.p.10">qop-options </p>
    752       <dl class="empty">
    753          <dd>This directive is optional, but is made so only for backward compatibility with RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.2"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>; it <em class="bcp14">SHOULD</em> be used by all implementations compliant with this version of the Digest scheme. If present, it is a quoted string of one
    754             or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication;
    755             the value "auth-int" indicates authentication with integrity protection; see the descriptions below for calculating the response
    756             directive value for the application of this choice. Unrecognized options <em class="bcp14">MUST</em> be ignored.
    757          </dd>
    758       </dl>
    759       <p id="rfc.section.3.2.1.p.11">auth-param </p>
    760       <dl class="empty">
    761          <dd>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
    762          </dd>
    763       </dl>
    764       <div id="rfc.iref.h.2"></div>
    765       <div id="rfc.iref.a.4"></div>
    766       <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="the.authorization.request.header" href="#the.authorization.request.header">The Authorization Request Header</a></h3>
    767       <p id="rfc.section.3.2.2.p.1">The client is expected to retry the request, passing an Authorization header line, which is defined according to the framework
    768          above, utilized as follows.
    769       </p>
    770       <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.c.6"></span>       credentials      = "Digest" digest-response
     366</pre><p id="rfc.section.3.2.1.p.3">The meanings of the values of the directives used above are as follows:</p><p id="rfc.section.3.2.1.p.4">realm </p><dl class="empty"><dd>A string to be displayed to users so they know which username and password to use. This string should contain at least the name of the host performing the authentication and might additionally indicate the collection of users who might have access. An example might be "registered_users@gotham.news.com".</dd></dl><p id="rfc.section.3.2.1.p.5">domain </p><dl class="empty"><dd>A quoted, space-separated list of URIs, as specified in RFC XURI <a href="#RFC2396" id="rfc.xref.RFC2396.2"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[7]</cite></a>, that define the protection space. If a URI is an abs_path, it is relative to the canonical root URL (see <a href="#access.authentication.framework" title="Access Authentication Framework">Section&nbsp;1.2</a> above) of the server being accessed. An absoluteURI in this list may refer to a different server than the one being accessed. The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI that has a URI in this list as a prefix (after both have been made absolute) may be assumed to be in the same protection space. If this directive is omitted or its value is empty, the client should assume that the protection space consists of all URIs on the responding server. This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is always the entire proxy; if present it should be ignored.</dd></dl><p id="rfc.section.3.2.1.p.6">nonce </p><dl class="empty"><dd>A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string, the double-quote character is not allowed.</dd><dd>The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce might, for example, be constructed as the base 64 encoding of</dd><dd><div id="rfc.figure.u.10"></div><pre class="text">         time-stamp H(time-stamp ":" ETag ":" private-key)
     367  </pre></dd><dd>where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from that header or if the time-stamp value is not recent enough. In this way the server can limit the time of the nonce's validity. The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address of the client in the nonce would appear to offer the server the ability to limit the reuse of the nonce to the same client that originally got it. However, that would break proxy farms, where requests from a single user often go through different proxies in the farm. Also, IP address spoofing is not that hard.)</dd><dd>An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp for GET requests. For more details on the issues involved see <a href="#security.considerations" title="Security Considerations">Section&nbsp;4</a> of this document.</dd><dd>The nonce is opaque to the client.</dd></dl><p id="rfc.section.3.2.1.p.7">opaque </p><dl class="empty"><dd>A string of data, specified by the server, which should be returned by the client unchanged in the Authorization header of subsequent requests with URIs in the same protection space. It is recommended that this string be base64 or hexadecimal data.</dd></dl><p id="rfc.section.3.2.1.p.8">stale </p><dl class="empty"><dd>A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE (case-insensitive), the client may wish to simply retry the request with a new encrypted response, without reprompting the user for a new username and password. The server should only set stale to TRUE if it receives a request for which the nonce is invalid but with a valid digest for that nonce (indicating that the client knows the correct username/password). If stale is FALSE, or anything other than TRUE, or the stale directive is not present, the username and/or password are invalid, and new values must be obtained.</dd></dl><p id="rfc.section.3.2.1.p.9">algorithm </p><dl class="empty"><dd>A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present it is assumed to be "MD5". If the algorithm is not understood, the challenge should be ignored (and a different one used, if there is more than one).</dd><dd>In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data). The notation unq(X) means the value of the quoted-string X without the surrounding quotes.</dd><dd>For the "MD5" and "MD5-sess" algorithms</dd><dd><div id="rfc.figure.u.11"></div><pre class="text">         H(data) = MD5(data)
     368    </pre></dd><dd>and</dd><dd><div id="rfc.figure.u.12"></div><pre class="text">         KD(secret, data) = H(concat(secret, ":", data))
     369    </pre></dd><dd>i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The "MD5-sess" algorithm is intended to allow efficient 3rd party authentication servers; for the difference in usage, see the description in <a href="#A1" title="A1">Section&nbsp;3.2.2.2</a>.</dd></dl><p id="rfc.section.3.2.1.p.10">qop-options </p><dl class="empty"><dd>This directive is optional, but is made so only for backward compatibility with RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.2"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>; it <em class="bcp14">SHOULD</em> be used by all implementations compliant with this version of the Digest scheme. If present, it is a quoted string of one or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication; the value "auth-int" indicates authentication with integrity protection; see the descriptions below for calculating the response directive value for the application of this choice. Unrecognized options <em class="bcp14">MUST</em> be ignored.</dd></dl><p id="rfc.section.3.2.1.p.11">auth-param </p><dl class="empty"><dd>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.</dd></dl><div id="rfc.iref.h.2"></div><div id="rfc.iref.a.4"></div><h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a id="the.authorization.request.header" href="#the.authorization.request.header">The Authorization Request Header</a></h3><p id="rfc.section.3.2.2.p.1">The client is expected to retry the request, passing an Authorization header line, which is defined according to the framework above, utilized as follows.</p><div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.c.6"></span>       credentials      = "Digest" digest-response
    771370<span id="rfc.iref.d.3"></span>       digest-response  = 1#( username | realm | nonce | digest-uri
    772371                       | response | [ algorithm ] | [cnonce] |
     
    789388                           "8" | "9" | "a" | "b" |
    790389                           "c" | "d" | "e" | "f"
    791 </pre><p id="rfc.section.3.2.2.p.3">The values of the opaque and algorithm fields must be those supplied in the WWW-Authenticate response header for the entity
    792          being requested.
    793       </p>
    794       <p id="rfc.section.3.2.2.p.4">response </p>
    795       <dl class="empty">
    796          <dd>A string of 32 hex digits computed as defined below, which proves that the user knows a password</dd>
    797       </dl>
    798       <p id="rfc.section.3.2.2.p.5">username </p>
    799       <dl class="empty">
    800          <dd>The user's name in the specified realm.</dd>
    801       </dl>
    802       <p id="rfc.section.3.2.2.p.6">digest-uri </p>
    803       <dl class="empty">
    804          <dd>The URI from Request-URI of the Request-Line; duplicated here because proxies are allowed to change the Request-Line in transit.</dd>
    805       </dl>
    806       <p id="rfc.section.3.2.2.p.7">qop </p>
    807       <dl class="empty">
    808          <dd>Indicates what "quality of protection" the client has applied to the message. If present, its value <em class="bcp14">MUST</em> be one of the alternatives the server indicated it supports in the WWW-Authenticate header. These values affect the computation
    809             of the request-digest. Note that this is a single token, not a quoted list of alternatives as in WWW-Authenticate. This directive
    810             is optional in order to preserve backward compatibility with a minimal implementation of RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.3"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>, but <em class="bcp14">SHOULD</em> be used if the server indicated that qop is supported by providing a qop directive in the WWW-Authenticate header field.
    811          </dd>
    812       </dl>
    813       <p id="rfc.section.3.2.2.p.8">cnonce </p>
    814       <dl class="empty">
    815          <dd>This <em class="bcp14">MUST</em> be specified if a qop directive is sent (see above), and <em class="bcp14">MUST NOT</em> be specified if the server did not send a qop directive in the WWW-Authenticate header field. The cnonce-value is an opaque
    816             quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide
    817             mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of
    818             the response-digest and request-digest values.
    819          </dd>
    820       </dl>
    821       <p id="rfc.section.3.2.2.p.9">nonce-count </p>
    822       <dl class="empty">
    823          <dd>This <em class="bcp14">MUST</em> be specified if a qop directive is sent (see above), and <em class="bcp14">MUST NOT</em> be specified if the server did not send a qop directive in the WWW-Authenticate header field. The nc-value is the hexadecimal
    824             count of the number of requests (including the current request) that the client has sent with the nonce value in this request.
    825             For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of
    826             this directive is to allow the server to detect request replays by maintaining its own copy of this count - if the same nc-value
    827             is seen twice, then the request is a replay. See the description below of the construction of the request-digest value.
    828          </dd>
    829       </dl>
    830       <p id="rfc.section.3.2.2.p.10">auth-param </p>
    831       <dl class="empty">
    832          <dd>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
    833          </dd>
    834       </dl>
    835       <p id="rfc.section.3.2.2.p.11">If a directive or its value is improper, or required directives are missing, the proper response is 400 Bad Request. If the
    836          request-digest is invalid, then a login failure should be logged, since repeated login failures from a single client may indicate
    837          an attacker attempting to guess passwords.
    838       </p>
    839       <p id="rfc.section.3.2.2.p.12">The definition of request-digest above indicates the encoding for its value. The following definitions show how the value
    840          is computed.
    841       </p>
    842       <h4 id="rfc.section.3.2.2.1"><a href="#rfc.section.3.2.2.1">3.2.2.1</a>&nbsp;<a id="request-digest" href="#request-digest">Request-Digest</a></h4>
    843       <p id="rfc.section.3.2.2.1.p.1">If the "qop" value is "auth" or "auth-int":</p>
    844       <div id="rfc.figure.u.14"></div><pre class="inline">      request-digest  = &lt;"&gt; &lt; KD ( H(A1),     unq(nonce-value)
     390</pre><p id="rfc.section.3.2.2.p.3">The values of the opaque and algorithm fields must be those supplied in the WWW-Authenticate response header for the entity being requested.</p><p id="rfc.section.3.2.2.p.4">response </p><dl class="empty"><dd>A string of 32 hex digits computed as defined below, which proves that the user knows a password</dd></dl><p id="rfc.section.3.2.2.p.5">username </p><dl class="empty"><dd>The user's name in the specified realm.</dd></dl><p id="rfc.section.3.2.2.p.6">digest-uri </p><dl class="empty"><dd>The URI from Request-URI of the Request-Line; duplicated here because proxies are allowed to change the Request-Line in transit.</dd></dl><p id="rfc.section.3.2.2.p.7">qop </p><dl class="empty"><dd>Indicates what "quality of protection" the client has applied to the message. If present, its value <em class="bcp14">MUST</em> be one of the alternatives the server indicated it supports in the WWW-Authenticate header. These values affect the computation of the request-digest. Note that this is a single token, not a quoted list of alternatives as in WWW-Authenticate. This directive is optional in order to preserve backward compatibility with a minimal implementation of RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.3"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>, but <em class="bcp14">SHOULD</em> be used if the server indicated that qop is supported by providing a qop directive in the WWW-Authenticate header field.</dd></dl><p id="rfc.section.3.2.2.p.8">cnonce </p><dl class="empty"><dd>This <em class="bcp14">MUST</em> be specified if a qop directive is sent (see above), and <em class="bcp14">MUST NOT</em> be specified if the server did not send a qop directive in the WWW-Authenticate header field. The cnonce-value is an opaque quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of the response-digest and request-digest values.</dd></dl><p id="rfc.section.3.2.2.p.9">nonce-count </p><dl class="empty"><dd>This <em class="bcp14">MUST</em> be specified if a qop directive is sent (see above), and <em class="bcp14">MUST NOT</em> be specified if the server did not send a qop directive in the WWW-Authenticate header field. The nc-value is the hexadecimal count of the number of requests (including the current request) that the client has sent with the nonce value in this request. For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of this directive is to allow the server to detect request replays by maintaining its own copy of this count - if the same nc-value is seen twice, then the request is a replay. See the description below of the construction of the request-digest value.</dd></dl><p id="rfc.section.3.2.2.p.10">auth-param </p><dl class="empty"><dd>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.</dd></dl><p id="rfc.section.3.2.2.p.11">If a directive or its value is improper, or required directives are missing, the proper response is 400 Bad Request. If the request-digest is invalid, then a login failure should be logged, since repeated login failures from a single client may indicate an attacker attempting to guess passwords.</p><p id="rfc.section.3.2.2.p.12">The definition of request-digest above indicates the encoding for its value. The following definitions show how the value is computed.</p><h4 id="rfc.section.3.2.2.1"><a href="#rfc.section.3.2.2.1">3.2.2.1</a>&nbsp;<a id="request-digest" href="#request-digest">Request-Digest</a></h4><p id="rfc.section.3.2.2.1.p.1">If the "qop" value is "auth" or "auth-int":</p><div id="rfc.figure.u.14"></div><pre class="inline">      request-digest  = &lt;"&gt; &lt; KD ( H(A1),     unq(nonce-value)
    845391                                          ":" nc-value
    846392                                          ":" unq(cnonce-value)
     
    848394                                          ":" H(A2)
    849395                                  ) &lt;"&gt;
    850 </pre><p id="rfc.section.3.2.2.1.p.3">If the "qop" directive is not present (this construction is for compatibility with RFC 2069):</p>
    851       <div id="rfc.figure.u.15"></div><pre class="inline">      request-digest  =
     396</pre><p id="rfc.section.3.2.2.1.p.3">If the "qop" directive is not present (this construction is for compatibility with RFC 2069):</p><div id="rfc.figure.u.15"></div><pre class="inline">      request-digest  =
    852397                 &lt;"&gt; &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) &gt;
    853398   &lt;"&gt;
    854 </pre><p id="rfc.section.3.2.2.1.p.5">See below for the definitions for A1 and A2.</p>
    855       <h4 id="rfc.section.3.2.2.2"><a href="#rfc.section.3.2.2.2">3.2.2.2</a>&nbsp;<a id="A1" href="#A1">A1</a></h4>
    856       <p id="rfc.section.3.2.2.2.p.1">If the "algorithm" directive's value is "MD5" or is unspecified, then A1 is:</p>
    857       <div id="rfc.figure.u.16"></div><pre class="inline">      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
    858 </pre><p id="rfc.section.3.2.2.2.p.3">where</p>
    859       <div id="rfc.figure.u.17"></div><pre class="inline">      passwd   = &lt; user's password &gt;
    860 </pre><p id="rfc.section.3.2.2.2.p.5">If the "algorithm" directive's value is "MD5-sess", then A1 is calculated only once - on the first request by the client following
    861          receipt of a WWW-Authenticate challenge from the server. It uses the server nonce from that challenge, and the first client
    862          nonce value to construct A1 as follows:
    863       </p>
    864       <div id="rfc.figure.u.18"></div><pre class="inline">      A1       = H( unq(username-value) ":" unq(realm-value)
     399</pre><p id="rfc.section.3.2.2.1.p.5">See below for the definitions for A1 and A2.</p><h4 id="rfc.section.3.2.2.2"><a href="#rfc.section.3.2.2.2">3.2.2.2</a>&nbsp;<a id="A1" href="#A1">A1</a></h4><p id="rfc.section.3.2.2.2.p.1">If the "algorithm" directive's value is "MD5" or is unspecified, then A1 is:</p><div id="rfc.figure.u.16"></div><pre class="inline">      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
     400</pre><p id="rfc.section.3.2.2.2.p.3">where</p><div id="rfc.figure.u.17"></div><pre class="inline">      passwd   = &lt; user's password &gt;
     401</pre><p id="rfc.section.3.2.2.2.p.5">If the "algorithm" directive's value is "MD5-sess", then A1 is calculated only once - on the first request by the client following receipt of a WWW-Authenticate challenge from the server. It uses the server nonce from that challenge, and the first client nonce value to construct A1 as follows:</p><div id="rfc.figure.u.18"></div><pre class="inline">      A1       = H( unq(username-value) ":" unq(realm-value)
    865402                     ":" passwd )
    866403                     ":" unq(nonce-value) ":" unq(cnonce-value)
    867 </pre><p id="rfc.section.3.2.2.2.p.7">This creates a 'session key' for the authentication of subsequent requests and responses which is different for each "authentication
    868          session", thus limiting the amount of material hashed with any one key. (Note: see further discussion of the authentication
    869          session in <a href="#digest.operation" title="Digest Operation">Section&nbsp;3.3</a>) Because the server need only use the hash of the user credentials in order to create the A1 value, this construction could
    870          be used in conjunction with a third party authentication service so that the web server would not need the actual password
    871          value. The specification of such a protocol is beyond the scope of this specification.
    872       </p>
    873       <h4 id="rfc.section.3.2.2.3"><a href="#rfc.section.3.2.2.3">3.2.2.3</a>&nbsp;A2
    874       </h4>
    875       <p id="rfc.section.3.2.2.3.p.1">If the "qop" directive's value is "auth" or is unspecified, then A2 is:</p>
    876       <div id="rfc.figure.u.19"></div><pre class="inline">      A2       = Method ":" digest-uri-value
    877 </pre><p id="rfc.section.3.2.2.3.p.3">If the "qop" value is "auth-int", then A2 is:</p>
    878       <div id="rfc.figure.u.20"></div><pre class="inline">      A2       = Method ":" digest-uri-value ":" H(entity-body)
    879 </pre><h4 id="rfc.section.3.2.2.4"><a href="#rfc.section.3.2.2.4">3.2.2.4</a>&nbsp;Directive values and quoted-string
    880       </h4>
    881       <p id="rfc.section.3.2.2.4.p.1">Note that the value of many of the directives, such as "username-value", are defined as a "quoted-string". However, the "unq"
    882          notation indicates that surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header
    883          includes the fields
    884       </p>
    885       <div id="rfc.figure.u.21"></div><pre class="text">     username="Mufasa", realm=myhost@testrealm.com
    886 </pre><p id="rfc.section.3.2.2.4.p.3">and the user Mufasa has password "Circle Of Life" then H(A1) would be H(Mufasa:myhost@testrealm.com:Circle Of Life) with no
    887          quotation marks in the digested string.
    888       </p>
    889       <p id="rfc.section.3.2.2.4.p.4">No white space is allowed in any of the strings to which the digest function H() is applied unless that white space exists
    890          in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated
    891          above must be
    892       </p>
    893       <div id="rfc.figure.u.22"></div><pre class="text">        Mufasa:myhost@testrealm.com:Circle Of Life
    894 </pre><p id="rfc.section.3.2.2.4.p.6">with no white space on either side of the colons, but with the white space between the words used in the password value. Likewise,
    895          the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless
    896          that white space was in the quoted strings or entity body being digested.
    897       </p>
    898       <p id="rfc.section.3.2.2.4.p.7">Also note that if integrity protection is applied (qop=auth-int), the H(entity-body) is the hash of the entity body, not the
    899          message body - it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient.
    900          Note that this includes multipart boundaries and embedded headers in each part of any multipart content-type.
    901       </p>
    902       <h4 id="rfc.section.3.2.2.5"><a href="#rfc.section.3.2.2.5">3.2.2.5</a>&nbsp;Various considerations
    903       </h4>
    904       <p id="rfc.section.3.2.2.5.p.1">The "Method" value is the HTTP request method as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.1" id="rfc.xref.RFC2616.9">5.1.1</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.10"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. The "request-uri" value is the Request-URI from the request line as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" id="rfc.xref.RFC2616.11">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.12"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. This may be "*", an "absoluteURL" or an "abs_path" as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" id="rfc.xref.RFC2616.13">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.14"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>, but it <em class="bcp14">MUST</em> agree with the Request-URI. In particular, it <em class="bcp14">MUST</em> be an "absoluteURL" if the Request-URI is an "absoluteURL". The "cnonce-value" is an optional client-chosen value whose purpose
    905          is to foil chosen plaintext attacks.
    906       </p>
    907       <p id="rfc.section.3.2.2.5.p.2">The authenticating server must assure that the resource designated by the "uri" directive is the same as the resource specified
    908          in the Request-Line; if they are not, the server <em class="bcp14">SHOULD</em> return a 400 Bad Request error. (Since this may be a symptom of an attack, server implementers may want to consider logging
    909          such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that
    910          an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would
    911          not result in the same digest as that calculated by the client.
    912       </p>
    913       <p id="rfc.section.3.2.2.5.p.3">Implementers should be aware of how authenticated transactions interact with shared caches. The HTTP/1.1 protocol specifies
    914          that when a shared cache (see section <a href="http://tools.ietf.org/html/rfc2616#section-13.7" id="rfc.xref.RFC2616.15">13.7</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.16"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) has received a request containing an Authorization header and a response from relaying that request, it <em class="bcp14">MUST NOT</em> return that response as a reply to any other request, unless one of two Cache-Control (see section <a href="http://tools.ietf.org/html/rfc2616#section-14.9" id="rfc.xref.RFC2616.17">14.9</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.18"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) directives was present in the response. If the original response included the "must-revalidate" Cache-Control directive,
    915          the cache <em class="bcp14">MAY</em> use the entity of that response in replying to a subsequent request, but <em class="bcp14">MUST</em> first revalidate it with the origin server, using the request headers from the new request to allow the origin server to authenticate
    916          the new request. Alternatively, if the original response included the "public" Cache-Control directive, the response entity <em class="bcp14">MAY</em> be returned in reply to any subsequent request.
    917       </p>
    918       <div id="rfc.iref.h.3"></div>
    919       <div id="rfc.iref.a.5"></div>
    920       <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;The Authentication-Info Header
    921       </h3>
    922       <p id="rfc.section.3.2.3.p.1">The Authentication-Info header is used by the server to communicate some information regarding the successful authentication
    923          in the response.
    924       </p>
    925       <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.a.6"></span>        AuthenticationInfo = "Authentication-Info" ":" auth-info
     404</pre><p id="rfc.section.3.2.2.2.p.7">This creates a 'session key' for the authentication of subsequent requests and responses which is different for each "authentication session", thus limiting the amount of material hashed with any one key. (Note: see further discussion of the authentication session in <a href="#digest.operation" title="Digest Operation">Section&nbsp;3.3</a>) Because the server need only use the hash of the user credentials in order to create the A1 value, this construction could be used in conjunction with a third party authentication service so that the web server would not need the actual password value. The specification of such a protocol is beyond the scope of this specification.</p><h4 id="rfc.section.3.2.2.3"><a href="#rfc.section.3.2.2.3">3.2.2.3</a>&nbsp;A2</h4><p id="rfc.section.3.2.2.3.p.1">If the "qop" directive's value is "auth" or is unspecified, then A2 is:</p><div id="rfc.figure.u.19"></div><pre class="inline">      A2       = Method ":" digest-uri-value
     405</pre><p id="rfc.section.3.2.2.3.p.3">If the "qop" value is "auth-int", then A2 is:</p><div id="rfc.figure.u.20"></div><pre class="inline">      A2       = Method ":" digest-uri-value ":" H(entity-body)
     406</pre><h4 id="rfc.section.3.2.2.4"><a href="#rfc.section.3.2.2.4">3.2.2.4</a>&nbsp;Directive values and quoted-string</h4><p id="rfc.section.3.2.2.4.p.1">Note that the value of many of the directives, such as "username-value", are defined as a "quoted-string". However, the "unq" notation indicates that surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header includes the fields</p><div id="rfc.figure.u.21"></div><pre class="text">     username="Mufasa", realm=myhost@testrealm.com
     407</pre><p id="rfc.section.3.2.2.4.p.3">and the user Mufasa has password "Circle Of Life" then H(A1) would be H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks in the digested string.</p><p id="rfc.section.3.2.2.4.p.4">No white space is allowed in any of the strings to which the digest function H() is applied unless that white space exists in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated above must be</p><div id="rfc.figure.u.22"></div><pre class="text">        Mufasa:myhost@testrealm.com:Circle Of Life
     408</pre><p id="rfc.section.3.2.2.4.p.6">with no white space on either side of the colons, but with the white space between the words used in the password value. Likewise, the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless that white space was in the quoted strings or entity body being digested.</p><p id="rfc.section.3.2.2.4.p.7">Also note that if integrity protection is applied (qop=auth-int), the H(entity-body) is the hash of the entity body, not the message body - it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient. Note that this includes multipart boundaries and embedded headers in each part of any multipart content-type.</p><h4 id="rfc.section.3.2.2.5"><a href="#rfc.section.3.2.2.5">3.2.2.5</a>&nbsp;Various considerations</h4><p id="rfc.section.3.2.2.5.p.1">The "Method" value is the HTTP request method as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.1" title="Method" id="rfc.xref.RFC2616.9">5.1.1</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.10"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. The "request-uri" value is the Request-URI from the request line as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" title="Request-URI" id="rfc.xref.RFC2616.11">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.12"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. This may be "*", an "absoluteURL" or an "abs_path" as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" title="Request-URI" id="rfc.xref.RFC2616.13">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.14"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>, but it <em class="bcp14">MUST</em> agree with the Request-URI. In particular, it <em class="bcp14">MUST</em> be an "absoluteURL" if the Request-URI is an "absoluteURL". The "cnonce-value" is an optional client-chosen value whose purpose is to foil chosen plaintext attacks.</p><p id="rfc.section.3.2.2.5.p.2">The authenticating server must assure that the resource designated by the "uri" directive is the same as the resource specified in the Request-Line; if they are not, the server <em class="bcp14">SHOULD</em> return a 400 Bad Request error. (Since this may be a symptom of an attack, server implementers may want to consider logging such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would not result in the same digest as that calculated by the client.</p><p id="rfc.section.3.2.2.5.p.3">Implementers should be aware of how authenticated transactions interact with shared caches. The HTTP/1.1 protocol specifies that when a shared cache (see section <a href="http://tools.ietf.org/html/rfc2616#section-13.7" title="Shared and Non-Shared Caches" id="rfc.xref.RFC2616.15">13.7</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.16"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) has received a request containing an Authorization header and a response from relaying that request, it <em class="bcp14">MUST NOT</em> return that response as a reply to any other request, unless one of two Cache-Control (see section <a href="http://tools.ietf.org/html/rfc2616#section-14.9" title="Cache-Control" id="rfc.xref.RFC2616.17">14.9</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.18"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) directives was present in the response. If the original response included the "must-revalidate" Cache-Control directive, the cache <em class="bcp14">MAY</em> use the entity of that response in replying to a subsequent request, but <em class="bcp14">MUST</em> first revalidate it with the origin server, using the request headers from the new request to allow the origin server to authenticate the new request. Alternatively, if the original response included the "public" Cache-Control directive, the response entity <em class="bcp14">MAY</em> be returned in reply to any subsequent request.</p><div id="rfc.iref.h.3"></div><div id="rfc.iref.a.5"></div><h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;The Authentication-Info Header</h3><p id="rfc.section.3.2.3.p.1">The Authentication-Info header is used by the server to communicate some information regarding the successful authentication in the response.</p><div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.a.6"></span>        AuthenticationInfo = "Authentication-Info" ":" auth-info
    926409<span id="rfc.iref.a.7"></span>        auth-info          = 1#(nextnonce | [ message-qop ]
    927410                               | [ response-auth ] | [ cnonce ]
     
    930413<span id="rfc.iref.r.5"></span>        response-auth      = "rspauth" "=" response-digest
    931414<span id="rfc.iref.r.6"></span>        response-digest    = &lt;"&gt; *LHEX &lt;"&gt;
    932 </pre><p id="rfc.section.3.2.3.p.3">The value of the nextnonce directive is the nonce the server wishes the client to use for a future authentication response.
    933          The server may send the Authentication-Info header with a nextnonce field as a means of implementing one-time or otherwise
    934          changing nonces. If the nextnonce field is present the client <em class="bcp14">SHOULD</em> use it when constructing the Authorization header for its next request. Failure of the client to do so may result in a request
    935          to re-authenticate from the server with the "stale=TRUE".
    936       </p>
    937       <p id="rfc.section.3.2.3.p.4"> </p>
    938       <dl class="empty">
    939          <dd>Server implementations should carefully consider the performance implications of the use of this mechanism; pipelined requests
    940             will not be possible if every response includes a nextnonce directive that must be used on the next request received by the
    941             server. Consideration should be given to the performance vs. security tradeoffs of allowing an old nonce value to be used
    942             for a limited time to permit request pipelining. Use of the nonce-count can retain most of the security advantages of a new
    943             server nonce without the deleterious affects on pipelining.
    944          </dd>
    945       </dl>
    946       <p id="rfc.section.3.2.3.p.5">message-qop</p>
    947       <dl class="empty">
    948          <dd>Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication;
    949             the value "auth-int" indicates authentication with integrity protection. The server <em class="bcp14">SHOULD</em> use the same value for the message-qop directive in the response as was sent by the client in the corresponding request.
    950          </dd>
    951       </dl>
    952       <p id="rfc.section.3.2.3.p.7">The optional response digest in the "response-auth" directive supports mutual authentication -- the server proves that it
    953          knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The "response-digest"
    954          value is calculated as for the "request-digest" in the Authorization header, except that if "qop=auth" or is not specified
    955          in the Authorization header for the request, A2 is
    956       </p>
    957       <div id="rfc.figure.u.24"></div><pre class="inline">      A2       = ":" digest-uri-value
    958 </pre><p id="rfc.section.3.2.3.p.9">and if "qop=auth-int", then A2 is</p>
    959       <div id="rfc.figure.u.25"></div><pre class="inline">      A2       = ":" digest-uri-value ":" H(entity-body)
    960 </pre><p id="rfc.section.3.2.3.p.11">where "digest-uri-value" is the value of the "uri" directive on the Authorization header in the request. The "cnonce-value"
    961          and "nc-value" <em class="bcp14">MUST</em> be the ones for the client request to which this message is the response. The "response-auth", "cnonce", and "nonce-count"
    962          directives <em class="bcp14">MUST</em> BE present if "qop=auth" or "qop=auth-int" is specified.
    963       </p>
    964       <p id="rfc.section.3.2.3.p.12">The Authentication-Info header is allowed in the trailer of an HTTP message transferred via chunked transfer-coding.</p>
    965       <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="digest.operation" href="#digest.operation">Digest Operation</a></h2>
    966       <p id="rfc.section.3.3.p.1">Upon receiving the Authorization header, the server may check its validity by looking up the password that corresponds to
    967          the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare
    968          the result to the given request-digest value.
    969       </p>
    970       <p id="rfc.section.3.3.p.2">Note that the HTTP server does not actually need to know the user's cleartext password. As long as H(A1) is available to the
    971          server, the validity of an Authorization header may be verified.
    972       </p>
    973       <p id="rfc.section.3.3.p.3">The client response to a WWW-Authenticate challenge for a protection space starts an authentication session with that protection
    974          space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the
    975          protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an
    976          authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization
    977          header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges.
    978          The server may choose to accept the old Authorization header information, even though the nonce value included might not be
    979          fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request;
    980          by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting
    981          for a new username and password.
    982       </p>
    983       <p id="rfc.section.3.3.p.4">Because the client is required to return the value of the opaque directive given to it by the server for the duration of a
    984          session, the opaque data may be used to transport authentication session state information. (Note that any such use can also
    985          be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for
    986          authenticating content that actually sits on another server. It would achieve this by having the first 401 response include
    987          a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state
    988          information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing
    989          to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the
    990          &lt;opaque&gt; data.
    991       </p>
    992       <p id="rfc.section.3.3.p.5">As with the basic scheme, proxies must be completely transparent in the Digest access authentication scheme. That is, they
    993          must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate
    994          a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization
    995          headers described in <a href="#proxy-authentication.and.proxy-authorization" title="Proxy-Authentication and Proxy-Authorization">Section&nbsp;3.6</a> below.
    996       </p>
    997       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Security Protocol Negotiation
    998       </h2>
    999       <p id="rfc.section.3.4.p.1">It is useful for a server to be able to know which security schemes a client is capable of handling.</p>
    1000       <p id="rfc.section.3.4.p.2">It is possible that a server may want to require Digest as its authentication method, even if the server does not know that
    1001          the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot
    1002          handle.
    1003       </p>
    1004       <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="specification.of.digest.headers.example" href="#specification.of.digest.headers.example">Example</a></h2>
    1005       <p id="rfc.section.3.5.p.1">The following example assumes that an access-protected document is being requested from the server via a GET request. The
    1006          URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document
    1007          is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).
    1008       </p>
    1009       <p id="rfc.section.3.5.p.2">The first time the client requests the document, no Authorization header is sent, so the server responds with:</p>
    1010       <div id="rfc.figure.u.26"></div><pre class="text">         HTTP/1.1 401 Unauthorized
     415</pre><p id="rfc.section.3.2.3.p.3">The value of the nextnonce directive is the nonce the server wishes the client to use for a future authentication response. The server may send the Authentication-Info header with a nextnonce field as a means of implementing one-time or otherwise changing nonces. If the nextnonce field is present the client <em class="bcp14">SHOULD</em> use it when constructing the Authorization header for its next request. Failure of the client to do so may result in a request to re-authenticate from the server with the "stale=TRUE".</p><p id="rfc.section.3.2.3.p.4"> </p><dl class="empty"><dd>Server implementations should carefully consider the performance implications of the use of this mechanism; pipelined requests will not be possible if every response includes a nextnonce directive that must be used on the next request received by the server. Consideration should be given to the performance vs. security tradeoffs of allowing an old nonce value to be used for a limited time to permit request pipelining. Use of the nonce-count can retain most of the security advantages of a new server nonce without the deleterious affects on pipelining.</dd></dl><p id="rfc.section.3.2.3.p.5">message-qop</p><dl class="empty"><dd>Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication; the value "auth-int" indicates authentication with integrity protection. The server <em class="bcp14">SHOULD</em> use the same value for the message-qop directive in the response as was sent by the client in the corresponding request.</dd></dl><p id="rfc.section.3.2.3.p.7">The optional response digest in the "response-auth" directive supports mutual authentication -- the server proves that it knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The "response-digest" value is calculated as for the "request-digest" in the Authorization header, except that if "qop=auth" or is not specified in the Authorization header for the request, A2 is</p><div id="rfc.figure.u.24"></div><pre class="inline">      A2       = ":" digest-uri-value
     416</pre><p id="rfc.section.3.2.3.p.9">and if "qop=auth-int", then A2 is</p><div id="rfc.figure.u.25"></div><pre class="inline">      A2       = ":" digest-uri-value ":" H(entity-body)
     417</pre><p id="rfc.section.3.2.3.p.11">where "digest-uri-value" is the value of the "uri" directive on the Authorization header in the request. The "cnonce-value" and "nc-value" <em class="bcp14">MUST</em> be the ones for the client request to which this message is the response. The "response-auth", "cnonce", and "nonce-count" directives <em class="bcp14">MUST</em> BE present if "qop=auth" or "qop=auth-int" is specified.</p><p id="rfc.section.3.2.3.p.12">The Authentication-Info header is allowed in the trailer of an HTTP message transferred via chunked transfer-coding.</p><h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a id="digest.operation" href="#digest.operation">Digest Operation</a></h2><p id="rfc.section.3.3.p.1">Upon receiving the Authorization header, the server may check its validity by looking up the password that corresponds to the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare the result to the given request-digest value.</p><p id="rfc.section.3.3.p.2">Note that the HTTP server does not actually need to know the user's cleartext password. As long as H(A1) is available to the server, the validity of an Authorization header may be verified.</p><p id="rfc.section.3.3.p.3">The client response to a WWW-Authenticate challenge for a protection space starts an authentication session with that protection space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges. The server may choose to accept the old Authorization header information, even though the nonce value included might not be fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request; by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting for a new username and password.</p><p id="rfc.section.3.3.p.4">Because the client is required to return the value of the opaque directive given to it by the server for the duration of a session, the opaque data may be used to transport authentication session state information. (Note that any such use can also be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for authenticating content that actually sits on another server. It would achieve this by having the first 401 response include a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the &lt;opaque&gt; data.</p><p id="rfc.section.3.3.p.5">As with the basic scheme, proxies must be completely transparent in the Digest access authentication scheme. That is, they must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization headers described in <a href="#proxy-authentication.and.proxy-authorization" title="Proxy-Authentication and Proxy-Authorization">Section&nbsp;3.6</a> below.</p><h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Security Protocol Negotiation</h2><p id="rfc.section.3.4.p.1">It is useful for a server to be able to know which security schemes a client is capable of handling.</p><p id="rfc.section.3.4.p.2">It is possible that a server may want to require Digest as its authentication method, even if the server does not know that the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot handle.</p><h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a id="specification.of.digest.headers.example" href="#specification.of.digest.headers.example">Example</a></h2><p id="rfc.section.3.5.p.1">The following example assumes that an access-protected document is being requested from the server via a GET request. The URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).</p><p id="rfc.section.3.5.p.2">The first time the client requests the document, no Authorization header is sent, so the server responds with:</p><div id="rfc.figure.u.26"></div><pre class="text">         HTTP/1.1 401 Unauthorized
    1011418         WWW-Authenticate: Digest
    1012419                 realm="testrealm@host.com",
     
    1014421                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
    1015422                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
    1016 </pre><p id="rfc.section.3.5.p.4">The client may prompt the user for the username and password, after which it will respond with a new request, including the
    1017          following Authorization header:
    1018       </p>
    1019       <div id="rfc.figure.u.27"></div><pre class="text">         Authorization: Digest username="Mufasa",
     423</pre><p id="rfc.section.3.5.p.4">The client may prompt the user for the username and password, after which it will respond with a new request, including the following Authorization header:</p><div id="rfc.figure.u.27"></div><pre class="text">         Authorization: Digest username="Mufasa",
    1020424                 realm="testrealm@host.com",
    1021425                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
     
    1026430                 response="6629fae49393a05397450978507c4ef1",
    1027431                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
    1028 </pre><h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="proxy-authentication.and.proxy-authorization" href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></h2>
    1029       <p id="rfc.section.3.6.p.1">The digest authentication scheme may also be used for authenticating users to proxies, proxies to proxies, or proxies to origin
    1030          servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate
    1031          and Proxy-Authorization headers specified in sections <a href="http://tools.ietf.org/html/rfc2616#section-10.33" id="rfc.xref.RFC2616.19">10.33</a> and <a href="http://tools.ietf.org/html/rfc2616#section-10.34" id="rfc.xref.RFC2616.20">10.34</a> of the HTTP/1.1 specification <a href="#RFC2616" id="rfc.xref.RFC2616.21"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a> and their behavior is subject to restrictions described there. The transactions for proxy authentication are very similar
    1032          to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy
    1033          Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate
    1034          header is the same as that for the WWW-Authenticate header as defined above in <a href="#the.www-authenticate.response.header" title="The WWW-Authenticate Response Header">Section&nbsp;3.2.1</a>.
    1035       </p>
    1036       <p id="rfc.section.3.6.p.2">The client/proxy must then re-issue the request with a Proxy-Authorization header, with directives as specified for the Authorization
    1037          header in <a href="#the.authorization.request.header" title="The Authorization Request Header">Section&nbsp;3.2.2</a> above.
    1038       </p>
    1039       <p id="rfc.section.3.6.p.3">On subsequent responses, the server sends Proxy-Authentication-Info with directives the same as those for the Authentication-Info
    1040          header field.
    1041       </p>
    1042       <p id="rfc.section.3.6.p.4">Note that in principle a client could be asked to authenticate itself to both a proxy and an end-server, but never in the
    1043          same response.
    1044       </p>
    1045       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
    1046       <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Authentication of Clients using Basic Authentication
    1047       </h2>
    1048       <p id="rfc.section.4.1.p.1">The Basic authentication scheme is not a secure method of user authentication, nor does it in any way protect the entity,
    1049          which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication
    1050          schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes
    1051          to use one-time passwords) to Basic authentication.
    1052       </p>
    1053       <p id="rfc.section.4.1.p.2">The most serious flaw in Basic authentication is that it results in the essentially cleartext transmission of the user's password
    1054          over the physical network. It is this problem which Digest Authentication attempts to address.
    1055       </p>
    1056       <p id="rfc.section.4.1.p.3">Because Basic authentication involves the cleartext transmission of passwords it <em class="bcp14">SHOULD NOT</em> be used (without enhancements) to protect sensitive or valuable information.
    1057       </p>
    1058       <p id="rfc.section.4.1.p.4">A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password
    1059          as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this
    1060          way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major
    1061          concern. This is only correct if the server issues both user name and password to the users and in particular does not allow
    1062          the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid
    1063          the task of maintaining multiple passwords.
    1064       </p>
    1065       <p id="rfc.section.4.1.p.5">If a server permits users to select their own passwords, then the threat is not only unauthorized access to documents on the
    1066          server but also unauthorized access to any other resources on other systems that the user protects with the same password.
    1067          Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner
    1068          or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all
    1069          those sites if this information is not maintained in a secure fashion.
    1070       </p>
    1071       <p id="rfc.section.4.1.p.6">Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting
    1072          to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or
    1073          gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not
    1074          possible with Digest Authentication. Server implementers <em class="bcp14">SHOULD</em> guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous
    1075          for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism
    1076          to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable
    1077          by the client.
    1078       </p>
    1079       <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;Authentication of Clients using Digest Authentication
    1080       </h2>
    1081       <p id="rfc.section.4.2.p.1">Digest Authentication does not provide a strong authentication mechanism, when compared to public key based mechanisms, for
    1082          example. However, it is significantly stronger than (e.g.) CRAM-MD5, which has been proposed for use with LDAP <a href="#ref10" id="rfc.xref.ref10.1"><cite title="Authentication Methods for LDAP">[10]</cite></a>, POP and IMAP (see RFC 2195 <a href="#RFC2195" id="rfc.xref.RFC2195.1"><cite title="IMAP/POP AUTHorize Extension for Simple Challenge/Response">[9]</cite></a>). It is intended to replace the much weaker and even more dangerous Basic mechanism.
    1083       </p>
    1084       <p id="rfc.section.4.2.p.2">Digest Authentication offers no confidentiality protection beyond protecting the actual password. All of the rest of the request
    1085          and response are available to an eavesdropper.
    1086       </p>
    1087       <p id="rfc.section.4.2.p.3">Digest Authentication offers only limited integrity protection for the messages in either direction. If qop=auth-int mechanism
    1088          is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response
    1089          directive values (see <a href="#specification.of.digest.headers" title="Specification of Digest Headers">Section&nbsp;3.2</a> above) are protected. Most header fields and their values could be modified as a part of a man-in-the-middle attack.
    1090       </p>
    1091       <p id="rfc.section.4.2.p.4">Many needs for secure HTTP transactions cannot be met by Digest Authentication. For those needs TLS or SHTTP are more appropriate
    1092          protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless
    1093          many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses
    1094          Basic should be switched to Digest as soon as practical.
    1095       </p>
    1096       <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Limited Use Nonce Values
    1097       </h2>
    1098       <p id="rfc.section.4.3.p.1">The Digest scheme uses a server-specified nonce to seed the generation of the request-digest value (as specified in <a href="#request-digest" title="Request-Digest">Section&nbsp;3.2.2.1</a> above). As shown in the example nonce in <a href="#the.www-authenticate.response.header" title="The WWW-Authenticate Response Header">Section&nbsp;3.2.1</a>, the server is free to construct the nonce such that it may only be used from a particular client, for a particular resource,
    1099          for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against,
    1100          for example, replay attacks (see 4.5). However, it should be noted that the method chosen for generating and checking the
    1101          nonce also has performance and resource implications. For example, a server may choose to allow each nonce value to be used
    1102          only once by maintaining a record of whether or not each recently issued nonce has been returned and sending a next-nonce
    1103          directive in the Authentication-Info header field of every response. This protects against even an immediate replay attack,
    1104          but has a high cost checking nonce values, and perhaps more important will cause authentication failures for any pipelined
    1105          requests (presumably returning a stale nonce indication). Similarly, incorporating a request-specific element such as the
    1106          Etag value for a resource limits the use of the nonce to that version of the resource and also defeats pipelining. Thus it
    1107          may be useful to do so for methods with side effects but have unacceptable performance for those that do not.
    1108       </p>
    1109       <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;Comparison of Digest with Basic Authentication
    1110       </h2>
    1111       <p id="rfc.section.4.4.p.1">Both Digest and Basic Authentication are very much on the weak end of the security strength spectrum. But a comparison between
    1112          the two points out the utility, even necessity, of replacing Basic by Digest.
    1113       </p>
    1114       <p id="rfc.section.4.4.p.2">The greatest threat to the type of transactions for which these protocols are used is network snooping. This kind of transaction
    1115          might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication
    1116          an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often
    1117          worse, will permit access to anything else the user protects with the same password.
    1118       </p>
    1119       <p id="rfc.section.4.4.p.3">By contrast, with Digest Authentication the eavesdropper only gets access to the transaction in question and not to the user's
    1120          password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document,
    1121          and even that may be limited by the server's choice of nonce.
    1122       </p>
    1123       <h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;Replay Attacks
    1124       </h2>
    1125       <p id="rfc.section.4.5.p.1">A replay attack against Digest authentication would usually be pointless for a simple GET request since an eavesdropper would
    1126          already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested
    1127          in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper
    1128          has the user's password, any document protected by that password is open to him.
    1129       </p>
    1130       <p id="rfc.section.4.5.p.2">Thus, for some purposes, it is necessary to protect against replay attacks. A good Digest implementation can do this in various
    1131          ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp,
    1132          the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince
    1133          the server that the request is coming from a false IP address and must cause the server to deliver the document to an IP address
    1134          different from the address to which it believes it is sending the document. An attack can only succeed in the period before
    1135          the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain
    1136          state between transactions.
    1137       </p>
    1138       <p id="rfc.section.4.5.p.3">For applications where no possibility of replay attack can be tolerated the server can use one-time nonce values which will
    1139          not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until
    1140          the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.
    1141       </p>
    1142       <p id="rfc.section.4.5.p.4">An implementation must give special attention to the possibility of replay attacks with POST and PUT requests. Unless the
    1143          server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int,
    1144          an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even
    1145          with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking
    1146          provides some protection against replay of previously used valid credentials, but see 4.8.
    1147       </p>
    1148       <h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;Weakness Created by Multiple Authentication Schemes
    1149       </h2>
    1150       <p id="rfc.section.4.6.p.1">An HTTP/1.1 server may return multiple challenges with a 401 (Authenticate) response, and each challenge may use a different
    1151          auth-scheme. A user agent <em class="bcp14">MUST</em> choose to use the strongest auth-scheme it understands and request credentials from the user based upon that challenge.
    1152       </p>
    1153       <p id="rfc.section.4.6.p.2"> </p>
    1154       <dl class="empty">
    1155          <dd>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
    1156             only include Basic if it is minimally acceptable.
    1157          </dd>
    1158       </dl>
    1159       <p id="rfc.section.4.6.p.3">When the server offers choices of authentication schemes using the WWW-Authenticate header, the strength of the resulting
    1160          authentication is only as good as that of the of the weakest of the authentication schemes. See <a href="#man.in.the.middle" title="Man in the Middle">Section&nbsp;4.8</a> below for discussion of particular attack scenarios that exploit multiple authentication schemes.
    1161       </p>
    1162       <h2 id="rfc.section.4.7"><a href="#rfc.section.4.7">4.7</a>&nbsp;Online dictionary attacks
    1163       </h2>
    1164       <p id="rfc.section.4.7.p.1">If the attacker can eavesdrop, then it can test any overheard nonce/response pairs against a list of common words. Such a
    1165          list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password
    1166          on the list is paid once for each challenge.
    1167       </p>
    1168       <p id="rfc.section.4.7.p.2">The server can mitigate this attack by not allowing users to select passwords that are in a dictionary.</p>
    1169       <h2 id="rfc.section.4.8"><a href="#rfc.section.4.8">4.8</a>&nbsp;<a id="man.in.the.middle" href="#man.in.the.middle">Man in the Middle</a></h2>
    1170       <p id="rfc.section.4.8.p.1">Both Basic and Digest authentication are vulnerable to "man in the middle" (MITM) attacks, for example, from a hostile or
    1171          compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities
    1172          to the attacker.
    1173       </p>
    1174       <p id="rfc.section.4.8.p.2">A possible man-in-the-middle attack would be to add a weak authentication scheme to the set of choices, hoping that the client
    1175          will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest
    1176          scheme that it understands from the choices offered.
    1177       </p>
    1178       <p id="rfc.section.4.8.p.3">An even better MITM attack would be to remove all offered choices, replacing them with a challenge that requests only Basic
    1179          authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using
    1180          the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy
    1181          caching service to gullible users.
    1182       </p>
    1183       <p id="rfc.section.4.8.p.4">User agents should consider measures such as presenting a visual indication at the time of the credentials request of what
    1184          authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce
    1185          a warning message before using a weaker one. It might also be a good idea for the user agent to be configured to demand Digest
    1186          authentication in general, or from specific sites.
    1187       </p>
    1188       <p id="rfc.section.4.8.p.5">Or, a hostile proxy might spoof the client into making a request the attacker wanted rather than one the client wanted. Of
    1189          course, this is still much harder than a comparable attack against Basic Authentication.
    1190       </p>
    1191       <h2 id="rfc.section.4.9"><a href="#rfc.section.4.9">4.9</a>&nbsp;Chosen plaintext attacks
    1192       </h2>
    1193       <p id="rfc.section.4.9.p.1">With Digest authentication, a MITM or a malicious server can arbitrarily choose the nonce that the client will use to compute
    1194          the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much
    1195          easier <a href="#ref8" id="rfc.xref.ref8.1"><cite title="Message Authentication with MD5">[8]</cite></a>.
    1196       </p>
    1197       <p id="rfc.section.4.9.p.2">However, no way to analyze the MD5 one-way function used by Digest using chosen plaintext is currently known.</p>
    1198       <p id="rfc.section.4.9.p.3">The countermeasure against this attack is for clients to be configured to require the use of the optional "cnonce" directive;
    1199          this allows the client to vary the input to the hash in a way not chosen by the attacker.
    1200       </p>
    1201       <h2 id="rfc.section.4.10"><a href="#rfc.section.4.10">4.10</a>&nbsp;Precomputed dictionary attacks
    1202       </h2>
    1203       <p id="rfc.section.4.10.p.1">With Digest authentication, if the attacker can execute a chosen plaintext attack, the attacker can precompute the response
    1204          for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation
    1205          can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding
    1206          to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some
    1207          might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list
    1208          can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes
    1209          of disk storage.
    1210       </p>
    1211       <p id="rfc.section.4.10.p.2">The countermeasure against this attack is to for clients to be configured to require the use of the optional "cnonce" directive.</p>
    1212       <h2 id="rfc.section.4.11"><a href="#rfc.section.4.11">4.11</a>&nbsp;Batch brute force attacks
    1213       </h2>
    1214       <p id="rfc.section.4.11.p.1">With Digest authentication, a MITM can execute a chosen plaintext attack, and can gather responses from many users to the
    1215          same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response
    1216          pairs in a single pass over that space. It also reduces the time to find the first password by a factor equal to the number
    1217          of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even
    1218          a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with
    1219          six or fewer letters in a few hours.
    1220       </p>
    1221       <p id="rfc.section.4.11.p.2">The countermeasure against this attack is to for clients to be configured to require the use of the optional "cnonce" directive.</p>
    1222       <h2 id="rfc.section.4.12"><a href="#rfc.section.4.12">4.12</a>&nbsp;Spoofing by Counterfeit Servers
    1223       </h2>
    1224       <p id="rfc.section.4.12.p.1">Basic Authentication is vulnerable to spoofing by counterfeit servers. If a user can be led to believe that she is connecting
    1225          to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then
    1226          the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult
    1227          with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of
    1228          the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack
    1229          by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of
    1230          each scheme.
    1231       </p>
    1232       <h2 id="rfc.section.4.13"><a href="#rfc.section.4.13">4.13</a>&nbsp;Storing passwords
    1233       </h2>
    1234       <p id="rfc.section.4.13.p.1">Digest authentication requires that the authenticating agent (usually the server) store some data derived from the user's
    1235          name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username
    1236          and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.
    1237       </p>
    1238       <p id="rfc.section.4.13.p.2">The security implications of this are that if this password file is compromised, then an attacker gains immediate access to
    1239          documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted
    1240          in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely
    1241          a brute force attack, would be necessary to obtain the user's password. This is the reason that the realm is part of the digested
    1242          data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically
    1243          compromise others with the same username and password (though it does expose them to brute force attack).
    1244       </p>
    1245       <p id="rfc.section.4.13.p.3">There are two important security consequences of this. First the password file must be protected as if it contained unencrypted
    1246          passwords, because for the purpose of accessing documents in its realm, it effectively does.
    1247       </p>
    1248       <p id="rfc.section.4.13.p.4">A second consequence of this is that the realm string should be unique among all realms which any single user is likely to
    1249          use. In particular a realm string should include the name of the host doing the authentication. The inability of the client
    1250          to authenticate the server is a weakness of Digest Authentication.
    1251       </p>
    1252       <h2 id="rfc.section.4.14"><a href="#rfc.section.4.14">4.14</a>&nbsp;Summary
    1253       </h2>
    1254       <p id="rfc.section.4.14.p.1">By modern cryptographic standards Digest Authentication is weak. But for a large range of purposes it is valuable as a replacement
    1255          for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending
    1256          on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect
    1257          the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may
    1258          be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may
    1259          satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited
    1260          lifetime.
    1261       </p>
    1262       <p id="rfc.section.4.14.p.2">The bottom line is that *any* compliant implementation will be relatively weak by cryptographic standards, but *any* compliant
    1263          implementation will be far superior to Basic Authentication.
    1264       </p>
    1265       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Sample implementation
    1266       </h1>
    1267       <p id="rfc.section.5.p.1">The following code implements the calculations of H(A1), H(A2), request-digest and response-digest, and a test program which
    1268          computes the values used in the example of <a href="#specification.of.digest.headers.example" title="Example">Section&nbsp;3.5</a>. It uses the MD5 implementation from RFC 1321.
    1269       </p>
    1270       <div id="rfc.figure.u.28"></div>
    1271       <p>File "digcalc.h":</p><pre class="text">#define HASHLEN 16
     432</pre><h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a id="proxy-authentication.and.proxy-authorization" href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></h2><p id="rfc.section.3.6.p.1">The digest authentication scheme may also be used for authenticating users to proxies, proxies to proxies, or proxies to origin servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate and Proxy-Authorization headers specified in sections <a href="http://tools.ietf.org/html/rfc2616#section-10.33" id="rfc.xref.RFC2616.19">10.33</a> and <a href="http://tools.ietf.org/html/rfc2616#section-10.34" id="rfc.xref.RFC2616.20">10.34</a> of the HTTP/1.1 specification <a href="#RFC2616" id="rfc.xref.RFC2616.21"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a> and their behavior is subject to restrictions described there. The transactions for proxy authentication are very similar to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate header is the same as that for the WWW-Authenticate header as defined above in <a href="#the.www-authenticate.response.header" title="The WWW-Authenticate Response Header">Section&nbsp;3.2.1</a>.</p><p id="rfc.section.3.6.p.2">The client/proxy must then re-issue the request with a Proxy-Authorization header, with directives as specified for the Authorization header in <a href="#the.authorization.request.header" title="The Authorization Request Header">Section&nbsp;3.2.2</a> above.</p><p id="rfc.section.3.6.p.3">On subsequent responses, the server sends Proxy-Authentication-Info with directives the same as those for the Authentication-Info header field.</p><p id="rfc.section.3.6.p.4">Note that in principle a client could be asked to authenticate itself to both a proxy and an end-server, but never in the same response.</p><h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1><h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Authentication of Clients using Basic Authentication</h2><p id="rfc.section.4.1.p.1">The Basic authentication scheme is not a secure method of user authentication, nor does it in any way protect the entity, which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes to use one-time passwords) to Basic authentication.</p><p id="rfc.section.4.1.p.2">The most serious flaw in Basic authentication is that it results in the essentially cleartext transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address.</p><p id="rfc.section.4.1.p.3">Because Basic authentication involves the cleartext transmission of passwords it <em class="bcp14">SHOULD NOT</em> be used (without enhancements) to protect sensitive or valuable information.</p><p id="rfc.section.4.1.p.4">A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major concern. This is only correct if the server issues both user name and password to the users and in particular does not allow the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid the task of maintaining multiple passwords.</p><p id="rfc.section.4.1.p.5">If a server permits users to select their own passwords, then the threat is not only unauthorized access to documents on the server but also unauthorized access to any other resources on other systems that the user protects with the same password. Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all those sites if this information is not maintained in a secure fashion.</p><p id="rfc.section.4.1.p.6">Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not possible with Digest Authentication. Server implementers <em class="bcp14">SHOULD</em> guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable by the client.</p><h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;Authentication of Clients using Digest Authentication</h2><p id="rfc.section.4.2.p.1">Digest Authentication does not provide a strong authentication mechanism, when compared to public key based mechanisms, for example. However, it is significantly stronger than (e.g.) CRAM-MD5, which has been proposed for use with LDAP <a href="#ref10" id="rfc.xref.ref10.1"><cite title="Authentication Methods for LDAP">[10]</cite></a>, POP and IMAP (see RFC 2195 <a href="#RFC2195" id="rfc.xref.RFC2195.1"><cite title="IMAP/POP AUTHorize Extension for Simple Challenge/Response">[9]</cite></a>). It is intended to replace the much weaker and even more dangerous Basic mechanism.</p><p id="rfc.section.4.2.p.2">Digest Authentication offers no confidentiality protection beyond protecting the actual password. All of the rest of the request and response are available to an eavesdropper.</p><p id="rfc.section.4.2.p.3">Digest Authentication offers only limited integrity protection for the messages in either direction. If qop=auth-int mechanism is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response directive values (see <a href="#specification.of.digest.headers" title="Specification of Digest Headers">Section&nbsp;3.2</a> above) are protected. Most header fields and their values could be modified as a part of a man-in-the-middle attack.</p><p id="rfc.section.4.2.p.4">Many needs for secure HTTP transactions cannot be met by Digest Authentication. For those needs TLS or SHTTP are more appropriate protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses Basic should be switched to Digest as soon as practical.</p><h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Limited Use Nonce Values</h2><p id="rfc.section.4.3.p.1">The Digest scheme uses a server-specified nonce to seed the generation of the request-digest value (as specified in <a href="#request-digest" title="Request-Digest">Section&nbsp;3.2.2.1</a> above). As shown in the example nonce in <a href="#the.www-authenticate.response.header" title="The WWW-Authenticate Response Header">Section&nbsp;3.2.1</a>, the server is free to construct the nonce such that it may only be used from a particular client, for a particular resource, for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against, for example, replay attacks (see <a href="#replay.attacks" title="Replay Attacks">4.5</a>). However, it should be noted that the method chosen for generating and checking the nonce also has performance and resource implications. For example, a server may choose to allow each nonce value to be used only once by maintaining a record of whether or not each recently issued nonce has been returned and sending a next-nonce directive in the Authentication-Info header field of every response. This protects against even an immediate replay attack, but has a high cost checking nonce values, and perhaps more important will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication). Similarly, incorporating a request-specific element such as the Etag value for a resource limits the use of the nonce to that version of the resource and also defeats pipelining. Thus it may be useful to do so for methods with side effects but have unacceptable performance for those that do not.</p><h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;Comparison of Digest with Basic Authentication</h2><p id="rfc.section.4.4.p.1">Both Digest and Basic Authentication are very much on the weak end of the security strength spectrum. But a comparison between the two points out the utility, even necessity, of replacing Basic by Digest.</p><p id="rfc.section.4.4.p.2">The greatest threat to the type of transactions for which these protocols are used is network snooping. This kind of transaction might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often worse, will permit access to anything else the user protects with the same password.</p><p id="rfc.section.4.4.p.3">By contrast, with Digest Authentication the eavesdropper only gets access to the transaction in question and not to the user's password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document, and even that may be limited by the server's choice of nonce.</p><h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;<a id="replay.attacks" href="#replay.attacks">Replay Attacks</a></h2><p id="rfc.section.4.5.p.1">A replay attack against Digest authentication would usually be pointless for a simple GET request since an eavesdropper would already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper has the user's password, any document protected by that password is open to him.</p><p id="rfc.section.4.5.p.2">Thus, for some purposes, it is necessary to protect against replay attacks. A good Digest implementation can do this in various ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp, the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince the server that the request is coming from a false IP address and must cause the server to deliver the document to an IP address different from the address to which it believes it is sending the document. An attack can only succeed in the period before the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain state between transactions.</p><p id="rfc.section.4.5.p.3">For applications where no possibility of replay attack can be tolerated the server can use one-time nonce values which will not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.</p><p id="rfc.section.4.5.p.4">An implementation must give special attention to the possibility of replay attacks with POST and PUT requests. Unless the server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int, an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking provides some protection against replay of previously used valid credentials, but see 4.8.</p><h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;Weakness Created by Multiple Authentication Schemes</h2><p id="rfc.section.4.6.p.1">An HTTP/1.1 server may return multiple challenges with a 401 (Authenticate) response, and each challenge may use a different auth-scheme. A user agent <em class="bcp14">MUST</em> choose to use the strongest auth-scheme it understands and request credentials from the user based upon that challenge.</p><p id="rfc.section.4.6.p.2"> </p><dl class="empty"><dd>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should only include Basic if it is minimally acceptable.</dd></dl><p id="rfc.section.4.6.p.3">When the server offers choices of authentication schemes using the WWW-Authenticate header, the strength of the resulting authentication is only as good as that of the of the weakest of the authentication schemes. See <a href="#man.in.the.middle" title="Man in the Middle">Section&nbsp;4.8</a> below for discussion of particular attack scenarios that exploit multiple authentication schemes.</p><h2 id="rfc.section.4.7"><a href="#rfc.section.4.7">4.7</a>&nbsp;Online dictionary attacks</h2><p id="rfc.section.4.7.p.1">If the attacker can eavesdrop, then it can test any overheard nonce/response pairs against a list of common words. Such a list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password on the list is paid once for each challenge.</p><p id="rfc.section.4.7.p.2">The server can mitigate this attack by not allowing users to select passwords that are in a dictionary.</p><h2 id="rfc.section.4.8"><a href="#rfc.section.4.8">4.8</a>&nbsp;<a id="man.in.the.middle" href="#man.in.the.middle">Man in the Middle</a></h2><p id="rfc.section.4.8.p.1">Both Basic and Digest authentication are vulnerable to "man in the middle" (MITM) attacks, for example, from a hostile or compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities to the attacker.</p><p id="rfc.section.4.8.p.2">A possible man-in-the-middle attack would be to add a weak authentication scheme to the set of choices, hoping that the client will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest scheme that it understands from the choices offered.</p><p id="rfc.section.4.8.p.3">An even better MITM attack would be to remove all offered choices, replacing them with a challenge that requests only Basic authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy caching service to gullible users.</p><p id="rfc.section.4.8.p.4">User agents should consider measures such as presenting a visual indication at the time of the credentials request of what authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce a warning message before using a weaker one. It might also be a good idea for the user agent to be configured to demand Digest authentication in general, or from specific sites.</p><p id="rfc.section.4.8.p.5">Or, a hostile proxy might spoof the client into making a request the attacker wanted rather than one the client wanted. Of course, this is still much harder than a comparable attack against Basic Authentication.</p><h2 id="rfc.section.4.9"><a href="#rfc.section.4.9">4.9</a>&nbsp;Chosen plaintext attacks</h2><p id="rfc.section.4.9.p.1">With Digest authentication, a MITM or a malicious server can arbitrarily choose the nonce that the client will use to compute the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much easier <a href="#ref8" id="rfc.xref.ref8.1"><cite title="Message Authentication with MD5">[8]</cite></a>.</p><p id="rfc.section.4.9.p.2">However, no way to analyze the MD5 one-way function used by Digest using chosen plaintext is currently known.</p><p id="rfc.section.4.9.p.3">The countermeasure against this attack is for clients to be configured to require the use of the optional "cnonce" directive; this allows the client to vary the input to the hash in a way not chosen by the attacker.</p><h2 id="rfc.section.4.10"><a href="#rfc.section.4.10">4.10</a>&nbsp;Precomputed dictionary attacks</h2><p id="rfc.section.4.10.p.1">With Digest authentication, if the attacker can execute a chosen plaintext attack, the attacker can precompute the response for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes of disk storage.</p><p id="rfc.section.4.10.p.2">The countermeasure against this attack is to for clients to be configured to require the use of the optional "cnonce" directive.</p><h2 id="rfc.section.4.11"><a href="#rfc.section.4.11">4.11</a>&nbsp;Batch brute force attacks</h2><p id="rfc.section.4.11.p.1">With Digest authentication, a MITM can execute a chosen plaintext attack, and can gather responses from many users to the same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response pairs in a single pass over that space. It also reduces the time to find the first password by a factor equal to the number of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with six or fewer letters in a few hours.</p><p id="rfc.section.4.11.p.2">The countermeasure against this attack is to for clients to be configured to require the use of the optional "cnonce" directive.</p><h2 id="rfc.section.4.12"><a href="#rfc.section.4.12">4.12</a>&nbsp;Spoofing by Counterfeit Servers</h2><p id="rfc.section.4.12.p.1">Basic Authentication is vulnerable to spoofing by counterfeit servers. If a user can be led to believe that she is connecting to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of each scheme.</p><h2 id="rfc.section.4.13"><a href="#rfc.section.4.13">4.13</a>&nbsp;Storing passwords</h2><p id="rfc.section.4.13.p.1">Digest authentication requires that the authenticating agent (usually the server) store some data derived from the user's name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.</p><p id="rfc.section.4.13.p.2">The security implications of this are that if this password file is compromised, then an attacker gains immediate access to documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely a brute force attack, would be necessary to obtain the user's password. This is the reason that the realm is part of the digested data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically compromise others with the same username and password (though it does expose them to brute force attack).</p><p id="rfc.section.4.13.p.3">There are two important security consequences of this. First the password file must be protected as if it contained unencrypted passwords, because for the purpose of accessing documents in its realm, it effectively does.</p><p id="rfc.section.4.13.p.4">A second consequence of this is that the realm string should be unique among all realms which any single user is likely to use. In particular a realm string should include the name of the host doing the authentication. The inability of the client to authenticate the server is a weakness of Digest Authentication.</p><h2 id="rfc.section.4.14"><a href="#rfc.section.4.14">4.14</a>&nbsp;Summary</h2><p id="rfc.section.4.14.p.1">By modern cryptographic standards Digest Authentication is weak. But for a large range of purposes it is valuable as a replacement for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited lifetime.</p><p id="rfc.section.4.14.p.2">The bottom line is that *any* compliant implementation will be relatively weak by cryptographic standards, but *any* compliant implementation will be far superior to Basic Authentication.</p><h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Sample implementation</h1><p id="rfc.section.5.p.1">The following code implements the calculations of H(A1), H(A2), request-digest and response-digest, and a test program which computes the values used in the example of <a href="#specification.of.digest.headers.example" title="Example">Section&nbsp;3.5</a>. It uses the MD5 implementation from RFC 1321.</p><div id="rfc.figure.u.28"></div><p>File "digcalc.h":</p><pre class="text">#define HASHLEN 16
    1272433typedef char HASH[HASHLEN];
    1273434#define HASHHEXLEN 32
     
    1299460    OUT HASHHEX Response      /* request-digest or response-digest */
    1300461    );
    1301 </pre><div id="rfc.figure.u.29"></div>
    1302       <p>File "digcalc.c":</p><pre class="text">#include &lt;global.h&gt;
     462</pre><div id="rfc.figure.u.29"></div><p>File "digcalc.c":</p><pre class="text">#include &lt;global.h&gt;
    1303463#include &lt;md5.h&gt;
    1304464#include &lt;string.h&gt;
     
    1409569      CvtHex(RespHash, Response);
    1410570};
    1411 </pre><div id="rfc.figure.u.30"></div>
    1412       <p>File "digtest.c":</p><pre class="text">#include &lt;stdio.h&gt;
     571</pre><div id="rfc.figure.u.30"></div><p>File "digtest.c":</p><pre class="text">#include &lt;stdio.h&gt;
    1413572#include "digcalc.h"
    1414573
     
    1435594      printf("Response = %s\n", Response);
    1436595};
    1437 </pre><h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Acknowledgments
    1438       </h1>
    1439       <p id="rfc.section.6.p.1">Eric W. Sink, of AbiSource, Inc., was one of the original authors before the specification underwent substantial revision.</p>
    1440       <p id="rfc.section.6.p.2">In addition to the authors, valuable discussion instrumental in creating this document has come from Peter J. Churchyard,
    1441          Ned Freed, and David M. Kristol.
    1442       </p>
    1443       <p id="rfc.section.6.p.3">Jim Gettys and Larry Masinter edited this document for update.</p>
    1444       <h1 id="rfc.references"><a href="#rfc.section.7" id="rfc.section.7">7.</a> References
    1445       </h1>
    1446       <table summary="References">
    1447          <tr>
    1448             <td class="reference"><b id="RFC1945">[1]</b></td>
    1449             <td class="top"><a href="mailto:timbl@w3.org" title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:frystyk@w3.org" title="W3 Consortium, MIT Laboratory for Computer Science">H.F. Nielsen</a>, “<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>”, RFC&nbsp;1945, May&nbsp;1996.
    1450             </td>
    1451          </tr> 
    1452          <tr>
    1453             <td class="reference"><b id="RFC2616">[2]</b></td>
    1454             <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Information and Computer Science">Fielding, R.T.</a>, <a href="mailto:jg@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation, Western Research Laboratory">Mogul, J.C.</a>, <a href="mailto:frystyk@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Nielsen, H.F.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.J.</a>, and <a href="mailto:timbl@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
    1455             </td>
    1456          </tr> 
    1457          <tr>
    1458             <td class="reference"><b id="RFC1321">[3]</b></td>
    1459             <td class="top"><a href="mailto:rivest@theory.lcs.mit.edu" title="Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science">Rivest, R.</a>, “<a href="http://tools.ietf.org/html/rfc1321">The MD5 Message-Digest Algorithm</a>”, RFC&nbsp;1321, April&nbsp;1992.
    1460             </td>
    1461          </tr> 
    1462          <tr>
    1463             <td class="reference"><b id="RFC2045">[4]</b></td>
    1464             <td class="top"><a href="mailto:ned@innosoft.com" title="Innosoft International, Inc.">Freed, N.</a> and <a href="mailto:nsb@nsb.fv.com" title="First Virtual Holdings">N.S. Borenstein</a>, “<a href="http://tools.ietf.org/html/rfc2045">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</a>”, RFC&nbsp;2045, November&nbsp;1996.
    1465             </td>
    1466          </tr> 
    1467          <tr>
    1468             <td class="reference"><b id="RFC2246">[5]</b></td>
    1469             <td class="top"><a href="mailto:tdierks@certicom.com" title="Certicom">Dierks, T.</a> and <a href="mailto:callen@certicom.com" title="Certicom">C. Allen</a>, “<a href="http://tools.ietf.org/html/rfc2246">The TLS Protocol Version 1.0</a>”, RFC&nbsp;2246, January&nbsp;1999.
    1470             </td>
    1471          </tr> 
    1472          <tr>
    1473             <td class="reference"><b id="RFC2069">[6]</b></td>
    1474             <td class="top"><a href="mailto:john@math.nwu.edu" title="Northwestern University,  Department of Mathematics">Franks, J.</a>, <a href="mailto:hallam@w3.org" title="CERN">Hallam-Baker, P.</a>, <a href="mailto:jeff@spyglass.com" title="Spyglass, Inc.">Hostetler, J.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:luotonen@netscape.com" title="Netscape Communications Corporation">Luotonen, A.</a>, <a href="mailto:eric@spyglass.com" title="Spyglass, Inc.">Sink, E.</a>, and <a href="mailto:stewart@OpenMarket.com" title="Open Market, Inc.">L. Stewart</a>, “<a href="http://tools.ietf.org/html/rfc2069">An Extension to HTTP : Digest Access Authentication</a>”, RFC&nbsp;2069, January&nbsp;1997.
    1475             </td>
    1476          </tr> 
    1477          <tr>
    1478             <td class="reference"><b id="RFC2396">[7]</b></td>
    1479             <td class="top"><a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:masinter@parc.xerox.com" title="Xerox PARC">L. Masinter</a>, “<a href="http://tools.ietf.org/html/rfc2396">Uniform Resource Identifiers (URI): Generic Syntax</a>”, RFC&nbsp;2396, August&nbsp;1998.
    1480             </td>
    1481          </tr> 
    1482          <tr>
    1483             <td class="reference"><b id="ref8">[8]</b></td>
    1484             <td class="top">Kaliski, B. and M. Robshaw, “<a href="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">Message Authentication with MD5</a>”, 1995, &lt;<a href="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm</a>&gt;.<br>CryptoBytes, Spring 1995
    1485             </td>
    1486          </tr> 
    1487          <tr>
    1488             <td class="reference"><b id="RFC2195">[9]</b></td>
    1489             <td class="top"><a href="mailto:klensin@mci.net" title="MCI">Klensin, J.C.</a>, <a href="mailto:randy@mci.net" title="MCI">Catoe, R.</a>, and <a href="mailto:paul@mci.net" title="MCI">P. Krumviede</a>, “<a href="http://tools.ietf.org/html/rfc2195">IMAP/POP AUTHorize Extension for Simple Challenge/Response</a>”, RFC&nbsp;2195, September&nbsp;1997.
    1490             </td>
    1491          </tr> 
    1492          <tr>
    1493             <td class="reference"><b id="ref10">[10]</b></td>
    1494             <td class="top">Morgan, B., Alvestrand, H., Hodges, J., and M. Wahl, “Authentication Methods for LDAP”.<br>Work in progress.
    1495             </td>
    1496          </tr>
    1497       </table>
    1498       <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
    1499       <address class="vcard"><span class="vcardline"><span class="fn">John Franks</span><span class="n hidden"><span class="family-name">Franks</span><span class="given-name">John</span></span></span><span class="org vcardline">Northwestern University, Department of Mathematics</span><span class="adr"><span class="street-address vcardline">Northwestern University</span><span class="vcardline"><span class="locality">Evanston</span>, <span class="region">IL</span>&nbsp;<span class="postal-code">60208-2730</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:john@math.nwu.edu"><span class="email">john@math.nwu.edu</span></a></span></address>
    1500       <address class="vcard"><span class="vcardline"><span class="fn">Phillip M. Hallam-Baker</span><span class="n hidden"><span class="family-name">Hallam-Baker</span><span class="given-name">Phillip M.</span></span></span><span class="org vcardline">Verisign Inc.</span><span class="adr"><span class="street-address vcardline">301 Edgewater Place</span><span class="street-address vcardline">Suite 210</span><span class="vcardline"><span class="locality">Wakefield</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01880</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:pbaker@verisign.com"><span class="email">pbaker@verisign.com</span></a></span></address>
    1501       <address class="vcard"><span class="vcardline"><span class="fn">Jeffery L. Hostetler</span><span class="n hidden"><span class="family-name">Hostetler</span><span class="given-name">Jeffery L.</span></span></span><span class="org vcardline">AbiSource, Inc.</span><span class="adr"><span class="street-address vcardline">6 Dunlap Court</span><span class="vcardline"><span class="locality">Savoy</span>, <span class="region">IL</span>&nbsp;<span class="postal-code">61874</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:jeff@AbiSource.com"><span class="email">jeff@AbiSource.com</span></a></span></address>
    1502       <address class="vcard"><span class="vcardline"><span class="fn">Scott D. Lawrence</span><span class="n hidden"><span class="family-name">Lawrence</span><span class="given-name">Scott D.</span></span></span><span class="org vcardline">Agranat Systems, Inc.</span><span class="adr"><span class="street-address vcardline">5 Clocktower Place</span><span class="street-address vcardline">Suite 400</span><span class="vcardline"><span class="locality">Maynard</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01754</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:lawrence@agranat.com"><span class="email">lawrence@agranat.com</span></a></span></address>
    1503       <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address>
    1504       <address class="vcard"><span class="vcardline"><span class="fn">Ari Luotonen</span><span class="n hidden"><span class="family-name">Luotonen</span><span class="given-name">Ari</span></span></span><span class="org vcardline">Netscape Communications Corporation</span><span class="adr"><span class="street-address vcardline">501 East Middlefield Road</span><span class="vcardline"><span class="locality">Mountain View</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94043</span></span><span class="country-name vcardline">USA</span></span></address>
    1505       <address class="vcard"><span class="vcardline"><span class="fn">Lawrence C. Stewart</span><span class="n hidden"><span class="family-name">Stewart</span><span class="given-name">Lawrence C.</span></span></span><span class="org vcardline">Open Market, Inc.</span><span class="adr"><span class="street-address vcardline">215 First Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02142</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:stewart@OpenMarket.com"><span class="email">stewart@OpenMarket.com</span></a></span></address>
    1506       <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
    1507       <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
    1508       <p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise
    1509          explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without
    1510          restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative
    1511          works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references
    1512          to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards
    1513          in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to
    1514          translate it into languages other than English.
    1515       </p>
    1516       <p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees.</p>
    1517       <p>This document and the information contained herein is provided on an “;AS IS” basis and THE INTERNET SOCIETY AND THE INTERNET
    1518          ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
    1519          OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
    1520          PURPOSE.
    1521       </p>
    1522       <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
    1523       <p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed
    1524          to pertain to the implementation or use of the technology described in this document or the extent to which any license under
    1525          such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights.
    1526          Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be
    1527          found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available,
    1528          or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors
    1529          or users of this specification can be obtained from the IETF Secretariat.
    1530       </p>
    1531       <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
    1532          rights which may cover technology that may be required to practice this standard. Please address the information to the IETF
    1533          Executive Director.
    1534       </p>
    1535       <h1>Acknowledgement</h1>
    1536       <p>Funding for the RFC Editor function is currently provided by the Internet Society.</p>
    1537       <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
    1538       <p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.B">B</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.D">D</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.Q">Q</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.U">U</a> <a href="#rfc.index.W">W</a>
    1539       </p>
    1540       <div class="print2col">
    1541          <ul class="ind">
    1542             <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
    1543                   <li class="indline1"><tt>algorithm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.3"><b>3.2.1</b></a></li>
    1544                   <li class="indline1"><tt>auth-info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.7"><b>3.2.3</b></a></li>
    1545                   <li class="indline1"><tt>auth-param</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>1.2</b></a></li>
    1546                   <li class="indline1"><tt>auth-scheme</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1"><b>1.2</b></a></li>
    1547                   <li class="indline1"><tt>Authentication-Info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.6"><b>3.2.3</b></a></li>
    1548                   <li class="indline1">Authentication-Info header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.5"><b>3.2.3</b></a></li>
    1549                   <li class="indline1">Authorization header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.4"><b>3.2.2</b></a></li>
    1550                </ul>
    1551             </li>
    1552             <li class="indline0"><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul class="ind">
    1553                   <li class="indline1"><tt>base64-user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.2"><b>2</b></a></li>
    1554                   <li class="indline1"><tt>basic-credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.1"><b>2</b></a></li>
    1555                </ul>
    1556             </li>
    1557             <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
    1558                   <li class="indline1"><tt>challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1"><b>1.2</b></a>, <a class="iref" href="#rfc.iref.c.3">2</a>, <a class="iref" href="#rfc.iref.c.5">3.2.1</a></li>
    1559                   <li class="indline1"><tt>cnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.2</b></a></li>
    1560                   <li class="indline1"><tt>cnonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.2</b></a></li>
    1561                   <li class="indline1"><tt>credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2"><b>1.2</b></a>, <a class="iref" href="#rfc.iref.c.4">2</a>, <a class="iref" href="#rfc.iref.c.6">3.2.2</a></li>
    1562                </ul>
    1563             </li>
    1564             <li class="indline0"><a id="rfc.index.D" href="#rfc.index.D"><b>D</b></a><ul class="ind">
    1565                   <li class="indline1"><tt>digest-challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.1"><b>3.2.1</b></a></li>
    1566                   <li class="indline1"><tt>digest-response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.3"><b>3.2.2</b></a></li>
    1567                   <li class="indline1"><tt>digest-uri</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.4"><b>3.2.2</b></a></li>
    1568                   <li class="indline1"><tt>digest-uri-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.5"><b>3.2.2</b></a></li>
    1569                   <li class="indline1"><tt>domain</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.2"><b>3.2.1</b></a></li>
    1570                </ul>
    1571             </li>
    1572             <li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind">
    1573                   <li class="indline1">Headers&nbsp;&nbsp;
    1574                      <ul class="ind">
    1575                         <li class="indline1">Authentication-Info&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.3"><b>3.2.3</b></a></li>
    1576                         <li class="indline1">Authorization&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>3.2.2</b></a></li>
    1577                         <li class="indline1">WWW-Authenticate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.1"><b>3.2.1</b></a></li>
    1578                      </ul>
    1579                   </li>
    1580                </ul>
    1581             </li>
    1582             <li class="indline0"><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul class="ind">
    1583                   <li class="indline1"><tt>LHEX</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.l.1"><b>3.2.2</b></a></li>
    1584                </ul>
    1585             </li>
    1586             <li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind">
    1587                   <li class="indline1"><tt>message-qop</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.2</b></a></li>
    1588                </ul>
    1589             </li>
    1590             <li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind">
    1591                   <li class="indline1"><tt>nc-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
    1592                   <li class="indline1"><tt>nextnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.5"><b>3.2.3</b></a></li>
    1593                   <li class="indline1"><tt>nonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a></li>
    1594                   <li class="indline1"><tt>nonce-count</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.2</b></a></li>
    1595                   <li class="indline1"><tt>nonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a></li>
    1596                </ul>
    1597             </li>
    1598             <li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind">
    1599                   <li class="indline1"><tt>opaque</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
    1600                </ul>
    1601             </li>
    1602             <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
    1603                   <li class="indline1"><tt>password</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>2</b></a></li>
    1604                </ul>
    1605             </li>
    1606             <li class="indline0"><a id="rfc.index.Q" href="#rfc.index.Q"><b>Q</b></a><ul class="ind">
    1607                   <li class="indline1"><tt>qop-options</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.1"><b>3.2.1</b></a></li>
    1608                   <li class="indline1"><tt>qop-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.2"><b>3.2.1</b></a></li>
    1609                </ul>
    1610             </li>
    1611             <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
    1612                   <li class="indline1"><tt>realm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.1"><b>1.2</b></a></li>
    1613                   <li class="indline1"><tt>realm-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.2"><b>1.2</b></a></li>
    1614                   <li class="indline1"><em>ref10</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ref10.1">4.2</a>, <a class="iref" href="#ref10"><b>7</b></a></li>
    1615                   <li class="indline1"><em>ref8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ref8.1">4.9</a>, <a class="iref" href="#ref8"><b>7</b></a></li>
    1616                   <li class="indline1"><tt>request-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.4"><b>3.2.2</b></a></li>
    1617                   <li class="indline1"><tt>response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.3"><b>3.2.2</b></a></li>
    1618                   <li class="indline1"><tt>response-auth</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.5"><b>3.2.3</b></a></li>
    1619                   <li class="indline1"><tt>response-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.6"><b>3.2.3</b></a></li>
    1620                   <li class="indline1"><em>RFC1321</em>&nbsp;&nbsp;<a class="iref" href="#RFC1321"><b>7</b></a></li>
    1621                   <li class="indline1"><em>RFC1945</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1945.1">3.1.1</a>, <a class="iref" href="#RFC1945"><b>7</b></a></li>
    1622                   <li class="indline1"><em>RFC2045</em>&nbsp;&nbsp;<a class="iref" href="#RFC2045"><b>7</b></a></li>
    1623                   <li class="indline1"><em>RFC2069</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2069.1">§</a>, <a class="iref" href="#rfc.xref.RFC2069.2">3.2.1</a>, <a class="iref" href="#rfc.xref.RFC2069.3">3.2.2</a>, <a class="iref" href="#RFC2069"><b>7</b></a></li>
    1624                   <li class="indline1"><em>RFC2195</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2195.1">4.2</a>, <a class="iref" href="#RFC2195"><b>7</b></a></li>
    1625                   <li class="indline1"><em>RFC2246</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2246.1">§</a>, <a class="iref" href="#RFC2246"><b>7</b></a></li>
    1626                   <li class="indline1"><em>RFC2396</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2396.1">2</a>, <a class="iref" href="#rfc.xref.RFC2396.2">3.2.1</a>, <a class="iref" href="#RFC2396"><b>7</b></a></li>
    1627                   <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.2">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.4">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.5">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.6">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.7">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.8">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.9">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.10">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.11">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.12">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.13">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.14">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.15">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.16">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.17">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.18">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.19">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.20">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.21">3.6</a>, <a class="iref" href="#RFC2616"><b>7</b></a><ul class="ind">
    1628                         <li class="indline1"><em>Section 2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.2">1.1</a></li>
    1629                         <li class="indline1"><em>Section 5.1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.11">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.13">3.2.2.5</a></li>
    1630                         <li class="indline1"><em>Section 5.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.9">3.2.2.5</a></li>
    1631                         <li class="indline1"><em>Section 10.33</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.19">3.6</a></li>
    1632                         <li class="indline1"><em>Section 10.34</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.20">3.6</a></li>
    1633                         <li class="indline1"><em>Section 13.5.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.7">1.2</a></li>
    1634                         <li class="indline1"><em>Section 13.7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.15">3.2.2.5</a></li>
    1635                         <li class="indline1"><em>Section 14.8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.5">1.2</a></li>
    1636                         <li class="indline1"><em>Section 14.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.17">3.2.2.5</a></li>
    1637                      </ul>
    1638                   </li>
    1639                </ul>
    1640             </li>
    1641             <li class="indline0"><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul class="ind">
    1642                   <li class="indline1"><tt>stale</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1"><b>3.2.1</b></a></li>
    1643                </ul>
    1644             </li>
    1645             <li class="indline0"><a id="rfc.index.U" href="#rfc.index.U"><b>U</b></a><ul class="ind">
    1646                   <li class="indline1"><tt>URI</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.3"><b>3.2.1</b></a></li>
    1647                   <li class="indline1"><tt>user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.1"><b>2</b></a></li>
    1648                   <li class="indline1"><tt>userid</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.2"><b>2</b></a></li>
    1649                   <li class="indline1"><tt>username</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.4"><b>3.2.2</b></a></li>
    1650                   <li class="indline1"><tt>username-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.5"><b>3.2.2</b></a></li>
    1651                </ul>
    1652             </li>
    1653             <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
    1654                   <li class="indline1">WWW-Authenticate header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.w.1"><b>3.2.1</b></a></li>
    1655                </ul>
    1656             </li>
    1657          </ul>
    1658       </div>
    1659    </body>
    1660 </html>
     596</pre><h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Acknowledgments</h1><p id="rfc.section.6.p.1">Eric W. Sink, of AbiSource, Inc., was one of the original authors before the specification underwent substantial revision.</p><p id="rfc.section.6.p.2">In addition to the authors, valuable discussion instrumental in creating this document has come from Peter J. Churchyard, Ned Freed, and David M. Kristol.</p><p id="rfc.section.6.p.3">Jim Gettys and Larry Masinter edited this document for update.</p><h1 id="rfc.references"><a href="#rfc.section.7" id="rfc.section.7">7.</a> References</h1><table summary="References"> <tr><td class="reference"><b id="RFC1945">[1]</b></td><td class="top"><a href="mailto:timbl@w3.org" title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:frystyk@w3.org" title="W3 Consortium, MIT Laboratory for Computer Science">H.F. Nielsen</a>, &#8220;<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>&#8221;, RFC&nbsp;1945, May&nbsp;1996.</td></tr>  <tr><td class="reference"><b id="RFC2616">[2]</b></td><td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Information and Computer Science">Fielding, R.T.</a>, <a href="mailto:jg@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation, Western Research Laboratory">Mogul, J.C.</a>, <a href="mailto:frystyk@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Nielsen, H.F.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.J.</a>, and <a href="mailto:timbl@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">T. Berners-Lee</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>&#8221;, RFC&nbsp;2616, June&nbsp;1999.</td></tr>  <tr><td class="reference"><b id="RFC1321">[3]</b></td><td class="top"><a href="mailto:rivest@theory.lcs.mit.edu" title="Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science">Rivest, R.</a>, &#8220;<a href="http://tools.ietf.org/html/rfc1321">The MD5 Message-Digest Algorithm</a>&#8221;, RFC&nbsp;1321, April&nbsp;1992.</td></tr>  <tr><td class="reference"><b id="RFC2045">[4]</b></td><td class="top"><a href="mailto:ned@innosoft.com" title="Innosoft International, Inc.">Freed, N.</a> and <a href="mailto:nsb@nsb.fv.com" title="First Virtual Holdings">N.S. Borenstein</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2045">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</a>&#8221;, RFC&nbsp;2045, November&nbsp;1996.</td></tr>  <tr><td class="reference"><b id="RFC2246">[5]</b></td><td class="top"><a href="mailto:tdierks@certicom.com" title="Certicom">Dierks, T.</a> and <a href="mailto:callen@certicom.com" title="Certicom">C. Allen</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2246">The TLS Protocol Version 1.0</a>&#8221;, RFC&nbsp;2246, January&nbsp;1999.</td></tr>  <tr><td class="reference"><b id="RFC2069">[6]</b></td><td class="top"><a href="mailto:john@math.nwu.edu" title="Northwestern University,  Department of Mathematics">Franks, J.</a>, <a href="mailto:hallam@w3.org" title="CERN">Hallam-Baker, P.</a>, <a href="mailto:jeff@spyglass.com" title="Spyglass, Inc.">Hostetler, J.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:luotonen@netscape.com" title="Netscape Communications Corporation">Luotonen, A.</a>, <a href="mailto:eric@spyglass.com" title="Spyglass, Inc.">Sink, E.</a>, and <a href="mailto:stewart@OpenMarket.com" title="Open Market, Inc.">L. Stewart</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2069">An Extension to HTTP : Digest Access Authentication</a>&#8221;, RFC&nbsp;2069, January&nbsp;1997.</td></tr>  <tr><td class="reference"><b id="RFC2396">[7]</b></td><td class="top"><a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:masinter@parc.xerox.com" title="Xerox PARC">L. Masinter</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2396">Uniform Resource Identifiers (URI): Generic Syntax</a>&#8221;, RFC&nbsp;2396, August&nbsp;1998.</td></tr>  <tr><td class="reference"><b id="ref8">[8]</b></td><td class="top">Kaliski, B. and M. Robshaw, &#8220;<a href="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">Message Authentication with MD5</a>&#8221;, 1995, &lt;<a href="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm</a>&gt;.<br>CryptoBytes, Spring 1995</td></tr>  <tr><td class="reference"><b id="RFC2195">[9]</b></td><td class="top"><a href="mailto:klensin@mci.net" title="MCI">Klensin, J.C.</a>, <a href="mailto:randy@mci.net" title="MCI">Catoe, R.</a>, and <a href="mailto:paul@mci.net" title="MCI">P. Krumviede</a>, &#8220;<a href="http://tools.ietf.org/html/rfc2195">IMAP/POP AUTHorize Extension for Simple Challenge/Response</a>&#8221;, RFC&nbsp;2195, September&nbsp;1997.</td></tr>  <tr><td class="reference"><b id="ref10">[10]</b></td><td class="top">Morgan, B., Alvestrand, H., Hodges, J., and M. Wahl, &#8220;Authentication Methods for LDAP&#8221;.<br>Work in progress.</td></tr> </table><h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1><address class="vcard"><span class="vcardline"><span class="fn">John Franks</span><span class="n hidden"><span class="family-name">Franks</span><span class="given-name">John</span></span></span><span class="org vcardline">Northwestern University, Department of Mathematics</span><span class="adr"><span class="street-address vcardline">Northwestern University</span><span class="vcardline"><span class="locality">Evanston</span>, <span class="region">IL</span>&nbsp;<span class="postal-code">60208-2730</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:john@math.nwu.edu"><span class="email">john@math.nwu.edu</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Phillip M. Hallam-Baker</span><span class="n hidden"><span class="family-name">Hallam-Baker</span><span class="given-name">Phillip M.</span></span></span><span class="org vcardline">Verisign Inc.</span><span class="adr"><span class="street-address vcardline">301 Edgewater Place</span><span class="street-address vcardline">Suite 210</span><span class="vcardline"><span class="locality">Wakefield</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01880</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:pbaker@verisign.com"><span class="email">pbaker@verisign.com</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Jeffery L. Hostetler</span><span class="n hidden"><span class="family-name">Hostetler</span><span class="given-name">Jeffery L.</span></span></span><span class="org vcardline">AbiSource, Inc.</span><span class="adr"><span class="street-address vcardline">6 Dunlap Court</span><span class="vcardline"><span class="locality">Savoy</span>, <span class="region">IL</span>&nbsp;<span class="postal-code">61874</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:jeff@AbiSource.com"><span class="email">jeff@AbiSource.com</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Scott D. Lawrence</span><span class="n hidden"><span class="family-name">Lawrence</span><span class="given-name">Scott D.</span></span></span><span class="org vcardline">Agranat Systems, Inc.</span><span class="adr"><span class="street-address vcardline">5 Clocktower Place</span><span class="street-address vcardline">Suite 400</span><span class="vcardline"><span class="locality">Maynard</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01754</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:lawrence@agranat.com"><span class="email">lawrence@agranat.com</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address><address class="vcard"><span class="vcardline"><span class="fn">Ari Luotonen</span><span class="n hidden"><span class="family-name">Luotonen</span><span class="given-name">Ari</span></span></span><span class="org vcardline">Netscape Communications Corporation</span><span class="adr"><span class="street-address vcardline">501 East Middlefield Road</span><span class="vcardline"><span class="locality">Mountain View</span>, <span class="region">CA</span>&nbsp;<span class="postal-code">94043</span></span><span class="country-name vcardline">USA</span></span></address><address class="vcard"><span class="vcardline"><span class="fn">Lawrence C. Stewart</span><span class="n hidden"><span class="family-name">Stewart</span><span class="given-name">Lawrence C.</span></span></span><span class="org vcardline">Open Market, Inc.</span><span class="adr"><span class="street-address vcardline">215 First Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02142</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:stewart@OpenMarket.com"><span class="email">stewart@OpenMarket.com</span></a></span></address><h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1><p>Copyright &copy; The Internet Society (1999). All Rights Reserved.</p><p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English.</p><p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees.</p><p>This document and the information contained herein is provided on an &#8220;AS IS&#8221; basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.</p><h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1><p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat.</p><p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.</p><h1>Acknowledgement</h1><p>Funding for the RFC Editor function is currently provided by the Internet Society.</p><h1 id="rfc.index"><a href="#rfc.index">Index</a></h1><p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.B">B</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.D">D</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.Q">Q</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.U">U</a> <a href="#rfc.index.W">W</a> </p><div class="print2col"><ul class="ind"><li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind"><li class="indline1"><tt>algorithm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.3"><b>3.2.1</b></a></li><li class="indline1"><tt>auth-info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.7"><b>3.2.3</b></a></li><li class="indline1"><tt>auth-param</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>1.2</b></a></li><li class="indline1"><tt>auth-scheme</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1"><b>1.2</b></a></li><li class="indline1"><tt>Authentication-Info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.6"><b>3.2.3</b></a></li><li class="indline1">Authentication-Info header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.5"><b>3.2.3</b></a></li><li class="indline1">Authorization header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.4"><b>3.2.2</b></a></li></ul></li><li class="indline0"><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul class="ind"><li class="indline1"><tt>base64-user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.2"><b>2</b></a></li><li class="indline1"><tt>basic-credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.1"><b>2</b></a></li></ul></li><li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind"><li class="indline1"><tt>challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1"><b>1.2</b></a>, <a class="iref" href="#rfc.iref.c.3">2</a>, <a class="iref" href="#rfc.iref.c.5">3.2.1</a></li><li class="indline1"><tt>cnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.2</b></a></li><li class="indline1"><tt>cnonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.2</b></a></li><li class="indline1"><tt>credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2"><b>1.2</b></a>, <a class="iref" href="#rfc.iref.c.4">2</a>, <a class="iref" href="#rfc.iref.c.6">3.2.2</a></li></ul></li><li class="indline0"><a id="rfc.index.D" href="#rfc.index.D"><b>D</b></a><ul class="ind"><li class="indline1"><tt>digest-challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.1"><b>3.2.1</b></a></li><li class="indline1"><tt>digest-response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.3"><b>3.2.2</b></a></li><li class="indline1"><tt>digest-uri</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.4"><b>3.2.2</b></a></li><li class="indline1"><tt>digest-uri-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.5"><b>3.2.2</b></a></li><li class="indline1"><tt>domain</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.2"><b>3.2.1</b></a></li></ul></li><li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind"><li class="indline1">Headers&nbsp;&nbsp;<ul class="ind"><li class="indline1">Authentication-Info&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.3"><b>3.2.3</b></a></li><li class="indline1">Authorization&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>3.2.2</b></a></li><li class="indline1">WWW-Authenticate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.1"><b>3.2.1</b></a></li></ul></li></ul></li><li class="indline0"><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul class="ind"><li class="indline1"><tt>LHEX</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.l.1"><b>3.2.2</b></a></li></ul></li><li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind"><li class="indline1"><tt>message-qop</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.2</b></a></li></ul></li><li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind"><li class="indline1"><tt>nc-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li><li class="indline1"><tt>nextnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.5"><b>3.2.3</b></a></li><li class="indline1"><tt>nonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a></li><li class="indline1"><tt>nonce-count</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.2</b></a></li><li class="indline1"><tt>nonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a></li></ul></li><li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind"><li class="indline1"><tt>opaque</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li></ul></li><li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind"><li class="indline1"><tt>password</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>2</b></a></li></ul></li><li class="indline0"><a id="rfc.index.Q" href="#rfc.index.Q"><b>Q</b></a><ul class="ind"><li class="indline1"><tt>qop-options</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.1"><b>3.2.1</b></a></li><li class="indline1"><tt>qop-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.2"><b>3.2.1</b></a></li></ul></li><li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind"><li class="indline1"><tt>realm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.1"><b>1.2</b></a></li><li class="indline1"><tt>realm-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.2"><b>1.2</b></a></li><li class="indline1"><em>ref10</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ref10.1">4.2</a>, <a class="iref" href="#ref10"><b>7</b></a></li><li class="indline1"><em>ref8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ref8.1">4.9</a>, <a class="iref" href="#ref8"><b>7</b></a></li><li class="indline1"><tt>request-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.4"><b>3.2.2</b></a></li><li class="indline1"><tt>response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.3"><b>3.2.2</b></a></li><li class="indline1"><tt>response-auth</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.5"><b>3.2.3</b></a></li><li class="indline1"><tt>response-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.6"><b>3.2.3</b></a></li><li class="indline1"><em>RFC1321</em>&nbsp;&nbsp;<a class="iref" href="#RFC1321"><b>7</b></a></li><li class="indline1"><em>RFC1945</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1945.1">3.1.1</a>, <a class="iref" href="#RFC1945"><b>7</b></a></li><li class="indline1"><em>RFC2045</em>&nbsp;&nbsp;<a class="iref" href="#RFC2045"><b>7</b></a></li><li class="indline1"><em>RFC2069</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2069.1">&sect;</a>, <a class="iref" href="#rfc.xref.RFC2069.2">3.2.1</a>, <a class="iref" href="#rfc.xref.RFC2069.3">3.2.2</a>, <a class="iref" href="#RFC2069"><b>7</b></a></li><li class="indline1"><em>RFC2195</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2195.1">4.2</a>, <a class="iref" href="#RFC2195"><b>7</b></a></li><li class="indline1"><em>RFC2246</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2246.1">&sect;</a>, <a class="iref" href="#RFC2246"><b>7</b></a></li><li class="indline1"><em>RFC2396</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2396.1">2</a>, <a class="iref" href="#rfc.xref.RFC2396.2">3.2.1</a>, <a class="iref" href="#RFC2396"><b>7</b></a></li><li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.2">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.4">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.5">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.6">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.7">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.8">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.9">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.10">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.11">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.12">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.13">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.14">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.15">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.16">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.17">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.18">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.19">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.20">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.21">3.6</a>, <a class="iref" href="#RFC2616"><b>7</b></a><ul class="ind"><li class="indline1"><em>Section 10.33</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.19">3.6</a></li><li class="indline1"><em>Section 10.34</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.20">3.6</a></li></ul><ul class="ind"><li class="indline1"><em>Section 2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.2">1.1</a></li><li class="indline1"><em>Section 5.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.9">3.2.2.5</a></li><li class="indline1"><em>Section 5.1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.11">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.13">3.2.2.5</a></li><li class="indline1"><em>Section 13.5.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.7">1.2</a></li><li class="indline1"><em>Section 13.7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.15">3.2.2.5</a></li><li class="indline1"><em>Section 14.8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.5">1.2</a></li><li class="indline1"><em>Section 14.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.17">3.2.2.5</a></li></ul></li></ul></li><li class="indline0"><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul class="ind"><li class="indline1"><tt>stale</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1"><b>3.2.1</b></a></li></ul></li><li class="indline0"><a id="rfc.index.U" href="#rfc.index.U"><b>U</b></a><ul class="ind"><li class="indline1"><tt>URI</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.3"><b>3.2.1</b></a></li><li class="indline1"><tt>user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.1"><b>2</b></a></li><li class="indline1"><tt>userid</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.2"><b>2</b></a></li><li class="indline1"><tt>username</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.4"><b>3.2.2</b></a></li><li class="indline1"><tt>username-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.5"><b>3.2.2</b></a></li></ul></li><li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind"><li class="indline1">WWW-Authenticate header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.w.1"><b>3.2.1</b></a></li></ul></li></ul></div></body></html>
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