Ignore:
Timestamp:
20/01/14 07:43:53 (8 years ago)
Author:
julian.reschke@…
Message:

update rfc2617.xml (ABNF alignment was off from published version), regen all HTML

File:
1 edited

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

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    342330      <link rel="Help" title="RFC-Editor's Status Page" href="http://www.rfc-editor.org/info/rfc2617">
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    424412      </table>
    425413      <p class="title">HTTP Authentication: Basic and Digest Access Authentication</p>
    426       <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
    427       <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
    428          for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
    429          state and status of this protocol. Distribution of this memo is unlimited.
    430       </p>
    431       <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
    432       <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
    433       <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
     414      <div id="rfc.status">
     415         <h1><a href="#rfc.status">Status of this Memo</a></h1>
     416         <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
     417            for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
     418            state and status of this protocol. Distribution of this memo is unlimited.
     419         </p>
     420      </div>
     421      <div id="rfc.copyrightnotice">
     422         <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1>
     423         <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
     424      </div>
     425      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
    434426      <p>"HTTP/1.0", includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure
    435427         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.
     
    444436         As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself
    445437         but in policies and procedures surrounding its use.
    446       </p> 
     438      </p>
    447439      <hr class="noprint">
    448440      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
    449441      <ul class="toc">
    450          <li>1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Access Authentication</a><ul>
    451                <li>1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Reliance on the HTTP/1.1 Specification</a></li>
    452                <li>1.2&nbsp;&nbsp;&nbsp;<a href="#access.authentication.framework">Access Authentication Framework</a></li>
     442         <li><a href="#rfc.section.1">1.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Access Authentication</a><ul>
     443               <li><a href="#rfc.section.1.1">1.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Reliance on the HTTP/1.1 Specification</a></li>
     444               <li><a href="#rfc.section.1.2">1.2</a>&nbsp;&nbsp;&nbsp;<a href="#access.authentication.framework">Access Authentication Framework</a></li>
    453445            </ul>
    454446         </li>
    455          <li>2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Basic Authentication Scheme</a></li>
    456          <li>3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Digest Access Authentication Scheme</a><ul>
    457                <li>3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Introduction</a><ul>
    458                      <li>3.1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.1">Purpose</a></li>
    459                      <li>3.1.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.2">Overall Operation</a></li>
    460                      <li>3.1.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.3">Representation of digest values</a></li>
    461                      <li>3.1.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.4">Limitations</a></li>
     447         <li><a href="#rfc.section.2">2.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Basic Authentication Scheme</a></li>
     448         <li><a href="#rfc.section.3">3.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Digest Access Authentication Scheme</a><ul>
     449               <li><a href="#rfc.section.3.1">3.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Introduction</a><ul>
     450                     <li><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.1">Purpose</a></li>
     451                     <li><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.2">Overall Operation</a></li>
     452                     <li><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.3">Representation of digest values</a></li>
     453                     <li><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.4">Limitations</a></li>
    462454                  </ul>
    463455               </li>
    464                <li>3.2&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a><ul>
    465                      <li>3.2.1&nbsp;&nbsp;&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></li>
    466                      <li>3.2.2&nbsp;&nbsp;&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a><ul>
    467                            <li>3.2.2.1&nbsp;&nbsp;&nbsp;<a href="#request-digest">Request-Digest</a></li>
    468                            <li>3.2.2.2&nbsp;&nbsp;&nbsp;<a href="#A1">A1</a></li>
    469                            <li>3.2.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.3">A2</a></li>
    470                            <li>3.2.2.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.4">Directive values and quoted-string</a></li>
    471                            <li>3.2.2.5&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.5">Various considerations</a></li>
     456               <li><a href="#rfc.section.3.2">3.2</a>&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a><ul>
     457                     <li><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></li>
     458                     <li><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a><ul>
     459                           <li><a href="#rfc.section.3.2.2.1">3.2.2.1</a>&nbsp;&nbsp;&nbsp;<a href="#request-digest">Request-Digest</a></li>
     460                           <li><a href="#rfc.section.3.2.2.2">3.2.2.2</a>&nbsp;&nbsp;&nbsp;<a href="#A1">A1</a></li>
     461                           <li><a href="#rfc.section.3.2.2.3">3.2.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.3">A2</a></li>
     462                           <li><a href="#rfc.section.3.2.2.4">3.2.2.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.4">Directive values and quoted-string</a></li>
     463                           <li><a href="#rfc.section.3.2.2.5">3.2.2.5</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.5">Various considerations</a></li>
    472464                        </ul>
    473465                     </li>
    474                      <li>3.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.3">The Authentication-Info Header</a></li>
     466                     <li><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.3">The Authentication-Info Header</a></li>
    475467                  </ul>
    476468               </li>
    477                <li>3.3&nbsp;&nbsp;&nbsp;<a href="#digest.operation">Digest Operation</a></li>
    478                <li>3.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Security Protocol Negotiation</a></li>
    479                <li>3.5&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers.example">Example</a></li>
    480                <li>3.6&nbsp;&nbsp;&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></li>
     469               <li><a href="#rfc.section.3.3">3.3</a>&nbsp;&nbsp;&nbsp;<a href="#digest.operation">Digest Operation</a></li>
     470               <li><a href="#rfc.section.3.4">3.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Security Protocol Negotiation</a></li>
     471               <li><a href="#rfc.section.3.5">3.5</a>&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers.example">Example</a></li>
     472               <li><a href="#rfc.section.3.6">3.6</a>&nbsp;&nbsp;&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></li>
    481473            </ul>
    482474         </li>
    483          <li>4.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul>
    484                <li>4.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Authentication of Clients using Basic Authentication</a></li>
    485                <li>4.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">Authentication of Clients using Digest Authentication</a></li>
    486                <li>4.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Limited Use Nonce Values</a></li>
    487                <li>4.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.4">Comparison of Digest with Basic Authentication</a></li>
    488                <li>4.5&nbsp;&nbsp;&nbsp;<a href="#replay.attacks">Replay Attacks</a></li>
    489                <li>4.6&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.6">Weakness Created by Multiple Authentication Schemes</a></li>
    490                <li>4.7&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.7">Online dictionary attacks</a></li>
    491                <li>4.8&nbsp;&nbsp;&nbsp;<a href="#man.in.the.middle">Man in the Middle</a></li>
    492                <li>4.9&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.9">Chosen plaintext attacks</a></li>
    493                <li>4.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.10">Precomputed dictionary attacks</a></li>
    494                <li>4.11&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.11">Batch brute force attacks</a></li>
    495                <li>4.12&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.12">Spoofing by Counterfeit Servers</a></li>
    496                <li>4.13&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.13">Storing passwords</a></li>
    497                <li>4.14&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.14">Summary</a></li>
     475         <li><a href="#rfc.section.4">4.</a>&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul>
     476               <li><a href="#rfc.section.4.1">4.1</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Authentication of Clients using Basic Authentication</a></li>
     477               <li><a href="#rfc.section.4.2">4.2</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">Authentication of Clients using Digest Authentication</a></li>
     478               <li><a href="#rfc.section.4.3">4.3</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Limited Use Nonce Values</a></li>
     479               <li><a href="#rfc.section.4.4">4.4</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.4">Comparison of Digest with Basic Authentication</a></li>
     480               <li><a href="#rfc.section.4.5">4.5</a>&nbsp;&nbsp;&nbsp;<a href="#replay.attacks">Replay Attacks</a></li>
     481               <li><a href="#rfc.section.4.6">4.6</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.6">Weakness Created by Multiple Authentication Schemes</a></li>
     482               <li><a href="#rfc.section.4.7">4.7</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.7">Online dictionary attacks</a></li>
     483               <li><a href="#rfc.section.4.8">4.8</a>&nbsp;&nbsp;&nbsp;<a href="#man.in.the.middle">Man in the Middle</a></li>
     484               <li><a href="#rfc.section.4.9">4.9</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.9">Chosen plaintext attacks</a></li>
     485               <li><a href="#rfc.section.4.10">4.10</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.10">Precomputed dictionary attacks</a></li>
     486               <li><a href="#rfc.section.4.11">4.11</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.11">Batch brute force attacks</a></li>
     487               <li><a href="#rfc.section.4.12">4.12</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.12">Spoofing by Counterfeit Servers</a></li>
     488               <li><a href="#rfc.section.4.13">4.13</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.13">Storing passwords</a></li>
     489               <li><a href="#rfc.section.4.14">4.14</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.14">Summary</a></li>
    498490            </ul>
    499491         </li>
    500          <li>5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Sample implementation</a></li>
    501          <li>6.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Acknowledgments</a></li>
    502          <li>7.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
     492         <li><a href="#rfc.section.5">5.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Sample implementation</a></li>
     493         <li><a href="#rfc.section.6">6.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Acknowledgments</a></li>
     494         <li><a href="#rfc.section.7">7.</a>&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
    503495         <li><a href="#rfc.authors">Authors' Addresses</a></li>
    504496         <li><a href="#rfc.index">Index</a></li>
    505497         <li><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
    506498      </ul>
    507       <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Access Authentication
    508       </h1>
    509       <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Reliance on the HTTP/1.1 Specification
    510       </h2>
    511       <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.
    512       </p>
    513       <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>
    514       <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,
    515          case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs
    516          which carry the parameters necessary for achieving authentication via that scheme.
    517       </p>
    518       <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.a.1"></span>      auth-scheme    = token
    519 <span id="rfc.iref.a.2"></span>      auth-param     = token "=" ( token | quoted-string )
     499      <div>
     500         <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Access Authentication
     501         </h1>
     502         <div>
     503            <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Reliance on the HTTP/1.1 Specification
     504            </h2>
     505            <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.
     506            </p>
     507         </div>
     508         <div id="access.authentication.framework">
     509            <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a>&nbsp;<a 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><span id="rfc.iref.a.2"></span>   auth-scheme    = token
     515   auth-param     = token "=" ( token | quoted-string )
    520516</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
    521          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.
    522       </p>
    523       <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
     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
    524520</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
    525          if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents
    526          of a challenge may itself contain a comma-separated list of authentication parameters.
    527       </p>
    528       <p id="rfc.section.1.2.p.6">The authentication parameter realm is defined for all authentication schemes:</p>
    529       <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.r.1"></span>      realm       = "realm" "=" realm-value
    530 <span id="rfc.iref.r.2"></span>      realm-value = quoted-string
     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><span id="rfc.iref.r.2"></span>   realm       = "realm" "=" realm-value
     526   realm-value = quoted-string
    531527</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
    532          (case-sensitive), in combination with the canonical root URL (the absoluteURI for the server whose abs_path is empty; see
    533          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
    534          partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm
    535          value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication
    536          scheme. Note that there may be multiple challenges with the same auth-scheme but different realms.
    537       </p>
    538       <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
    539          (Unauthorized)--MAY do so by including an Authorization header field with the request. A client that wishes to authenticate
    540          itself with a proxy--usually, but not necessarily, after receiving a 407 (Proxy Authentication Required)--MAY do so by including
    541          a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization field
    542          value consist of credentials containing the authentication information of the client for the realm of the resource being requested.
    543          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
    544          upon that challenge.
    545       </p>
    546       <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.c.2"></span>   credentials = auth-scheme #auth-param
    547 </pre><p id="rfc.section.1.2.p.11"> </p>
    548       <ul class="empty">
    549          <li>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
    550             only include Basic if it is minimally acceptable.
    551          </li>
    552       </ul>
    553       <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
    554          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,
    555          parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot
    556          extend outside the scope of its server.
    557       </p>
    558       <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.
    559          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
    560          resource.
    561       </p>
    562       <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
    563          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
    564          authentication information. However, these additional mechanisms are not defined by this specification.
    565       </p>
    566       <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
    567          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>).
    568       </p>
    569       <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;Basic Authentication Scheme
    570       </h1>
    571       <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
    572          for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms
    573          on that server. The server will service the request only if it can validate the user-ID and password for the protection space
    574          of the Request-URI. There are no optional authentication parameters.
    575       </p>
    576       <p id="rfc.section.2.p.2">For Basic, the framework above is utilized as follows:</p>
    577       <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.c.3"></span>      challenge   = "Basic" realm
    578 <span id="rfc.iref.c.4"></span>      credentials = "Basic" basic-credentials
     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" 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
     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)--<em class="bcp14">MAY</em> do so by including an Authorization header field with the request. A client that wishes to authenticate itself with a proxy--usually,
     536               but not necessarily, after receiving a 407 (Proxy Authentication Required)--<em class="bcp14">MAY</em> do so by including a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization
     537               field value consist of credentials containing the authentication information of the client for the realm of the resource being
     538               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
     539               upon that challenge.
     540            </p>
     541            <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.c.2"></span>   credentials = auth-scheme #auth-param
     542</pre><p id="rfc.section.1.2.p.11"></p>
     543            <ul class="empty">
     544               <li>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
     545                  only include Basic if it is minimally acceptable.
     546               </li>
     547            </ul>
     548            <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
     549               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,
     550               parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot
     551               extend outside the scope of its server.
     552            </p>
     553            <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.
     554               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
     555               resource.
     556            </p>
     557            <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
     558               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
     559               authentication information. However, these additional mechanisms are not defined by this specification.
     560            </p>
     561            <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
     562               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>).
     563            </p>
     564         </div>
     565      </div>
     566      <div>
     567         <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;Basic Authentication Scheme
     568         </h1>
     569         <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
     570            for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms
     571            on that server. The server will service the request only if it can validate the user-ID and password for the protection space
     572            of the Request-URI. There are no optional authentication parameters.
     573         </p>
     574         <p id="rfc.section.2.p.2">For Basic, the framework above is utilized as follows:</p>
     575         <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.c.3"></span><span id="rfc.iref.c.4"></span>   challenge   = "Basic" realm
     576   credentials = "Basic" basic-credentials
    579577</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:
    580       </p>
    581       <div id="rfc.figure.u.6"></div><pre class="text">      WWW-Authenticate: Basic realm="WallyWorld"
     578         </p>
     579         <div id="rfc.figure.u.6"></div><pre class="text">   WWW-Authenticate: Basic realm="WallyWorld"
    582580</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
    583          with the same challenge using the Proxy-Authenticate header field.
    584       </p>
    585       <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
    586          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.
    587       </p>
    588       <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.b.1"></span>      basic-credentials = base64-user-pass
    589 <span id="rfc.iref.b.2"></span>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
    590                        except not limited to 76 char/line&gt;
    591 <span id="rfc.iref.u.1"></span>      user-pass   = userid ":" password
    592 <span id="rfc.iref.u.2"></span>      userid      = *&lt;TEXT excluding ":"&gt;
    593 <span id="rfc.iref.p.1"></span>      password    = *TEXT
     581            with the same challenge using the Proxy-Authenticate header field.
     582         </p>
     583         <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
     584            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.
     585         </p>
     586         <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.b.1"></span><span id="rfc.iref.b.2"></span><span id="rfc.iref.u.1"></span><span id="rfc.iref.u.2"></span><span id="rfc.iref.p.1"></span>   basic-credentials = base64-user-pass
     587   base64-user-pass  = &lt;base64 <a href="#RFC2045" id="rfc.xref.RFC2045.1"><cite title="Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies">[4]</cite></a> encoding of user-pass,
     588                    except not limited to 76 char/line&gt;
     589   user-pass   = userid ":" password
     590   userid      = *&lt;TEXT excluding ":"&gt;
     591   password    = *TEXT
    594592</pre><p id="rfc.section.2.p.9">Userids might be case sensitive.</p>
    595       <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>
    596       <div id="rfc.figure.u.8"></div><pre class="text">      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
     593         <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>
     594         <div id="rfc.figure.u.8"></div><pre class="text">   Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
    597595</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
    598          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
    599          challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the
    600          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.
    601       </p>
    602       <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Digest Access Authentication Scheme
    603       </h1>
    604       <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Introduction
    605       </h2>
    606       <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;Purpose
    607       </h3>
    608       <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
    609          the network in an unencrypted form. This section provides the specification for a scheme that does not send the password in
    610          cleartext, referred to as "Digest Access Authentication".
    611       </p>
    612       <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
    613          Web. This scheme provides no encryption of message content. The intent is simply to create an access authentication method
    614          that avoids the most serious flaws of Basic authentication.
    615       </p>
    616       <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;Overall Operation
    617       </h3>
    618       <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
    619          using a nonce value. A valid response contains a checksum (by default, the MD5 checksum) of the username, the password, the
    620          given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with
    621          the Basic scheme, the username and password must be prearranged in some fashion not addressed by this document.
    622       </p>
    623       <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;Representation of digest values
    624       </h3>
    625       <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
    626          is used and that is the only algorithm described in this document.
    627       </p>
    628       <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
    629          the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation
    630          as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That
    631          is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'.
    632       </p>
    633       <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;Limitations
    634       </h3>
    635       <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
    636          for Basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same
    637          problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between
    638          user and server to establish the user's password.
    639       </p>
    640       <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
    641          private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better
    642          than Basic authentication.
    643       </p>
    644       <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>
    645       <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
    646          header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.
    647       </p>
    648       <div id="rfc.iref.h.1"></div>
    649       <div id="rfc.iref.w.1"></div>
    650       <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>
    651       <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
    652          responds with a "401 Unauthorized" status code, and a WWW-Authenticate header as per the framework defined above, which for
    653          the digest scheme is utilized as follows:
    654       </p>
    655       <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.c.5"></span>      challenge        =  "Digest" digest-challenge
    656 
    657 <span id="rfc.iref.d.1"></span>      digest-challenge  = 1#( realm | [ domain ] | nonce |
    658                           [ opaque ] |[ stale ] | [ algorithm ] |
    659                           [ qop-options ] | [auth-param] )
    660 
    661 
    662 <span id="rfc.iref.d.2"></span>      domain            = "domain" "=" &lt;"&gt; URI ( 1*SP URI ) &lt;"&gt;
    663 <span id="rfc.iref.u.3"></span>      URI               = absoluteURI | abs_path
    664 <span id="rfc.iref.n.1"></span>      nonce             = "nonce" "=" nonce-value
    665 <span id="rfc.iref.n.2"></span>      nonce-value       = quoted-string
    666 <span id="rfc.iref.o.1"></span>      opaque            = "opaque" "=" quoted-string
    667 <span id="rfc.iref.s.1"></span>      stale             = "stale" "=" ( "true" | "false" )
    668 <span id="rfc.iref.a.3"></span>      algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
    669                            token )
    670 <span id="rfc.iref.q.1"></span>      qop-options       = "qop" "=" &lt;"&gt; 1#qop-value &lt;"&gt;
    671 <span id="rfc.iref.q.2"></span>      qop-value         = "auth" | "auth-int" | token
     596            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
     597            challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the
     598            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.
     599         </p>
     600      </div>
     601      <div>
     602         <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Digest Access Authentication Scheme
     603         </h1>
     604         <div>
     605            <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Introduction
     606            </h2>
     607            <div>
     608               <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;Purpose
     609               </h3>
     610               <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
     611                  the network in an unencrypted form. This section provides the specification for a scheme that does not send the password in
     612                  cleartext, referred to as "Digest Access Authentication".
     613               </p>
     614               <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
     615                  Web. This scheme provides no encryption of message content. The intent is simply to create an access authentication method
     616                  that avoids the most serious flaws of Basic authentication.
     617               </p>
     618            </div>
     619            <div>
     620               <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;Overall Operation
     621               </h3>
     622               <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
     623                  using a nonce value. A valid response contains a checksum (by default, the MD5 checksum) of the username, the password, the
     624                  given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with
     625                  the Basic scheme, the username and password must be prearranged in some fashion not addressed by this document.
     626               </p>
     627            </div>
     628            <div>
     629               <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;Representation of digest values
     630               </h3>
     631               <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
     632                  is used and that is the only algorithm described in this document.
     633               </p>
     634               <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
     635                  the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation
     636                  as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That
     637                  is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'.
     638               </p>
     639            </div>
     640            <div>
     641               <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;Limitations
     642               </h3>
     643               <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
     644                  for Basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same
     645                  problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between
     646                  user and server to establish the user's password.
     647               </p>
     648               <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
     649                  private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better
     650                  than Basic authentication.
     651               </p>
     652            </div>
     653         </div>
     654         <div id="specification.of.digest.headers">
     655            <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a></h2>
     656            <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
     657               header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.
     658            </p>
     659            <div id="the.www-authenticate.response.header">
     660               <div id="rfc.iref.h.1"></div>
     661               <div id="rfc.iref.w.1"></div>
     662               <h3 id="rfc.section.3.2.1"><a href="#rfc.section.3.2.1">3.2.1</a>&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></h3>
     663               <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
     664                  responds with a "401 Unauthorized" status code, and a WWW-Authenticate header as per the framework defined above, which for
     665                  the digest scheme is utilized as follows:
     666               </p>
     667               <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.c.5"></span><span id="rfc.iref.d.1"></span><span id="rfc.iref.d.2"></span><span id="rfc.iref.u.3"></span><span id="rfc.iref.n.1"></span><span id="rfc.iref.n.2"></span><span id="rfc.iref.o.1"></span><span id="rfc.iref.s.1"></span><span id="rfc.iref.a.3"></span><span id="rfc.iref.q.1"></span><span id="rfc.iref.q.2"></span>   challenge        =  "Digest" digest-challenge
     668 
     669   digest-challenge  = 1#( realm | [ domain ] | nonce |
     670                       [ opaque ] |[ stale ] | [ algorithm ] |
     671                       [ qop-options ] | [auth-param] )
     672 
     673 
     674   domain            = "domain" "=" &lt;"&gt; URI ( 1*SP URI ) &lt;"&gt;
     675   URI               = absoluteURI | abs_path
     676   nonce             = "nonce" "=" nonce-value
     677   nonce-value       = quoted-string
     678   opaque            = "opaque" "=" quoted-string
     679   stale             = "stale" "=" ( "true" | "false" )
     680   algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
     681                        token )
     682   qop-options       = "qop" "=" &lt;"&gt; 1#qop-value &lt;"&gt;
     683   qop-value         = "auth" | "auth-int" | token
    672684</pre><p id="rfc.section.3.2.1.p.3">The meanings of the values of the directives used above are as follows:</p>
    673       <p id="rfc.section.3.2.1.p.4">realm </p>
    674       <ul class="empty">
    675          <li>A string to be displayed to users so they know which username and password to use. This string should contain at least the
    676             name of the host performing the authentication and might additionally indicate the collection of users who might have access.
    677             An example might be "registered_users@gotham.news.com".
    678          </li>
    679       </ul>
    680       <p id="rfc.section.3.2.1.p.5">domain </p>
    681       <ul class="empty">
    682          <li>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.
    683             The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI
    684             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.
    685             If this directive is omitted or its value is empty, the client should assume that the protection space consists of all URIs
    686             on the responding server. This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is
    687             always the entire proxy; if present it should be ignored.
    688          </li>
    689       </ul>
    690       <p id="rfc.section.3.2.1.p.6">nonce </p>
    691       <ul class="empty">
    692          <li>A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that
    693             this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string,
    694             the double-quote character is not allowed.
    695          </li>
    696          <li>The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce
    697             might, for example, be constructed as the base 64 encoding of
    698          </li>
    699          <li>
    700             <div id="rfc.figure.u.10"></div><pre class="text">         time-stamp H(time-stamp ":" ETag ":" private-key)
    701   </pre></li>
    702          <li>where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated
    703             with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate
    704             the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from
    705             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.
    706             The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address
    707             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
    708             that originally got it. However, that would break proxy farms, where requests from a single user often go through different
    709             proxies in the farm. Also, IP address spoofing is not that hard.)
    710          </li>
    711          <li>An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against
    712             a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp
    713             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.
    714          </li>
    715          <li>The nonce is opaque to the client.</li>
    716       </ul>
    717       <p id="rfc.section.3.2.1.p.7">opaque </p>
    718       <ul class="empty">
    719          <li>A string of data, specified by the server, which should be returned by the client unchanged in the Authorization header of
    720             subsequent requests with URIs in the same protection space. It is recommended that this string be base64 or hexadecimal data.
    721          </li>
    722       </ul>
    723       <p id="rfc.section.3.2.1.p.8">stale </p>
    724       <ul class="empty">
    725          <li>A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE
    726             (case-insensitive), the client may wish to simply retry the request with a new encrypted response, without reprompting the
    727             user for a new username and password. The server should only set stale to TRUE if it receives a request for which the nonce
    728             is invalid but with a valid digest for that nonce (indicating that the client knows the correct username/password). If stale
    729             is FALSE, or anything other than TRUE, or the stale directive is not present, the username and/or password are invalid, and
    730             new values must be obtained.
    731          </li>
    732       </ul>
    733       <p id="rfc.section.3.2.1.p.9">algorithm </p>
    734       <ul class="empty">
    735          <li>A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present it is assumed to
    736             be "MD5". If the algorithm is not understood, the challenge should be ignored (and a different one used, if there is more
    737             than one).
    738          </li>
    739          <li>In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted
    740             by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data).
    741             The notation unq(X) means the value of the quoted-string X without the surrounding quotes.
    742          </li>
    743          <li>For the "MD5" and "MD5-sess" algorithms</li>
    744          <li>
    745             <div id="rfc.figure.u.11"></div><pre class="text">         H(data) = MD5(data)
    746     </pre></li>
    747          <li>and</li>
    748          <li>
    749             <div id="rfc.figure.u.12"></div><pre class="text">         KD(secret, data) = H(concat(secret, ":", data))
    750     </pre></li>
    751          <li>i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The "MD5-sess" algorithm is
    752             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>.
    753          </li>
    754       </ul>
    755       <p id="rfc.section.3.2.1.p.10">qop-options </p>
    756       <ul class="empty">
    757          <li>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
    758             or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication;
    759             the value "auth-int" indicates authentication with integrity protection; see the descriptions below for calculating the response
    760             directive value for the application of this choice. Unrecognized options <em class="bcp14">MUST</em> be ignored.
    761          </li>
    762       </ul>
    763       <p id="rfc.section.3.2.1.p.11">auth-param </p>
    764       <ul class="empty">
    765          <li>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
    766          </li>
    767       </ul>
    768       <div id="rfc.iref.h.2"></div>
    769       <div id="rfc.iref.a.4"></div>
    770       <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>
    771       <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
    772          above, utilized as follows.
    773       </p>
    774       <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.c.6"></span>       credentials      = "Digest" digest-response
    775 <span id="rfc.iref.d.3"></span>       digest-response  = 1#( username | realm | nonce | digest-uri
    776                        | response | [ algorithm ] | [cnonce] |
    777                        [opaque] | [message-qop] |
    778                            [nonce-count]  | [auth-param] )
    779 
    780 <span id="rfc.iref.u.4"></span>       username         = "username" "=" username-value
    781 <span id="rfc.iref.u.5"></span>       username-value   = quoted-string
    782 <span id="rfc.iref.d.4"></span>       digest-uri       = "uri" "=" digest-uri-value
    783 <span id="rfc.iref.d.5"></span>       digest-uri-value = request-uri   ; As specified by HTTP/1.1
    784 <span id="rfc.iref.m.1"></span>       message-qop      = "qop" "=" qop-value
    785 <span id="rfc.iref.c.7"></span>       cnonce           = "cnonce" "=" cnonce-value
    786 <span id="rfc.iref.c.8"></span>       cnonce-value     = nonce-value
    787 <span id="rfc.iref.n.3"></span>       nonce-count      = "nc" "=" nc-value
    788 <span id="rfc.iref.n.4"></span>       nc-value         = 8LHEX
    789 <span id="rfc.iref.r.3"></span>       response         = "response" "=" request-digest
    790 <span id="rfc.iref.r.4"></span>       request-digest = &lt;"&gt; 32LHEX &lt;"&gt;
    791 <span id="rfc.iref.l.1"></span>       LHEX             =  "0" | "1" | "2" | "3" |
    792                            "4" | "5" | "6" | "7" |
    793                            "8" | "9" | "a" | "b" |
    794                            "c" | "d" | "e" | "f"
     685               <p id="rfc.section.3.2.1.p.4">realm </p>
     686               <ul class="empty">
     687                  <li>A string to be displayed to users so they know which username and password to use. This string should contain at least the
     688                     name of the host performing the authentication and might additionally indicate the collection of users who might have access.
     689                     An example might be "registered_users@gotham.news.com".
     690                  </li>
     691               </ul>
     692               <p id="rfc.section.3.2.1.p.5">domain </p>
     693               <ul class="empty">
     694                  <li>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.
     695                     The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI
     696                     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.
     697                     If this directive is omitted or its value is empty, the client should assume that the protection space consists of all URIs
     698                     on the responding server. This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is
     699                     always the entire proxy; if present it should be ignored.
     700                  </li>
     701               </ul>
     702               <p id="rfc.section.3.2.1.p.6">nonce </p>
     703               <ul class="empty">
     704                  <li>A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that
     705                     this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string,
     706                     the double-quote character is not allowed.
     707                  </li>
     708                  <li>The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce
     709                     might, for example, be constructed as the base 64 encoding of
     710                  </li>
     711                  <li>
     712                     <div id="rfc.figure.u.10"></div><pre class="text">   time-stamp H(time-stamp ":" ETag ":" private-key)
     713</pre></li>
     714                  <li>where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated
     715                     with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate
     716                     the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from
     717                     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.
     718                     The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address
     719                     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
     720                     that originally got it. However, that would break proxy farms, where requests from a single user often go through different
     721                     proxies in the farm. Also, IP address spoofing is not that hard.)
     722                  </li>
     723                  <li>An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against
     724                     a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp
     725                     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.
     726                  </li>
     727                  <li>The nonce is opaque to the client.</li>
     728               </ul>
     729               <p id="rfc.section.3.2.1.p.7">opaque </p>
     730               <ul class="empty">
     731                  <li>A string of data, specified by the server, which should be returned by the client unchanged in the Authorization header of
     732                     subsequent requests with URIs in the same protection space. It is recommended that this string be base64 or hexadecimal data.
     733                  </li>
     734               </ul>
     735               <p id="rfc.section.3.2.1.p.8">stale </p>
     736               <ul class="empty">
     737                  <li>A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE
     738                     (case-insensitive), the client may wish to simply retry the request with a new encrypted response, without reprompting the
     739                     user for a new username and password. The server should only set stale to TRUE if it receives a request for which the nonce
     740                     is invalid but with a valid digest for that nonce (indicating that the client knows the correct username/password). If stale
     741                     is FALSE, or anything other than TRUE, or the stale directive is not present, the username and/or password are invalid, and
     742                     new values must be obtained.
     743                  </li>
     744               </ul>
     745               <p id="rfc.section.3.2.1.p.9">algorithm </p>
     746               <ul class="empty">
     747                  <li>A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present it is assumed to
     748                     be "MD5". If the algorithm is not understood, the challenge should be ignored (and a different one used, if there is more
     749                     than one).
     750                  </li>
     751                  <li>In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted
     752                     by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data).
     753                     The notation unq(X) means the value of the quoted-string X without the surrounding quotes.
     754                  </li>
     755                  <li>For the "MD5" and "MD5-sess" algorithms</li>
     756                  <li>
     757                     <div id="rfc.figure.u.11"></div><pre class="text">       H(data) = MD5(data)
     758</pre></li>
     759                  <li>and</li>
     760                  <li>
     761                     <div id="rfc.figure.u.12"></div><pre class="text">       KD(secret, data) = H(concat(secret, ":", data))
     762</pre></li>
     763                  <li>i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The "MD5-sess" algorithm is
     764                     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>.
     765                  </li>
     766               </ul>
     767               <p id="rfc.section.3.2.1.p.10">qop-options </p>
     768               <ul class="empty">
     769                  <li>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
     770                     or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication;
     771                     the value "auth-int" indicates authentication with integrity protection; see the descriptions below for calculating the response
     772                     directive value for the application of this choice. Unrecognized options <em class="bcp14">MUST</em> be ignored.
     773                  </li>
     774               </ul>
     775               <p id="rfc.section.3.2.1.p.11">auth-param </p>
     776               <ul class="empty">
     777                  <li>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
     778                  </li>
     779               </ul>
     780            </div>
     781            <div id="the.authorization.request.header">
     782               <div id="rfc.iref.h.2"></div>
     783               <div id="rfc.iref.a.4"></div>
     784               <h3 id="rfc.section.3.2.2"><a href="#rfc.section.3.2.2">3.2.2</a>&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a></h3>
     785               <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
     786                  above, utilized as follows.
     787               </p>
     788               <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.c.6"></span><span id="rfc.iref.d.3"></span><span id="rfc.iref.u.4"></span><span id="rfc.iref.u.5"></span><span id="rfc.iref.d.4"></span><span id="rfc.iref.d.5"></span><span id="rfc.iref.m.1"></span><span id="rfc.iref.c.7"></span><span id="rfc.iref.c.8"></span><span id="rfc.iref.n.3"></span><span id="rfc.iref.n.4"></span><span id="rfc.iref.r.3"></span><span id="rfc.iref.r.4"></span><span id="rfc.iref.l.1"></span>    credentials      = "Digest" digest-response
     789    digest-response  = 1#( username | realm | nonce | digest-uri
     790                    | response | [ algorithm ] | [cnonce] |
     791                    [opaque] | [message-qop] |
     792                       [nonce-count]  | [auth-param] )
     793   
     794    username         = "username" "=" username-value
     795    username-value   = quoted-string
     796    digest-uri       = "uri" "=" digest-uri-value
     797    digest-uri-value = request-uri   ; As specified by HTTP/1.1
     798    message-qop      = "qop" "=" qop-value
     799    cnonce           = "cnonce" "=" cnonce-value
     800    cnonce-value     = nonce-value
     801    nonce-count      = "nc" "=" nc-value
     802    nc-value         = 8LHEX
     803    response         = "response" "=" request-digest
     804    request-digest = &lt;"&gt; 32LHEX &lt;"&gt;
     805    LHEX             =  "0" | "1" | "2" | "3" |
     806                       "4" | "5" | "6" | "7" |
     807                       "8" | "9" | "a" | "b" |
     808                       "c" | "d" | "e" | "f"
    795809</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
    796          being requested.
    797       </p>
    798       <p id="rfc.section.3.2.2.p.4">response </p>
    799       <ul class="empty">
    800          <li>A string of 32 hex digits computed as defined below, which proves that the user knows a password</li>
    801       </ul>
    802       <p id="rfc.section.3.2.2.p.5">username </p>
    803       <ul class="empty">
    804          <li>The user's name in the specified realm.</li>
    805       </ul>
    806       <p id="rfc.section.3.2.2.p.6">digest-uri </p>
    807       <ul class="empty">
    808          <li>The URI from Request-URI of the Request-Line; duplicated here because proxies are allowed to change the Request-Line in transit.</li>
    809       </ul>
    810       <p id="rfc.section.3.2.2.p.7">qop </p>
    811       <ul class="empty">
    812          <li>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
    813             of the request-digest. Note that this is a single token, not a quoted list of alternatives as in WWW-Authenticate. This directive
    814             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.
    815          </li>
    816       </ul>
    817       <p id="rfc.section.3.2.2.p.8">cnonce </p>
    818       <ul class="empty">
    819          <li>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
    820             quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide
    821             mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of
    822             the response-digest and request-digest values.
    823          </li>
    824       </ul>
    825       <p id="rfc.section.3.2.2.p.9">nonce-count </p>
    826       <ul class="empty">
    827          <li>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
    828             count of the number of requests (including the current request) that the client has sent with the nonce value in this request.
    829             For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of
    830             this directive is to allow the server to detect request replays by maintaining its own copy of this count - if the same nc-value
    831             is seen twice, then the request is a replay. See the description below of the construction of the request-digest value.
    832          </li>
    833       </ul>
    834       <p id="rfc.section.3.2.2.p.10">auth-param </p>
    835       <ul class="empty">
    836          <li>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
    837          </li>
    838       </ul>
    839       <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
    840          request-digest is invalid, then a login failure should be logged, since repeated login failures from a single client may indicate
    841          an attacker attempting to guess passwords.
    842       </p>
    843       <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
    844          is computed.
    845       </p>
    846       <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>
    847       <p id="rfc.section.3.2.2.1.p.1">If the "qop" value is "auth" or "auth-int":</p>
    848       <div id="rfc.figure.u.14"></div><pre class="inline">      request-digest  = &lt;"&gt; &lt; KD ( H(A1),     unq(nonce-value)
    849                                           ":" nc-value
    850                                           ":" unq(cnonce-value)
    851                                           ":" unq(qop-value)
    852                                           ":" H(A2)
    853                                   ) &lt;"&gt;
     810                  being requested.
     811               </p>
     812               <p id="rfc.section.3.2.2.p.4">response </p>
     813               <ul class="empty">
     814                  <li>A string of 32 hex digits computed as defined below, which proves that the user knows a password</li>
     815               </ul>
     816               <p id="rfc.section.3.2.2.p.5">username </p>
     817               <ul class="empty">
     818                  <li>The user's name in the specified realm.</li>
     819               </ul>
     820               <p id="rfc.section.3.2.2.p.6">digest-uri </p>
     821               <ul class="empty">
     822                  <li>The URI from Request-URI of the Request-Line; duplicated here because proxies are allowed to change the Request-Line in transit.</li>
     823               </ul>
     824               <p id="rfc.section.3.2.2.p.7">qop </p>
     825               <ul class="empty">
     826                  <li>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
     827                     of the request-digest. Note that this is a single token, not a quoted list of alternatives as in WWW-Authenticate. This directive
     828                     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.
     829                  </li>
     830               </ul>
     831               <p id="rfc.section.3.2.2.p.8">cnonce </p>
     832               <ul class="empty">
     833                  <li>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
     834                     quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide
     835                     mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of
     836                     the response-digest and request-digest values.
     837                  </li>
     838               </ul>
     839               <p id="rfc.section.3.2.2.p.9">nonce-count </p>
     840               <ul class="empty">
     841                  <li>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
     842                     count of the number of requests (including the current request) that the client has sent with the nonce value in this request.
     843                     For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of
     844                     this directive is to allow the server to detect request replays by maintaining its own copy of this count - if the same nc-value
     845                     is seen twice, then the request is a replay. See the description below of the construction of the request-digest value.
     846                  </li>
     847               </ul>
     848               <p id="rfc.section.3.2.2.p.10">auth-param </p>
     849               <ul class="empty">
     850                  <li>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
     851                  </li>
     852               </ul>
     853               <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
     854                  request-digest is invalid, then a login failure should be logged, since repeated login failures from a single client may indicate
     855                  an attacker attempting to guess passwords.
     856               </p>
     857               <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
     858                  is computed.
     859               </p>
     860               <div id="request-digest">
     861                  <h4 id="rfc.section.3.2.2.1"><a href="#rfc.section.3.2.2.1">3.2.2.1</a>&nbsp;<a href="#request-digest">Request-Digest</a></h4>
     862                  <p id="rfc.section.3.2.2.1.p.1">If the "qop" value is "auth" or "auth-int":</p>
     863                  <div id="rfc.figure.u.14"></div><pre class="inline">   request-digest  = &lt;"&gt; &lt; KD ( H(A1),     unq(nonce-value)
     864                                       ":" nc-value
     865                                       ":" unq(cnonce-value)
     866                                       ":" unq(qop-value)
     867                                       ":" H(A2)
     868                               ) &lt;"&gt;
    854869</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>
    855       <div id="rfc.figure.u.15"></div><pre class="inline">      request-digest  =
    856                  &lt;"&gt; &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) &gt;
    857    &lt;"&gt;
     870                  <div id="rfc.figure.u.15"></div><pre class="inline">  request-digest  =
     871             &lt;"&gt; &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) &gt;
     872&lt;"&gt;
    858873</pre><p id="rfc.section.3.2.2.1.p.5">See below for the definitions for A1 and A2.</p>
    859       <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>
    860       <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>
    861       <div id="rfc.figure.u.16"></div><pre class="inline">      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
     874               </div>
     875               <div id="A1">
     876                  <h4 id="rfc.section.3.2.2.2"><a href="#rfc.section.3.2.2.2">3.2.2.2</a>&nbsp;<a href="#A1">A1</a></h4>
     877                  <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>
     878                  <div id="rfc.figure.u.16"></div><pre class="inline">   A1       = unq(username-value) ":" unq(realm-value) ":" passwd
    862879</pre><p id="rfc.section.3.2.2.2.p.3">where</p>
    863       <div id="rfc.figure.u.17"></div><pre class="inline">      passwd   = &lt; user's password &gt;
     880                  <div id="rfc.figure.u.17"></div><pre class="inline">   passwd   = &lt; user's password &gt;
    864881</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
    865          receipt of a WWW-Authenticate challenge from the server. It uses the server nonce from that challenge, and the first client
    866          nonce value to construct A1 as follows:
    867       </p>
    868       <div id="rfc.figure.u.18"></div><pre class="inline">      A1       = H( unq(username-value) ":" unq(realm-value)
    869                      ":" passwd )
    870                      ":" unq(nonce-value) ":" unq(cnonce-value)
     882                     receipt of a WWW-Authenticate challenge from the server. It uses the server nonce from that challenge, and the first client
     883                     nonce value to construct A1 as follows:
     884                  </p>
     885                  <div id="rfc.figure.u.18"></div><pre class="inline">   A1       = H( unq(username-value) ":" unq(realm-value)
     886                  ":" passwd )
     887                  ":" unq(nonce-value) ":" unq(cnonce-value)
    871888</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
    872          session", thus limiting the amount of material hashed with any one key. (Note: see further discussion of the authentication
    873          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
    874          be used in conjunction with a third party authentication service so that the web server would not need the actual password
    875          value. The specification of such a protocol is beyond the scope of this specification.
    876       </p>
    877       <h4 id="rfc.section.3.2.2.3"><a href="#rfc.section.3.2.2.3">3.2.2.3</a>&nbsp;A2
    878       </h4>
    879       <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>
    880       <div id="rfc.figure.u.19"></div><pre class="inline">      A2       = Method ":" digest-uri-value
     889                     session", thus limiting the amount of material hashed with any one key. (Note: see further discussion of the authentication
     890                     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
     891                     be used in conjunction with a third party authentication service so that the web server would not need the actual password
     892                     value. The specification of such a protocol is beyond the scope of this specification.
     893                  </p>
     894               </div>
     895               <div>
     896                  <h4 id="rfc.section.3.2.2.3"><a href="#rfc.section.3.2.2.3">3.2.2.3</a>&nbsp;A2
     897                  </h4>
     898                  <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>
     899                  <div id="rfc.figure.u.19"></div><pre class="inline">   A2       = Method ":" digest-uri-value
    881900</pre><p id="rfc.section.3.2.2.3.p.3">If the "qop" value is "auth-int", then A2 is:</p>
    882       <div id="rfc.figure.u.20"></div><pre class="inline">      A2       = Method ":" digest-uri-value ":" H(entity-body)
    883 </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
    884       </h4>
    885       <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"
    886          notation indicates that surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header
    887          includes the fields
    888       </p>
    889       <div id="rfc.figure.u.21"></div><pre class="text">     username="Mufasa", realm=myhost@testrealm.com
     901                  <div id="rfc.figure.u.20"></div><pre class="inline">   A2       = Method ":" digest-uri-value ":" H(entity-body)
     902</pre></div>
     903               <div>
     904                  <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
     905                  </h4>
     906                  <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"
     907                     notation indicates that surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header
     908                     includes the fields
     909                  </p>
     910                  <div id="rfc.figure.u.21"></div><pre class="text">  username="Mufasa", realm=myhost@testrealm.com
    890911</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
    891          quotation marks in the digested string.
    892       </p>
    893       <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
    894          in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated
    895          above must be
    896       </p>
    897       <div id="rfc.figure.u.22"></div><pre class="text">        Mufasa:myhost@testrealm.com:Circle Of Life
     912                     quotation marks in the digested string.
     913                  </p>
     914                  <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
     915                     in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated
     916                     above must be
     917                  </p>
     918                  <div id="rfc.figure.u.22"></div><pre class="text">     Mufasa:myhost@testrealm.com:Circle Of Life
    898919</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,
    899          the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless
    900          that white space was in the quoted strings or entity body being digested.
    901       </p>
    902       <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
    903          message body - it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient.
    904          Note that this includes multipart boundaries and embedded headers in each part of any multipart content-type.
    905       </p>
    906       <h4 id="rfc.section.3.2.2.5"><a href="#rfc.section.3.2.2.5">3.2.2.5</a>&nbsp;Various considerations
    907       </h4>
    908       <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
    909          is to foil chosen plaintext attacks.
    910       </p>
    911       <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
    912          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
    913          such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that
    914          an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would
    915          not result in the same digest as that calculated by the client.
    916       </p>
    917       <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
    918          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,
    919          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
    920          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.
    921       </p>
    922       <div id="rfc.iref.h.3"></div>
    923       <div id="rfc.iref.a.5"></div>
    924       <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;The Authentication-Info Header
    925       </h3>
    926       <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
    927          in the response.
    928       </p>
    929       <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.a.6"></span>        AuthenticationInfo = "Authentication-Info" ":" auth-info
    930 <span id="rfc.iref.a.7"></span>        auth-info          = 1#(nextnonce | [ message-qop ]
    931                                | [ response-auth ] | [ cnonce ]
    932                                | [nonce-count] )
    933 <span id="rfc.iref.n.5"></span>        nextnonce          = "nextnonce" "=" nonce-value
    934 <span id="rfc.iref.r.5"></span>        response-auth      = "rspauth" "=" response-digest
    935 <span id="rfc.iref.r.6"></span>        response-digest    = &lt;"&gt; *LHEX &lt;"&gt;
     920                     the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless
     921                     that white space was in the quoted strings or entity body being digested.
     922                  </p>
     923                  <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
     924                     message body - it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient.
     925                     Note that this includes multipart boundaries and embedded headers in each part of any multipart content-type.
     926                  </p>
     927               </div>
     928               <div>
     929                  <h4 id="rfc.section.3.2.2.5"><a href="#rfc.section.3.2.2.5">3.2.2.5</a>&nbsp;Various considerations
     930                  </h4>
     931                  <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
     932                     is to foil chosen plaintext attacks.
     933                  </p>
     934                  <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
     935                     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
     936                     such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that
     937                     an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would
     938                     not result in the same digest as that calculated by the client.
     939                  </p>
     940                  <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
     941                     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,
     942                     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
     943                     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.
     944                  </p>
     945               </div>
     946            </div>
     947            <div>
     948               <div id="rfc.iref.h.3"></div>
     949               <div id="rfc.iref.a.5"></div>
     950               <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;The Authentication-Info Header
     951               </h3>
     952               <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
     953                  in the response.
     954               </p>
     955               <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.a.6"></span><span id="rfc.iref.a.7"></span><span id="rfc.iref.n.5"></span><span id="rfc.iref.r.5"></span><span id="rfc.iref.r.6"></span>     AuthenticationInfo = "Authentication-Info" ":" auth-info
     956     auth-info          = 1#(nextnonce | [ message-qop ]
     957                            | [ response-auth ] | [ cnonce ]
     958                            | [nonce-count] )
     959     nextnonce          = "nextnonce" "=" nonce-value
     960     response-auth      = "rspauth" "=" response-digest
     961     response-digest    = &lt;"&gt; *LHEX &lt;"&gt;
    936962</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.
    937          The server may send the Authentication-Info header with a nextnonce field as a means of implementing one-time or otherwise
    938          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
    939          to re-authenticate from the server with the "stale=TRUE".
    940       </p>
    941       <p id="rfc.section.3.2.3.p.4"> </p>
    942       <ul class="empty">
    943          <li>Server implementations should carefully consider the performance implications of the use of this mechanism; pipelined requests
    944             will not be possible if every response includes a nextnonce directive that must be used on the next request received by the
    945             server. Consideration should be given to the performance vs. security tradeoffs of allowing an old nonce value to be used
    946             for a limited time to permit request pipelining. Use of the nonce-count can retain most of the security advantages of a new
    947             server nonce without the deleterious affects on pipelining.
    948          </li>
    949       </ul>
    950       <p id="rfc.section.3.2.3.p.5">message-qop</p>
    951       <ul class="empty">
    952          <li>Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication;
    953             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.
    954          </li>
    955       </ul>
    956       <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
    957          knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The "response-digest"
    958          value is calculated as for the "request-digest" in the Authorization header, except that if "qop=auth" or is not specified
    959          in the Authorization header for the request, A2 is
    960       </p>
    961       <div id="rfc.figure.u.24"></div><pre class="inline">      A2       = ":" digest-uri-value
     963                  The server may send the Authentication-Info header with a nextnonce field as a means of implementing one-time or otherwise
     964                  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
     965                  to re-authenticate from the server with the "stale=TRUE".
     966               </p>
     967               <p id="rfc.section.3.2.3.p.4"></p>
     968               <ul class="empty">
     969                  <li>Server implementations should carefully consider the performance implications of the use of this mechanism; pipelined requests
     970                     will not be possible if every response includes a nextnonce directive that must be used on the next request received by the
     971                     server. Consideration should be given to the performance vs. security tradeoffs of allowing an old nonce value to be used
     972                     for a limited time to permit request pipelining. Use of the nonce-count can retain most of the security advantages of a new
     973                     server nonce without the deleterious affects on pipelining.
     974                  </li>
     975               </ul>
     976               <p id="rfc.section.3.2.3.p.5">message-qop</p>
     977               <ul class="empty">
     978                  <li>Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication;
     979                     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.
     980                  </li>
     981               </ul>
     982               <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
     983                  knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The "response-digest"
     984                  value is calculated as for the "request-digest" in the Authorization header, except that if "qop=auth" or is not specified
     985                  in the Authorization header for the request, A2 is
     986               </p>
     987               <div id="rfc.figure.u.24"></div><pre class="inline">   A2       = ":" digest-uri-value
    962988</pre><p id="rfc.section.3.2.3.p.9">and if "qop=auth-int", then A2 is</p>
    963       <div id="rfc.figure.u.25"></div><pre class="inline">      A2       = ":" digest-uri-value ":" H(entity-body)
     989               <div id="rfc.figure.u.25"></div><pre class="inline">   A2       = ":" digest-uri-value ":" H(entity-body)
    964990</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"
    965          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"
    966          directives <em class="bcp14">MUST</em> BE present if "qop=auth" or "qop=auth-int" is specified.
    967       </p>
    968       <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>
    969       <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>
    970       <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
    971          the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare
    972          the result to the given request-digest value.
    973       </p>
    974       <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
    975          server, the validity of an Authorization header may be verified.
    976       </p>
    977       <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
    978          space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the
    979          protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an
    980          authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization
    981          header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges.
    982          The server may choose to accept the old Authorization header information, even though the nonce value included might not be
    983          fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request;
    984          by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting
    985          for a new username and password.
    986       </p>
    987       <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
    988          session, the opaque data may be used to transport authentication session state information. (Note that any such use can also
    989          be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for
    990          authenticating content that actually sits on another server. It would achieve this by having the first 401 response include
    991          a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state
    992          information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing
    993          to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the
    994          &lt;opaque&gt; data.
    995       </p>
    996       <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
    997          must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate
    998          a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization
    999          headers described in <a href="#proxy-authentication.and.proxy-authorization" title="Proxy-Authentication and Proxy-Authorization">Section&nbsp;3.6</a> below.
    1000       </p>
    1001       <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Security Protocol Negotiation
    1002       </h2>
    1003       <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>
    1004       <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
    1005          the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot
    1006          handle.
    1007       </p>
    1008       <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>
    1009       <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
    1010          URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document
    1011          is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).
    1012       </p>
    1013       <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>
    1014       <div id="rfc.figure.u.26"></div><pre class="text">         HTTP/1.1 401 Unauthorized
    1015          WWW-Authenticate: Digest
    1016                  realm="testrealm@host.com",
    1017                  qop="auth,auth-int",
    1018                  nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
    1019                  opaque="5ccc069c403ebaf9f0171e9517f40e41"
     991                  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"
     992                  directives <em class="bcp14">MUST</em> BE present if "qop=auth" or "qop=auth-int" is specified.
     993               </p>
     994               <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>
     995            </div>
     996         </div>
     997         <div id="digest.operation">
     998            <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a>&nbsp;<a href="#digest.operation">Digest Operation</a></h2>
     999            <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
     1000               the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare
     1001               the result to the given request-digest value.
     1002            </p>
     1003            <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
     1004               server, the validity of an Authorization header may be verified.
     1005            </p>
     1006            <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
     1007               space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the
     1008               protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an
     1009               authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization
     1010               header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges.
     1011               The server may choose to accept the old Authorization header information, even though the nonce value included might not be
     1012               fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request;
     1013               by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting
     1014               for a new username and password.
     1015            </p>
     1016            <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
     1017               session, the opaque data may be used to transport authentication session state information. (Note that any such use can also
     1018               be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for
     1019               authenticating content that actually sits on another server. It would achieve this by having the first 401 response include
     1020               a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state
     1021               information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing
     1022               to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the
     1023               &lt;opaque&gt; data.
     1024            </p>
     1025            <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
     1026               must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate
     1027               a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization
     1028               headers described in <a href="#proxy-authentication.and.proxy-authorization" title="Proxy-Authentication and Proxy-Authorization">Section&nbsp;3.6</a> below.
     1029            </p>
     1030         </div>
     1031         <div>
     1032            <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Security Protocol Negotiation
     1033            </h2>
     1034            <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>
     1035            <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
     1036               the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot
     1037               handle.
     1038            </p>
     1039         </div>
     1040         <div id="specification.of.digest.headers.example">
     1041            <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a>&nbsp;<a href="#specification.of.digest.headers.example">Example</a></h2>
     1042            <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
     1043               URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document
     1044               is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).
     1045            </p>
     1046            <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>
     1047            <div id="rfc.figure.u.26"></div><pre class="text">      HTTP/1.1 401 Unauthorized
     1048      WWW-Authenticate: Digest
     1049              realm="testrealm@host.com",
     1050              qop="auth,auth-int",
     1051              nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
     1052              opaque="5ccc069c403ebaf9f0171e9517f40e41"
    10201053</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
    1021          following Authorization header:
    1022       </p>
    1023       <div id="rfc.figure.u.27"></div><pre class="text">         Authorization: Digest username="Mufasa",
    1024                  realm="testrealm@host.com",
    1025                  nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
    1026                  uri="/dir/index.html",
    1027                  qop=auth,
    1028                  nc=00000001,
    1029                  cnonce="0a4f113b",
    1030                  response="6629fae49393a05397450978507c4ef1",
    1031                  opaque="5ccc069c403ebaf9f0171e9517f40e41"
    1032 </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>
    1033       <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
    1034          servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate
    1035          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
    1036          to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy
    1037          Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate
    1038          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>.
    1039       </p>
    1040       <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
    1041          header in <a href="#the.authorization.request.header" title="The Authorization Request Header">Section&nbsp;3.2.2</a> above.
    1042       </p>
    1043       <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
    1044          header field.
    1045       </p>
    1046       <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
    1047          same response.
    1048       </p>
    1049       <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
    1050       <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Authentication of Clients using Basic Authentication
    1051       </h2>
    1052       <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,
    1053          which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication
    1054          schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes
    1055          to use one-time passwords) to Basic authentication.
    1056       </p>
    1057       <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
    1058          over the physical network. It is this problem which Digest Authentication attempts to address.
    1059       </p>
    1060       <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.
    1061       </p>
    1062       <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
    1063          as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this
    1064          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
    1065          concern. This is only correct if the server issues both user name and password to the users and in particular does not allow
    1066          the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid
    1067          the task of maintaining multiple passwords.
    1068       </p>
    1069       <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
    1070          server but also unauthorized access to any other resources on other systems that the user protects with the same password.
    1071          Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner
    1072          or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all
    1073          those sites if this information is not maintained in a secure fashion.
    1074       </p>
    1075       <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
    1076          to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or
    1077          gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not
    1078          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
    1079          for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism
    1080          to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable
    1081          by the client.
    1082       </p>
    1083       <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;Authentication of Clients using Digest Authentication
    1084       </h2>
    1085       <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
    1086          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.
    1087       </p>
    1088       <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
    1089          and response are available to an eavesdropper.
    1090       </p>
    1091       <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
    1092          is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response
    1093          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.
    1094       </p>
    1095       <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
    1096          protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless
    1097          many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses
    1098          Basic should be switched to Digest as soon as practical.
    1099       </p>
    1100       <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Limited Use Nonce Values
    1101       </h2>
    1102       <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,
    1103          for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against,
    1104          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
    1105          implications. For example, a server may choose to allow each nonce value to be used only once by maintaining a record of whether
    1106          or not each recently issued nonce has been returned and sending a next-nonce directive in the Authentication-Info header field
    1107          of every response. This protects against even an immediate replay attack, but has a high cost checking nonce values, and perhaps
    1108          more important will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication).
    1109          Similarly, incorporating a request-specific element such as the Etag value for a resource limits the use of the nonce to that
    1110          version of the resource and also defeats pipelining. Thus it may be useful to do so for methods with side effects but have
    1111          unacceptable performance for those that do not.
    1112       </p>
    1113       <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;Comparison of Digest with Basic Authentication
    1114       </h2>
    1115       <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
    1116          the two points out the utility, even necessity, of replacing Basic by Digest.
    1117       </p>
    1118       <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
    1119          might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication
    1120          an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often
    1121          worse, will permit access to anything else the user protects with the same password.
    1122       </p>
    1123       <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
    1124          password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document,
    1125          and even that may be limited by the server's choice of nonce.
    1126       </p>
    1127       <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>
    1128       <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
    1129          already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested
    1130          in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper
    1131          has the user's password, any document protected by that password is open to him.
    1132       </p>
    1133       <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
    1134          ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp,
    1135          the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince
    1136          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
    1137          different from the address to which it believes it is sending the document. An attack can only succeed in the period before
    1138          the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain
    1139          state between transactions.
    1140       </p>
    1141       <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
    1142          not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until
    1143          the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.
    1144       </p>
    1145       <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
    1146          server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int,
    1147          an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even
    1148          with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking
    1149          provides some protection against replay of previously used valid credentials, but see 4.8.
    1150       </p>
    1151       <h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;Weakness Created by Multiple Authentication Schemes
    1152       </h2>
    1153       <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
    1154          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.
    1155       </p>
    1156       <p id="rfc.section.4.6.p.2"> </p>
    1157       <ul class="empty">
    1158          <li>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
    1159             only include Basic if it is minimally acceptable.
    1160          </li>
    1161       </ul>
    1162       <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
    1163          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.
    1164       </p>
    1165       <h2 id="rfc.section.4.7"><a href="#rfc.section.4.7">4.7</a>&nbsp;Online dictionary attacks
    1166       </h2>
    1167       <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
    1168          list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password
    1169          on the list is paid once for each challenge.
    1170       </p>
    1171       <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>
    1172       <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>
    1173       <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
    1174          compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities
    1175          to the attacker.
    1176       </p>
    1177       <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
    1178          will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest
    1179          scheme that it understands from the choices offered.
    1180       </p>
    1181       <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
    1182          authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using
    1183          the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy
    1184          caching service to gullible users.
    1185       </p>
    1186       <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
    1187          authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce
    1188          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
    1189          authentication in general, or from specific sites.
    1190       </p>
    1191       <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
    1192          course, this is still much harder than a comparable attack against Basic Authentication.
    1193       </p>
    1194       <h2 id="rfc.section.4.9"><a href="#rfc.section.4.9">4.9</a>&nbsp;Chosen plaintext attacks
    1195       </h2>
    1196       <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
    1197          the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much
    1198          easier <a href="#ref8" id="rfc.xref.ref8.1"><cite title="Message Authentication with MD5">[8]</cite></a>.
    1199       </p>
    1200       <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>
    1201       <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;
    1202          this allows the client to vary the input to the hash in a way not chosen by the attacker.
    1203       </p>
    1204       <h2 id="rfc.section.4.10"><a href="#rfc.section.4.10">4.10</a>&nbsp;Precomputed dictionary attacks
    1205       </h2>
    1206       <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
    1207          for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation
    1208          can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding
    1209          to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some
    1210          might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list
    1211          can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes
    1212          of disk storage.
    1213       </p>
    1214       <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>
    1215       <h2 id="rfc.section.4.11"><a href="#rfc.section.4.11">4.11</a>&nbsp;Batch brute force attacks
    1216       </h2>
    1217       <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
    1218          same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response
    1219          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
    1220          of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even
    1221          a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with
    1222          six or fewer letters in a few hours.
    1223       </p>
    1224       <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>
    1225       <h2 id="rfc.section.4.12"><a href="#rfc.section.4.12">4.12</a>&nbsp;Spoofing by Counterfeit Servers
    1226       </h2>
    1227       <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
    1228          to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then
    1229          the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult
    1230          with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of
    1231          the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack
    1232          by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of
    1233          each scheme.
    1234       </p>
    1235       <h2 id="rfc.section.4.13"><a href="#rfc.section.4.13">4.13</a>&nbsp;Storing passwords
    1236       </h2>
    1237       <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
    1238          name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username
    1239          and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.
    1240       </p>
    1241       <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
    1242          documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted
    1243          in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely
    1244          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
    1245          data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically
    1246          compromise others with the same username and password (though it does expose them to brute force attack).
    1247       </p>
    1248       <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
    1249          passwords, because for the purpose of accessing documents in its realm, it effectively does.
    1250       </p>
    1251       <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
    1252          use. In particular a realm string should include the name of the host doing the authentication. The inability of the client
    1253          to authenticate the server is a weakness of Digest Authentication.
    1254       </p>
    1255       <h2 id="rfc.section.4.14"><a href="#rfc.section.4.14">4.14</a>&nbsp;Summary
    1256       </h2>
    1257       <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
    1258          for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending
    1259          on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect
    1260          the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may
    1261          be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may
    1262          satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited
    1263          lifetime.
    1264       </p>
    1265       <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
    1266          implementation will be far superior to Basic Authentication.
    1267       </p>
    1268       <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Sample implementation
    1269       </h1>
    1270       <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
    1271          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.
    1272       </p>
    1273       <div id="rfc.figure.u.28"></div>
    1274       <p>File "digcalc.h":</p><pre class="text">#define HASHLEN 16
     1054               following Authorization header:
     1055            </p>
     1056            <div id="rfc.figure.u.27"></div><pre class="text">      Authorization: Digest username="Mufasa",
     1057              realm="testrealm@host.com",
     1058              nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
     1059              uri="/dir/index.html",
     1060              qop=auth,
     1061              nc=00000001,
     1062              cnonce="0a4f113b",
     1063              response="6629fae49393a05397450978507c4ef1",
     1064              opaque="5ccc069c403ebaf9f0171e9517f40e41"
     1065</pre></div>
     1066         <div id="proxy-authentication.and.proxy-authorization">
     1067            <h2 id="rfc.section.3.6"><a href="#rfc.section.3.6">3.6</a>&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></h2>
     1068            <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
     1069               servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate
     1070               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
     1071               to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy
     1072               Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate
     1073               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>.
     1074            </p>
     1075            <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
     1076               header in <a href="#the.authorization.request.header" title="The Authorization Request Header">Section&nbsp;3.2.2</a> above.
     1077            </p>
     1078            <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
     1079               header field.
     1080            </p>
     1081            <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
     1082               same response.
     1083            </p>
     1084         </div>
     1085      </div>
     1086      <div id="security.considerations">
     1087         <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a href="#security.considerations">Security Considerations</a></h1>
     1088         <div>
     1089            <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Authentication of Clients using Basic Authentication
     1090            </h2>
     1091            <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,
     1092               which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication
     1093               schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes
     1094               to use one-time passwords) to Basic authentication.
     1095            </p>
     1096            <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
     1097               over the physical network. It is this problem which Digest Authentication attempts to address.
     1098            </p>
     1099            <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.
     1100            </p>
     1101            <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
     1102               as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this
     1103               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
     1104               concern. This is only correct if the server issues both user name and password to the users and in particular does not allow
     1105               the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid
     1106               the task of maintaining multiple passwords.
     1107            </p>
     1108            <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
     1109               server but also unauthorized access to any other resources on other systems that the user protects with the same password.
     1110               Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner
     1111               or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all
     1112               those sites if this information is not maintained in a secure fashion.
     1113            </p>
     1114            <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
     1115               to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or
     1116               gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not
     1117               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
     1118               for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism
     1119               to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable
     1120               by the client.
     1121            </p>
     1122         </div>
     1123         <div>
     1124            <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;Authentication of Clients using Digest Authentication
     1125            </h2>
     1126            <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
     1127               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.
     1128            </p>
     1129            <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
     1130               and response are available to an eavesdropper.
     1131            </p>
     1132            <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
     1133               is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response
     1134               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.
     1135            </p>
     1136            <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
     1137               protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless
     1138               many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses
     1139               Basic should be switched to Digest as soon as practical.
     1140            </p>
     1141         </div>
     1142         <div>
     1143            <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Limited Use Nonce Values
     1144            </h2>
     1145            <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,
     1146               for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against,
     1147               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
     1148               implications. For example, a server may choose to allow each nonce value to be used only once by maintaining a record of whether
     1149               or not each recently issued nonce has been returned and sending a next-nonce directive in the Authentication-Info header field
     1150               of every response. This protects against even an immediate replay attack, but has a high cost checking nonce values, and perhaps
     1151               more important will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication).
     1152               Similarly, incorporating a request-specific element such as the Etag value for a resource limits the use of the nonce to that
     1153               version of the resource and also defeats pipelining. Thus it may be useful to do so for methods with side effects but have
     1154               unacceptable performance for those that do not.
     1155            </p>
     1156         </div>
     1157         <div>
     1158            <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;Comparison of Digest with Basic Authentication
     1159            </h2>
     1160            <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
     1161               the two points out the utility, even necessity, of replacing Basic by Digest.
     1162            </p>
     1163            <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
     1164               might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication
     1165               an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often
     1166               worse, will permit access to anything else the user protects with the same password.
     1167            </p>
     1168            <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
     1169               password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document,
     1170               and even that may be limited by the server's choice of nonce.
     1171            </p>
     1172         </div>
     1173         <div id="replay.attacks">
     1174            <h2 id="rfc.section.4.5"><a href="#rfc.section.4.5">4.5</a>&nbsp;<a href="#replay.attacks">Replay Attacks</a></h2>
     1175            <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
     1176               already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested
     1177               in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper
     1178               has the user's password, any document protected by that password is open to him.
     1179            </p>
     1180            <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
     1181               ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp,
     1182               the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince
     1183               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
     1184               different from the address to which it believes it is sending the document. An attack can only succeed in the period before
     1185               the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain
     1186               state between transactions.
     1187            </p>
     1188            <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
     1189               not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until
     1190               the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.
     1191            </p>
     1192            <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
     1193               server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int,
     1194               an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even
     1195               with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking
     1196               provides some protection against replay of previously used valid credentials, but see 4.8.
     1197            </p>
     1198         </div>
     1199         <div>
     1200            <h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;Weakness Created by Multiple Authentication Schemes
     1201            </h2>
     1202            <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
     1203               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.
     1204            </p>
     1205            <p id="rfc.section.4.6.p.2"></p>
     1206            <ul class="empty">
     1207               <li>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
     1208                  only include Basic if it is minimally acceptable.
     1209               </li>
     1210            </ul>
     1211            <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
     1212               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.
     1213            </p>
     1214         </div>
     1215         <div>
     1216            <h2 id="rfc.section.4.7"><a href="#rfc.section.4.7">4.7</a>&nbsp;Online dictionary attacks
     1217            </h2>
     1218            <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
     1219               list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password
     1220               on the list is paid once for each challenge.
     1221            </p>
     1222            <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>
     1223         </div>
     1224         <div id="man.in.the.middle">
     1225            <h2 id="rfc.section.4.8"><a href="#rfc.section.4.8">4.8</a>&nbsp;<a href="#man.in.the.middle">Man in the Middle</a></h2>
     1226            <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
     1227               compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities
     1228               to the attacker.
     1229            </p>
     1230            <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
     1231               will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest
     1232               scheme that it understands from the choices offered.
     1233            </p>
     1234            <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
     1235               authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using
     1236               the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy
     1237               caching service to gullible users.
     1238            </p>
     1239            <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
     1240               authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce
     1241               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
     1242               authentication in general, or from specific sites.
     1243            </p>
     1244            <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
     1245               course, this is still much harder than a comparable attack against Basic Authentication.
     1246            </p>
     1247         </div>
     1248         <div>
     1249            <h2 id="rfc.section.4.9"><a href="#rfc.section.4.9">4.9</a>&nbsp;Chosen plaintext attacks
     1250            </h2>
     1251            <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
     1252               the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much
     1253               easier <a href="#ref8" id="rfc.xref.ref8.1"><cite title="Message Authentication with MD5">[8]</cite></a>.
     1254            </p>
     1255            <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>
     1256            <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;
     1257               this allows the client to vary the input to the hash in a way not chosen by the attacker.
     1258            </p>
     1259         </div>
     1260         <div>
     1261            <h2 id="rfc.section.4.10"><a href="#rfc.section.4.10">4.10</a>&nbsp;Precomputed dictionary attacks
     1262            </h2>
     1263            <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
     1264               for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation
     1265               can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding
     1266               to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some
     1267               might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list
     1268               can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes
     1269               of disk storage.
     1270            </p>
     1271            <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>
     1272         </div>
     1273         <div>
     1274            <h2 id="rfc.section.4.11"><a href="#rfc.section.4.11">4.11</a>&nbsp;Batch brute force attacks
     1275            </h2>
     1276            <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
     1277               same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response
     1278               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
     1279               of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even
     1280               a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with
     1281               six or fewer letters in a few hours.
     1282            </p>
     1283            <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>
     1284         </div>
     1285         <div>
     1286            <h2 id="rfc.section.4.12"><a href="#rfc.section.4.12">4.12</a>&nbsp;Spoofing by Counterfeit Servers
     1287            </h2>
     1288            <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
     1289               to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then
     1290               the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult
     1291               with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of
     1292               the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack
     1293               by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of
     1294               each scheme.
     1295            </p>
     1296         </div>
     1297         <div>
     1298            <h2 id="rfc.section.4.13"><a href="#rfc.section.4.13">4.13</a>&nbsp;Storing passwords
     1299            </h2>
     1300            <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
     1301               name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username
     1302               and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.
     1303            </p>
     1304            <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
     1305               documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted
     1306               in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely
     1307               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
     1308               data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically
     1309               compromise others with the same username and password (though it does expose them to brute force attack).
     1310            </p>
     1311            <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
     1312               passwords, because for the purpose of accessing documents in its realm, it effectively does.
     1313            </p>
     1314            <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
     1315               use. In particular a realm string should include the name of the host doing the authentication. The inability of the client
     1316               to authenticate the server is a weakness of Digest Authentication.
     1317            </p>
     1318         </div>
     1319         <div>
     1320            <h2 id="rfc.section.4.14"><a href="#rfc.section.4.14">4.14</a>&nbsp;Summary
     1321            </h2>
     1322            <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
     1323               for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending
     1324               on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect
     1325               the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may
     1326               be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may
     1327               satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited
     1328               lifetime.
     1329            </p>
     1330            <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
     1331               implementation will be far superior to Basic Authentication.
     1332            </p>
     1333         </div>
     1334      </div>
     1335      <div>
     1336         <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Sample implementation
     1337         </h1>
     1338         <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
     1339            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.
     1340         </p>
     1341         <div id="rfc.figure.u.28"></div>
     1342         <p>File "digcalc.h":</p><pre class="text">#define HASHLEN 16
    12751343typedef char HASH[HASHLEN];
    12761344#define HASHHEXLEN 32
     
    13031371    );
    13041372</pre><div id="rfc.figure.u.29"></div>
    1305       <p>File "digcalc.c":</p><pre class="text">#include &lt;global.h&gt;
     1373         <p>File "digcalc.c":</p><pre class="text">#include &lt;global.h&gt;
    13061374#include &lt;md5.h&gt;
    13071375#include &lt;string.h&gt;
     
    14131481};
    14141482</pre><div id="rfc.figure.u.30"></div>
    1415       <p>File "digtest.c":</p><pre class="text">#include &lt;stdio.h&gt;
     1483         <p>File "digtest.c":</p><pre class="text">#include &lt;stdio.h&gt;
    14161484#include "digcalc.h"
    14171485
     
    14381506      printf("Response = %s\n", Response);
    14391507};
    1440 </pre><h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Acknowledgments
    1441       </h1>
    1442       <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>
    1443       <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,
    1444          Ned Freed, and David M. Kristol.
    1445       </p>
    1446       <p id="rfc.section.6.p.3">Jim Gettys and Larry Masinter edited this document for update.</p>
     1508</pre></div>
     1509      <div>
     1510         <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Acknowledgments
     1511         </h1>
     1512         <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>
     1513         <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,
     1514            Ned Freed, and David M. Kristol.
     1515         </p>
     1516         <p id="rfc.section.6.p.3">Jim Gettys and Larry Masinter edited this document for update.</p>
     1517      </div>
    14471518      <h1 id="rfc.references"><a href="#rfc.section.7" id="rfc.section.7">7.</a> References
    14481519      </h1>
    1449       <table> 
     1520      <table>
    14501521         <tr>
    14511522            <td class="reference"><b id="RFC1945">[1]</b></td>
    14521523            <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.</a>, and <a href="mailto:frystyk@w3.org" title="W3 Consortium, MIT Laboratory for Computer Science">H. Nielsen</a>, “<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>”, RFC&nbsp;1945, May&nbsp;1996.
    14531524            </td>
    1454          </tr> 
     1525         </tr>
    14551526         <tr>
    14561527            <td class="reference"><b id="RFC2616">[2]</b></td>
    14571528            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Information and Computer Science">Fielding, R.</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.</a>, <a href="mailto:frystyk@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Nielsen, H.</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.</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.
    14581529            </td>
    1459          </tr> 
     1530         </tr>
    14601531         <tr>
    14611532            <td class="reference"><b id="RFC1321">[3]</b></td>
    14621533            <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.
    14631534            </td>
    1464          </tr> 
     1535         </tr>
    14651536         <tr>
    14661537            <td class="reference"><b id="RFC2045">[4]</b></td>
    14671538            <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. 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.
    14681539            </td>
    1469          </tr> 
     1540         </tr>
    14701541         <tr>
    14711542            <td class="reference"><b id="RFC2246">[5]</b></td>
    14721543            <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.
    14731544            </td>
    1474          </tr> 
     1545         </tr>
    14751546         <tr>
    14761547            <td class="reference"><b id="RFC2069">[6]</b></td>
    14771548            <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.
    14781549            </td>
    1479          </tr> 
     1550         </tr>
    14801551         <tr>
    14811552            <td class="reference"><b id="RFC2396">[7]</b></td>
    14821553            <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.</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.
    14831554            </td>
    1484          </tr> 
     1555         </tr>
    14851556         <tr>
    14861557            <td class="reference"><b id="ref8">[8]</b></td>
    14871558            <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
    14881559            </td>
    1489          </tr> 
     1560         </tr>
    14901561         <tr>
    14911562            <td class="reference"><b id="RFC2195">[9]</b></td>
    14921563            <td class="top"><a href="mailto:klensin@mci.net" title="MCI">Klensin, J.</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.
    14931564            </td>
    1494          </tr> 
     1565         </tr>
    14951566         <tr>
    14961567            <td class="reference"><b id="ref10">[10]</b></td>
    14971568            <td class="top">Morgan, B., Alvestrand, H., Hodges, J., and M. Wahl, “Authentication Methods for LDAP”.<br>Work in progress.
    14981569            </td>
    1499          </tr> 
     1570         </tr>
    15001571      </table>
    15011572      <div class="avoidbreak">
    15021573         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
    1503          <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>
    1504          <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>
    1505          <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>
    1506          <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>
    1507          <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>
    1508          <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>
    1509          <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>
     1574         <p><b>John Franks</b><br>Northwestern University, Department of Mathematics<br>Northwestern University<br>Evanston, IL&nbsp;60208-2730<br>USA<br>EMail: <a href="mailto:john@math.nwu.edu">john@math.nwu.edu</a></p>
     1575         <p><b>Phillip M. Hallam-Baker</b><br>Verisign Inc.<br>301 Edgewater Place<br>Suite 210<br>Wakefield, MA&nbsp;01880<br>USA<br>EMail: <a href="mailto:pbaker@verisign.com">pbaker@verisign.com</a></p>
     1576         <p><b>Jeffery L. Hostetler</b><br>AbiSource, Inc.<br>6 Dunlap Court<br>Savoy, IL&nbsp;61874<br>USA<br>EMail: <a href="mailto:jeff@AbiSource.com">jeff@AbiSource.com</a></p>
     1577         <p><b>Scott D. Lawrence</b><br>Agranat Systems, Inc.<br>5 Clocktower Place<br>Suite 400<br>Maynard, MA&nbsp;01754<br>USA<br>EMail: <a href="mailto:lawrence@agranat.com">lawrence@agranat.com</a></p>
     1578         <p><b>Paul J. Leach</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA&nbsp;98052<br>USA<br>EMail: <a href="mailto:paulle@microsoft.com">paulle@microsoft.com</a></p>
     1579         <p><b>Ari Luotonen</b><br>Netscape Communications Corporation<br>501 East Middlefield Road<br>Mountain View, CA&nbsp;94043<br>USA
     1580         </p>
     1581         <p><b>Lawrence C. Stewart</b><br>Open Market, Inc.<br>215 First Street<br>Cambridge, MA&nbsp;02142<br>USA<br>EMail: <a href="mailto:stewart@OpenMarket.com">stewart@OpenMarket.com</a></p>
    15101582      </div>
    15111583      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
     
    15941666                  <li><em>RFC1321</em>&nbsp;&nbsp;<a href="#RFC1321"><b>7</b></a></li>
    15951667                  <li><em>RFC1945</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1945.1">3.1.1</a>, <a href="#RFC1945"><b>7</b></a></li>
    1596                   <li><em>RFC2045</em>&nbsp;&nbsp;<a href="#RFC2045"><b>7</b></a></li>
     1668                  <li><em>RFC2045</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2045.1">2</a>, <a href="#RFC2045"><b>7</b></a></li>
    15971669                  <li><em>RFC2069</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2069.1">§</a>, <a href="#rfc.xref.RFC2069.2">3.2.1</a>, <a href="#rfc.xref.RFC2069.3">3.2.2</a>, <a href="#RFC2069"><b>7</b></a></li>
    15981670                  <li><em>RFC2195</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2195.1">4.2</a>, <a href="#RFC2195"><b>7</b></a></li>
     
    16331705         </ul>
    16341706      </div>
    1635       <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
    1636       <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
    1637       <p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise
    1638          explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without
    1639          restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative
    1640          works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references
    1641          to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards
    1642          in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to
    1643          translate it into languages other than English.
    1644       </p>
    1645       <p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.</p>
    1646       <p>This document and the information contained herein is provided on an “AS IS” basis and THE INTERNET SOCIETY AND THE INTERNET
    1647          ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
    1648          OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
    1649          PURPOSE.
    1650       </p>
    1651       <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
    1652       <p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed
    1653          to pertain to the implementation or use of the technology described in this document or the extent to which any license under
    1654          such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights.
    1655          Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be
    1656          found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available,
    1657          or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors
    1658          or users of this specification can be obtained from the IETF Secretariat.
    1659       </p>
    1660       <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
    1661          rights which may cover technology that may be required to practice this standard. Please address the information to the IETF
    1662          Executive Director.
    1663       </p>
    1664       <h1>Acknowledgment</h1>
    1665       <p>Funding for the RFC Editor function is currently provided by the Internet Society.</p>
     1707      <div id="rfc.copyright">
     1708         <h1><a href="#rfc.copyright">Full Copyright Statement</a></h1>
     1709         <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
     1710         <p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise
     1711            explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without
     1712            restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative
     1713            works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references
     1714            to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards
     1715            in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to
     1716            translate it into languages other than English.
     1717         </p>
     1718         <p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.</p>
     1719         <p>This document and the information contained herein is provided on an “AS IS” basis and THE INTERNET SOCIETY AND THE INTERNET
     1720            ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
     1721            OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
     1722            PURPOSE.
     1723         </p>
     1724      </div>
     1725      <div id="rfc.ipr">
     1726         <h1><a href="#rfc.ipr">Intellectual Property</a></h1>
     1727         <p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed
     1728            to pertain to the implementation or use of the technology described in this document or the extent to which any license under
     1729            such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights.
     1730            Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be
     1731            found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available,
     1732            or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors
     1733            or users of this specification can be obtained from the IETF Secretariat.
     1734         </p>
     1735         <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
     1736            rights which may cover technology that may be required to practice this standard. Please address the information to the IETF
     1737            Executive Director.
     1738         </p>
     1739      </div>
     1740      <div>
     1741         <h1>Acknowledgment</h1>
     1742         <p>Funding for the RFC Editor function is currently provided by the Internet Society.</p>
     1743      </div>
    16661744   </body>
    16671745</html>
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