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294       content: "RFC 2617";
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297       content: "June 1999";
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300       content: "HTTP Authentication";
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324</style><link rel="Contents" href="#rfc.toc">
325      <link rel="Author" href="#rfc.authors">
326      <link rel="Copyright" href="#rfc.copyright">
327      <link rel="Index" href="#rfc.index">
328      <link rel="Chapter" title="1 Access Authentication" href="#rfc.section.1">
329      <link rel="Chapter" title="2 Basic Authentication Scheme" href="#rfc.section.2">
330      <link rel="Chapter" title="3 Digest Access Authentication Scheme" href="#rfc.section.3">
331      <link rel="Chapter" title="4 Security Considerations" href="#rfc.section.4">
332      <link rel="Chapter" title="5 Sample implementation" href="#rfc.section.5">
333      <link rel="Chapter" title="6 Acknowledgments" href="#rfc.section.6">
334      <link rel="Chapter" href="#rfc.section.7" title="7 References">
335      <link rel="Alternate" title="Authorative ASCII Version" href="">
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338      <link rel="schema.dct" href="">
339      <meta name="dct.creator" content="Franks, J.">
340      <meta name="dct.creator" content="Hallam-Baker, P.M.">
341      <meta name="dct.creator" content="Hostetler, J.L.">
342      <meta name="dct.creator" content="Lawrence, S.D.">
343      <meta name="dct.creator" content="Leach, P.J.">
344      <meta name="dct.creator" content="Luotonen, A.">
345      <meta name="dct.creator" content="Stewart, L.">
346      <meta name="dct.identifier" content="urn:ietf:rfc:2617">
347      <meta name="dct.issued" scheme="ISO8601" content="1999-06">
348      <meta name="dct.replaces" content="urn:ietf:rfc:2069">
349      <meta name="dct.abstract" content="&#34;HTTP/1.0&#34;, includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication (unless used in conjunction with some external secure system such as SSL ), as the user name and password are passed over the network as cleartext. This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme and a scheme based on cryptographic hashes, referred to as &#34;Digest Access Authentication&#34;. It is therefore also intended to serve as a replacement for RFC 2069 . Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication; other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended. Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use.">
350      <meta name="dct.isPartOf" content="urn:issn:2070-1721">
351      <meta name="description" content="&#34;HTTP/1.0&#34;, includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication (unless used in conjunction with some external secure system such as SSL ), as the user name and password are passed over the network as cleartext. This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme and a scheme based on cryptographic hashes, referred to as &#34;Digest Access Authentication&#34;. It is therefore also intended to serve as a replacement for RFC 2069 . Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication; other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended. Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use.">
352   </head>
353   <body>
354      <table class="header">
355         <tbody>
356            <tr>
357               <td class="left">Network Working Group</td>
358               <td class="right">J. Franks</td>
359            </tr>
360            <tr>
361               <td class="left">Request for Comments: 2617</td>
362               <td class="right">Northwestern University, Department of Mathematics</td>
363            </tr>
364            <tr>
365               <td class="left">Obsoletes: <a href="">2069</a></td>
366               <td class="right">P. Hallam-Baker</td>
367            </tr>
368            <tr>
369               <td class="left">Category: Standards Track</td>
370               <td class="right">Verisign Inc.</td>
371            </tr>
372            <tr>
373               <td class="left"></td>
374               <td class="right">J. Hostetler</td>
375            </tr>
376            <tr>
377               <td class="left"></td>
378               <td class="right">AbiSource, Inc.</td>
379            </tr>
380            <tr>
381               <td class="left"></td>
382               <td class="right">S. Lawrence</td>
383            </tr>
384            <tr>
385               <td class="left"></td>
386               <td class="right">Agranat Systems, Inc.</td>
387            </tr>
388            <tr>
389               <td class="left"></td>
390               <td class="right">P. Leach</td>
391            </tr>
392            <tr>
393               <td class="left"></td>
394               <td class="right">Microsoft Corporation</td>
395            </tr>
396            <tr>
397               <td class="left"></td>
398               <td class="right">A. Luotonen</td>
399            </tr>
400            <tr>
401               <td class="left"></td>
402               <td class="right">Netscape Communications Corporation</td>
403            </tr>
404            <tr>
405               <td class="left"></td>
406               <td class="right">L. Stewart</td>
407            </tr>
408            <tr>
409               <td class="left"></td>
410               <td class="right">Open Market, Inc.</td>
411            </tr>
412            <tr>
413               <td class="left"></td>
414               <td class="right">June 1999</td>
415            </tr>
416         </tbody>
417      </table>
418      <p class="title">HTTP Authentication: Basic and Digest Access Authentication</p>
419      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
420      <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
421         for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
422         state and status of this protocol. Distribution of this memo is unlimited.
423      </p>
424      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
425      <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
426      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
427      <p>"HTTP/1.0", includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure
428         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.
429      </p>
430      <p>This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme
431         and a scheme based on cryptographic hashes, referred to as "Digest Access Authentication". It is therefore also intended to
432         serve as a replacement for RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.1"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>. Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication;
433         other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended.
434      </p>
435      <p>Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password);
436         unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness.
437         As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself
438         but in policies and procedures surrounding its use.
439      </p>
440      <hr class="noprint">
441      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
442      <ul class="toc">
443         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Access Authentication</a><ul class="toc">
444               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Reliance on the HTTP/1.1 Specification</a></li>
445               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#access.authentication.framework">Access Authentication Framework</a></li>
446            </ul>
447         </li>
448         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Basic Authentication Scheme</a></li>
449         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Digest Access Authentication Scheme</a><ul class="toc">
450               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Introduction</a><ul class="toc">
451                     <li class="tocline1">3.1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.1">Purpose</a></li>
452                     <li class="tocline1">3.1.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.2">Overall Operation</a></li>
453                     <li class="tocline1">3.1.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.3">Representation of digest values</a></li>
454                     <li class="tocline1">3.1.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.4">Limitations</a></li>
455                  </ul>
456               </li>
457               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a><ul class="toc">
458                     <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></li>
459                     <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a><ul class="toc">
460                           <li class="tocline1">;&nbsp;&nbsp;<a href="#request-digest">Request-Digest</a></li>
461                           <li class="tocline1">;&nbsp;&nbsp;<a href="#A1">A1</a></li>
462                           <li class="tocline1">;&nbsp;&nbsp;<a href="#rfc.section.">A2</a></li>
463                           <li class="tocline1">;&nbsp;&nbsp;<a href="#rfc.section.">Directive values and quoted-string</a></li>
464                           <li class="tocline1">;&nbsp;&nbsp;<a href="#rfc.section.">Various considerations</a></li>
465                        </ul>
466                     </li>
467                     <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.3">The Authentication-Info Header</a></li>
468                  </ul>
469               </li>
470               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#digest.operation">Digest Operation</a></li>
471               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Security Protocol Negotiation</a></li>
472               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers.example">Example</a></li>
473               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></li>
474            </ul>
475         </li>
476         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc">
477               <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Authentication of Clients using Basic Authentication</a></li>
478               <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">Authentication of Clients using Digest Authentication</a></li>
479               <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Limited Use Nonce Values</a></li>
480               <li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.4">Comparison of Digest with Basic Authentication</a></li>
481               <li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#replay.attacks">Replay Attacks</a></li>
482               <li class="tocline1">4.6&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.6">Weakness Created by Multiple Authentication Schemes</a></li>
483               <li class="tocline1">4.7&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.7">Online dictionary attacks</a></li>
484               <li class="tocline1">4.8&nbsp;&nbsp;&nbsp;<a href="">Man in the Middle</a></li>
485               <li class="tocline1">4.9&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.9">Chosen plaintext attacks</a></li>
486               <li class="tocline1">4.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.10">Precomputed dictionary attacks</a></li>
487               <li class="tocline1">4.11&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.11">Batch brute force attacks</a></li>
488               <li class="tocline1">4.12&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.12">Spoofing by Counterfeit Servers</a></li>
489               <li class="tocline1">4.13&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.13">Storing passwords</a></li>
490               <li class="tocline1">4.14&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.14">Summary</a></li>
491            </ul>
492         </li>
493         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Sample implementation</a></li>
494         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Acknowledgments</a></li>
495         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
496         <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
497         <li class="tocline0"><a href="#rfc.index">Index</a></li>
498         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
499      </ul>
500      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Access Authentication
501      </h1>
502      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Reliance on the HTTP/1.1 Specification
503      </h2>
504      <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="" 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.
505      </p>
506      <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>
507      <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,
508         case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs
509         which carry the parameters necessary for achieving authentication via that scheme.
510      </p>
511      <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.a.1"></span>      auth-scheme    = token
512<span id="rfc.iref.a.2"></span>      auth-param     = token "=" ( token | quoted-string )
513</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
514         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.
515      </p>
516      <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</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
518         if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents
519         of a challenge may itself contain a comma-separated list of authentication parameters.
520      </p>
521      <p id="rfc.section.1.2.p.6">The authentication parameter realm is defined for all authentication schemes:</p>
522      <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.r.1"></span>      realm       = "realm" "=" realm-value
523<span id="rfc.iref.r.2"></span>      realm-value = quoted-string
524</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
525         (case-sensitive), in combination with the canonical root URL (the absoluteURI for the server whose abs_path is empty; see
526         section <a href="" 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
527         partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm
528         value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication
529         scheme. Note that there may be multiple challenges with the same auth-scheme but different realms.
530      </p>
531      <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
532         (Unauthorized)--MAY do so by including an Authorization header field with the request. A client that wishes to authenticate
533         itself with a proxy--usually, but not necessarily, after receiving a 407 (Proxy Authentication Required)--MAY do so by including
534         a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization field
535         value consist of credentials containing the authentication information of the client for the realm of the resource being requested.
536         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
537         upon that challenge.
538      </p>
539      <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.c.2"></span>   credentials = auth-scheme #auth-param
540</pre><p id="rfc.section.1.2.p.11"> </p>
541      <ul class="empty">
542         <li>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
543            only include Basic if it is minimally acceptable.
544         </li>
545      </ul>
546      <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
547         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,
548         parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot
549         extend outside the scope of its server.
550      </p>
551      <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.
552         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
553         resource.
554      </p>
555      <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
556         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
557         authentication information. However, these additional mechanisms are not defined by this specification.
558      </p>
559      <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
560         and Authorization headers untouched, and follow the rules found in section <a href="" 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="" 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>).
561      </p>
562      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;Basic Authentication Scheme
563      </h1>
564      <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
565         for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms
566         on that server. The server will service the request only if it can validate the user-ID and password for the protection space
567         of the Request-URI. There are no optional authentication parameters.
568      </p>
569      <p id="rfc.section.2.p.2">For Basic, the framework above is utilized as follows:</p>
570      <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.c.3"></span>      challenge   = "Basic" realm
571<span id="rfc.iref.c.4"></span>      credentials = "Basic" basic-credentials
572</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:
573      </p>
574      <div id="rfc.figure.u.6"></div><pre class="text">      WWW-Authenticate: Basic realm="WallyWorld"
575</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
576         with the same challenge using the Proxy-Authenticate header field.
577      </p>
578      <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
579         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.
580      </p>
581      <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.b.1"></span>      basic-credentials = base64-user-pass
582<span id="rfc.iref.b.2"></span>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
583                       except not limited to 76 char/line&gt;
584<span id="rfc.iref.u.1"></span>      user-pass   = userid ":" password
585<span id="rfc.iref.u.2"></span>      userid      = *&lt;TEXT excluding ":"&gt;
586<span id="rfc.iref.p.1"></span>      password    = *TEXT
587</pre><p id="rfc.section.2.p.9">Userids might be case sensitive.</p>
588      <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>
589      <div id="rfc.figure.u.8"></div><pre class="text">      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
590</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
591         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
592         challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the
593         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.
594      </p>
595      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Digest Access Authentication Scheme
596      </h1>
597      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Introduction
598      </h2>
599      <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;Purpose
600      </h3>
601      <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
602         the network in an unencrypted form. This section provides the specification for a scheme that does not send the password in
603         cleartext, referred to as "Digest Access Authentication".
604      </p>
605      <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
606         Web. This scheme provides no encryption of message content. The intent is simply to create an access authentication method
607         that avoids the most serious flaws of Basic authentication.
608      </p>
609      <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;Overall Operation
610      </h3>
611      <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
612         using a nonce value. A valid response contains a checksum (by default, the MD5 checksum) of the username, the password, the
613         given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with
614         the Basic scheme, the username and password must be prearranged in some fashion not addressed by this document.
615      </p>
616      <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;Representation of digest values
617      </h3>
618      <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
619         is used and that is the only algorithm described in this document.
620      </p>
621      <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
622         the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation
623         as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That
624         is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'.
625      </p>
626      <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;Limitations
627      </h3>
628      <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
629         for Basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same
630         problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between
631         user and server to establish the user's password.
632      </p>
633      <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
634         private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better
635         than Basic authentication.
636      </p>
637      <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>
638      <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
639         header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.
640      </p>
641      <div id="rfc.iref.h.1"></div>
642      <div id="rfc.iref.w.1"></div>
643      <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>
644      <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
645         responds with a "401 Unauthorized" status code, and a WWW-Authenticate header as per the framework defined above, which for
646         the digest scheme is utilized as follows:
647      </p>
648      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.c.5"></span>      challenge        =  "Digest" digest-challenge
650<span id="rfc.iref.d.1"></span>      digest-challenge  = 1#( realm | [ domain ] | nonce |
651                          [ opaque ] |[ stale ] | [ algorithm ] |
652                          [ qop-options ] | [auth-param] )
655<span id="rfc.iref.d.2"></span>      domain            = "domain" "=" &lt;"&gt; URI ( 1*SP URI ) &lt;"&gt;
656<span id="rfc.iref.u.3"></span>      URI               = absoluteURI | abs_path
657<span id="rfc.iref.n.1"></span>      nonce             = "nonce" "=" nonce-value
658<span id="rfc.iref.n.2"></span>      nonce-value       = quoted-string
659<span id="rfc.iref.o.1"></span>      opaque            = "opaque" "=" quoted-string
660<span id="rfc.iref.s.1"></span>      stale             = "stale" "=" ( "true" | "false" )
661<span id="rfc.iref.a.3"></span>      algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
662                           token )
663<span id="rfc.iref.q.1"></span>      qop-options       = "qop" "=" &lt;"&gt; 1#qop-value &lt;"&gt;
664<span id="rfc.iref.q.2"></span>      qop-value         = "auth" | "auth-int" | token
665</pre><p id="rfc.section.3.2.1.p.3">The meanings of the values of the directives used above are as follows:</p>
666      <p id="rfc.section.3.2.1.p.4">realm </p>
667      <ul class="empty">
668         <li>A string to be displayed to users so they know which username and password to use. This string should contain at least the
669            name of the host performing the authentication and might additionally indicate the collection of users who might have access.
670            An example might be "".
671         </li>
672      </ul>
673      <p id="rfc.section.3.2.1.p.5">domain </p>
674      <ul class="empty">
675         <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.
676            The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI
677            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.
678            If this directive is omitted or its value is empty, the client should assume that the protection space consists of all URIs
679            on the responding server. This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is
680            always the entire proxy; if present it should be ignored.
681         </li>
682      </ul>
683      <p id="rfc.section.3.2.1.p.6">nonce </p>
684      <ul class="empty">
685         <li>A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that
686            this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string,
687            the double-quote character is not allowed.
688         </li>
689         <li>The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce
690            might, for example, be constructed as the base 64 encoding of
691         </li>
692         <li>
693            <div id="rfc.figure.u.10"></div><pre class="text">         time-stamp H(time-stamp ":" ETag ":" private-key)
694  </pre></li>
695         <li>where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated
696            with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate
697            the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from
698            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.
699            The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address
700            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
701            that originally got it. However, that would break proxy farms, where requests from a single user often go through different
702            proxies in the farm. Also, IP address spoofing is not that hard.)
703         </li>
704         <li>An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against
705            a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp
706            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.
707         </li>
708         <li>The nonce is opaque to the client.</li>
709      </ul>
710      <p id="rfc.section.3.2.1.p.7">opaque </p>
711      <ul class="empty">
712         <li>A string of data, specified by the server, which should be returned by the client unchanged in the Authorization header of
713            subsequent requests with URIs in the same protection space. It is recommended that this string be base64 or hexadecimal data.
714         </li>
715      </ul>
716      <p id="rfc.section.3.2.1.p.8">stale </p>
717      <ul class="empty">
718         <li>A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE
719            (case-insensitive), the client may wish to simply retry the request with a new encrypted response, without reprompting the
720            user for a new username and password. The server should only set stale to TRUE if it receives a request for which the nonce
721            is invalid but with a valid digest for that nonce (indicating that the client knows the correct username/password). If stale
722            is FALSE, or anything other than TRUE, or the stale directive is not present, the username and/or password are invalid, and
723            new values must be obtained.
724         </li>
725      </ul>
726      <p id="rfc.section.3.2.1.p.9">algorithm </p>
727      <ul class="empty">
728         <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
729            be "MD5". If the algorithm is not understood, the challenge should be ignored (and a different one used, if there is more
730            than one).
731         </li>
732         <li>In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted
733            by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data).
734            The notation unq(X) means the value of the quoted-string X without the surrounding quotes.
735         </li>
736         <li>For the "MD5" and "MD5-sess" algorithms</li>
737         <li>
738            <div id="rfc.figure.u.11"></div><pre class="text">         H(data) = MD5(data)
739    </pre></li>
740         <li>and</li>
741         <li>
742            <div id="rfc.figure.u.12"></div><pre class="text">         KD(secret, data) = H(concat(secret, ":", data))
743    </pre></li>
744         <li>i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The "MD5-sess" algorithm is
745            intended to allow efficient 3rd party authentication servers; for the difference in usage, see the description in <a href="#A1" title="A1">Section&nbsp;</a>.
746         </li>
747      </ul>
748      <p id="rfc.section.3.2.1.p.10">qop-options </p>
749      <ul class="empty">
750         <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
751            or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication;
752            the value "auth-int" indicates authentication with integrity protection; see the descriptions below for calculating the response
753            directive value for the application of this choice. Unrecognized options <em class="bcp14">MUST</em> be ignored.
754         </li>
755      </ul>
756      <p id="rfc.section.3.2.1.p.11">auth-param </p>
757      <ul class="empty">
758         <li>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
759         </li>
760      </ul>
761      <div id="rfc.iref.h.2"></div>
762      <div id="rfc.iref.a.4"></div>
763      <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>
764      <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
765         above, utilized as follows.
766      </p>
767      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.c.6"></span>       credentials      = "Digest" digest-response
768<span id="rfc.iref.d.3"></span>       digest-response  = 1#( username | realm | nonce | digest-uri
769                       | response | [ algorithm ] | [cnonce] |
770                       [opaque] | [message-qop] |
771                           [nonce-count]  | [auth-param] )
773<span id="rfc.iref.u.4"></span>       username         = "username" "=" username-value
774<span id="rfc.iref.u.5"></span>       username-value   = quoted-string
775<span id="rfc.iref.d.4"></span>       digest-uri       = "uri" "=" digest-uri-value
776<span id="rfc.iref.d.5"></span>       digest-uri-value = request-uri   ; As specified by HTTP/1.1
777<span id="rfc.iref.m.1"></span>       message-qop      = "qop" "=" qop-value
778<span id="rfc.iref.c.7"></span>       cnonce           = "cnonce" "=" cnonce-value
779<span id="rfc.iref.c.8"></span>       cnonce-value     = nonce-value
780<span id="rfc.iref.n.3"></span>       nonce-count      = "nc" "=" nc-value
781<span id="rfc.iref.n.4"></span>       nc-value         = 8LHEX
782<span id="rfc.iref.r.3"></span>       response         = "response" "=" request-digest
783<span id="rfc.iref.r.4"></span>       request-digest = &lt;"&gt; 32LHEX &lt;"&gt;
784<span id="rfc.iref.l.1"></span>       LHEX             =  "0" | "1" | "2" | "3" |
785                           "4" | "5" | "6" | "7" |
786                           "8" | "9" | "a" | "b" |
787                           "c" | "d" | "e" | "f"
788</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
789         being requested.
790      </p>
791      <p id="rfc.section.3.2.2.p.4">response </p>
792      <ul class="empty">
793         <li>A string of 32 hex digits computed as defined below, which proves that the user knows a password</li>
794      </ul>
795      <p id="rfc.section.3.2.2.p.5">username </p>
796      <ul class="empty">
797         <li>The user's name in the specified realm.</li>
798      </ul>
799      <p id="rfc.section.3.2.2.p.6">digest-uri </p>
800      <ul class="empty">
801         <li>The URI from Request-URI of the Request-Line; duplicated here because proxies are allowed to change the Request-Line in transit.</li>
802      </ul>
803      <p id="rfc.section.3.2.2.p.7">qop </p>
804      <ul class="empty">
805         <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
806            of the request-digest. Note that this is a single token, not a quoted list of alternatives as in WWW-Authenticate. This directive
807            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.
808         </li>
809      </ul>
810      <p id="rfc.section.3.2.2.p.8">cnonce </p>
811      <ul class="empty">
812         <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
813            quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide
814            mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of
815            the response-digest and request-digest values.
816         </li>
817      </ul>
818      <p id="rfc.section.3.2.2.p.9">nonce-count </p>
819      <ul class="empty">
820         <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
821            count of the number of requests (including the current request) that the client has sent with the nonce value in this request.
822            For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of
823            this directive is to allow the server to detect request replays by maintaining its own copy of this count - if the same nc-value
824            is seen twice, then the request is a replay. See the description below of the construction of the request-digest value.
825         </li>
826      </ul>
827      <p id="rfc.section.3.2.2.p.10">auth-param </p>
828      <ul class="empty">
829         <li>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
830         </li>
831      </ul>
832      <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
833         request-digest is invalid, then a login failure should be logged, since repeated login failures from a single client may indicate
834         an attacker attempting to guess passwords.
835      </p>
836      <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
837         is computed.
838      </p>
839      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="request-digest" href="#request-digest">Request-Digest</a></h4>
840      <p id="rfc.section.">If the "qop" value is "auth" or "auth-int":</p>
841      <div id="rfc.figure.u.14"></div><pre class="inline">      request-digest  = &lt;"&gt; &lt; KD ( H(A1),     unq(nonce-value)
842                                          ":" nc-value
843                                          ":" unq(cnonce-value)
844                                          ":" unq(qop-value)
845                                          ":" H(A2)
846                                  ) &lt;"&gt;
847</pre><p id="rfc.section.">If the "qop" directive is not present (this construction is for compatibility with RFC 2069):</p>
848      <div id="rfc.figure.u.15"></div><pre class="inline">      request-digest  =
849                 &lt;"&gt; &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) &gt;
850   &lt;"&gt;
851</pre><p id="rfc.section.">See below for the definitions for A1 and A2.</p>
852      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;<a id="A1" href="#A1">A1</a></h4>
853      <p id="rfc.section.">If the "algorithm" directive's value is "MD5" or is unspecified, then A1 is:</p>
854      <div id="rfc.figure.u.16"></div><pre class="inline">      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
855</pre><p id="rfc.section.">where</p>
856      <div id="rfc.figure.u.17"></div><pre class="inline">      passwd   = &lt; user's password &gt;
857</pre><p id="rfc.section.">If the "algorithm" directive's value is "MD5-sess", then A1 is calculated only once - on the first request by the client following
858         receipt of a WWW-Authenticate challenge from the server. It uses the server nonce from that challenge, and the first client
859         nonce value to construct A1 as follows:
860      </p>
861      <div id="rfc.figure.u.18"></div><pre class="inline">      A1       = H( unq(username-value) ":" unq(realm-value)
862                     ":" passwd )
863                     ":" unq(nonce-value) ":" unq(cnonce-value)
864</pre><p id="rfc.section.">This creates a 'session key' for the authentication of subsequent requests and responses which is different for each "authentication
865         session", thus limiting the amount of material hashed with any one key. (Note: see further discussion of the authentication
866         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
867         be used in conjunction with a third party authentication service so that the web server would not need the actual password
868         value. The specification of such a protocol is beyond the scope of this specification.
869      </p>
870      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;A2
871      </h4>
872      <p id="rfc.section.">If the "qop" directive's value is "auth" or is unspecified, then A2 is:</p>
873      <div id="rfc.figure.u.19"></div><pre class="inline">      A2       = Method ":" digest-uri-value
874</pre><p id="rfc.section.">If the "qop" value is "auth-int", then A2 is:</p>
875      <div id="rfc.figure.u.20"></div><pre class="inline">      A2       = Method ":" digest-uri-value ":" H(entity-body)
876</pre><h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Directive values and quoted-string
877      </h4>
878      <p id="rfc.section.">Note that the value of many of the directives, such as "username-value", are defined as a "quoted-string". However, the "unq"
879         notation indicates that surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header
880         includes the fields
881      </p>
882      <div id="rfc.figure.u.21"></div><pre class="text">     username="Mufasa",
883</pre><p id="rfc.section.">and the user Mufasa has password "Circle Of Life" then H(A1) would be H( Of Life) with no
884         quotation marks in the digested string.
885      </p>
886      <p id="rfc.section.">No white space is allowed in any of the strings to which the digest function H() is applied unless that white space exists
887         in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated
888         above must be
889      </p>
890      <div id="rfc.figure.u.22"></div><pre class="text"> Of Life
891</pre><p id="rfc.section.">with no white space on either side of the colons, but with the white space between the words used in the password value. Likewise,
892         the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless
893         that white space was in the quoted strings or entity body being digested.
894      </p>
895      <p id="rfc.section.">Also note that if integrity protection is applied (qop=auth-int), the H(entity-body) is the hash of the entity body, not the
896         message body - it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient.
897         Note that this includes multipart boundaries and embedded headers in each part of any multipart content-type.
898      </p>
899      <h4 id="rfc.section."><a href="#rfc.section."></a>&nbsp;Various considerations
900      </h4>
901      <p id="rfc.section.">The "Method" value is the HTTP request method as specified in section <a href="" 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="" 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="" 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
902         is to foil chosen plaintext attacks.
903      </p>
904      <p id="rfc.section.">The authenticating server must assure that the resource designated by the "uri" directive is the same as the resource specified
905         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
906         such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that
907         an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would
908         not result in the same digest as that calculated by the client.
909      </p>
910      <p id="rfc.section.">Implementers should be aware of how authenticated transactions interact with shared caches. The HTTP/1.1 protocol specifies
911         that when a shared cache (see section <a href="" 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="" 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,
912         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
913         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.
914      </p>
915      <div id="rfc.iref.h.3"></div>
916      <div id="rfc.iref.a.5"></div>
917      <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;The Authentication-Info Header
918      </h3>
919      <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
920         in the response.
921      </p>
922      <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.a.6"></span>        AuthenticationInfo = "Authentication-Info" ":" auth-info
923<span id="rfc.iref.a.7"></span>        auth-info          = 1#(nextnonce | [ message-qop ]
924                               | [ response-auth ] | [ cnonce ]
925                               | [nonce-count] )
926<span id="rfc.iref.n.5"></span>        nextnonce          = "nextnonce" "=" nonce-value
927<span id="rfc.iref.r.5"></span>        response-auth      = "rspauth" "=" response-digest
928<span id="rfc.iref.r.6"></span>        response-digest    = &lt;"&gt; *LHEX &lt;"&gt;
929</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.
930         The server may send the Authentication-Info header with a nextnonce field as a means of implementing one-time or otherwise
931         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
932         to re-authenticate from the server with the "stale=TRUE".
933      </p>
934      <p id="rfc.section.3.2.3.p.4"> </p>
935      <ul class="empty">
936         <li>Server implementations should carefully consider the performance implications of the use of this mechanism; pipelined requests
937            will not be possible if every response includes a nextnonce directive that must be used on the next request received by the
938            server. Consideration should be given to the performance vs. security tradeoffs of allowing an old nonce value to be used
939            for a limited time to permit request pipelining. Use of the nonce-count can retain most of the security advantages of a new
940            server nonce without the deleterious affects on pipelining.
941         </li>
942      </ul>
943      <p id="rfc.section.3.2.3.p.5">message-qop</p>
944      <ul class="empty">
945         <li>Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication;
946            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.
947         </li>
948      </ul>
949      <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
950         knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The "response-digest"
951         value is calculated as for the "request-digest" in the Authorization header, except that if "qop=auth" or is not specified
952         in the Authorization header for the request, A2 is
953      </p>
954      <div id="rfc.figure.u.24"></div><pre class="inline">      A2       = ":" digest-uri-value
955</pre><p id="rfc.section.3.2.3.p.9">and if "qop=auth-int", then A2 is</p>
956      <div id="rfc.figure.u.25"></div><pre class="inline">      A2       = ":" digest-uri-value ":" H(entity-body)
957</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"
958         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"
959         directives <em class="bcp14">MUST</em> BE present if "qop=auth" or "qop=auth-int" is specified.
960      </p>
961      <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>
962      <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>
963      <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
964         the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare
965         the result to the given request-digest value.
966      </p>
967      <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
968         server, the validity of an Authorization header may be verified.
969      </p>
970      <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
971         space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the
972         protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an
973         authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization
974         header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges.
975         The server may choose to accept the old Authorization header information, even though the nonce value included might not be
976         fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request;
977         by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting
978         for a new username and password.
979      </p>
980      <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
981         session, the opaque data may be used to transport authentication session state information. (Note that any such use can also
982         be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for
983         authenticating content that actually sits on another server. It would achieve this by having the first 401 response include
984         a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state
985         information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing
986         to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the
987         &lt;opaque&gt; data.
988      </p>
989      <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
990         must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate
991         a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization
992         headers described in <a href="#proxy-authentication.and.proxy-authorization" title="Proxy-Authentication and Proxy-Authorization">Section&nbsp;3.6</a> below.
993      </p>
994      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Security Protocol Negotiation
995      </h2>
996      <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>
997      <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
998         the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot
999         handle.
1000      </p>
1001      <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>
1002      <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
1003         URI of the document is "". Both client and server know that the username for this document
1004         is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).
1005      </p>
1006      <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>
1007      <div id="rfc.figure.u.26"></div><pre class="text">         HTTP/1.1 401 Unauthorized
1008         WWW-Authenticate: Digest
1009                 realm="",
1010                 qop="auth,auth-int",
1011                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1012                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1013</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
1014         following Authorization header:
1015      </p>
1016      <div id="rfc.figure.u.27"></div><pre class="text">         Authorization: Digest username="Mufasa",
1017                 realm="",
1018                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1019                 uri="/dir/index.html",
1020                 qop=auth,
1021                 nc=00000001,
1022                 cnonce="0a4f113b",
1023                 response="6629fae49393a05397450978507c4ef1",
1024                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1025</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>
1026      <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
1027         servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate
1028         and Proxy-Authorization headers specified in sections <a href="" id="rfc.xref.RFC2616.19">10.33</a> and <a href="" 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
1029         to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy
1030         Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate
1031         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>.
1032      </p>
1033      <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
1034         header in <a href="#the.authorization.request.header" title="The Authorization Request Header">Section&nbsp;3.2.2</a> above.
1035      </p>
1036      <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
1037         header field.
1038      </p>
1039      <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
1040         same response.
1041      </p>
1042      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1043      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Authentication of Clients using Basic Authentication
1044      </h2>
1045      <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,
1046         which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication
1047         schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes
1048         to use one-time passwords) to Basic authentication.
1049      </p>
1050      <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
1051         over the physical network. It is this problem which Digest Authentication attempts to address.
1052      </p>
1053      <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.
1054      </p>
1055      <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
1056         as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this
1057         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
1058         concern. This is only correct if the server issues both user name and password to the users and in particular does not allow
1059         the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid
1060         the task of maintaining multiple passwords.
1061      </p>
1062      <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
1063         server but also unauthorized access to any other resources on other systems that the user protects with the same password.
1064         Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner
1065         or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all
1066         those sites if this information is not maintained in a secure fashion.
1067      </p>
1068      <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
1069         to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or
1070         gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not
1071         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
1072         for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism
1073         to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable
1074         by the client.
1075      </p>
1076      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;Authentication of Clients using Digest Authentication
1077      </h2>
1078      <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
1079         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.
1080      </p>
1081      <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
1082         and response are available to an eavesdropper.
1083      </p>
1084      <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
1085         is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response
1086         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.
1087      </p>
1088      <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
1089         protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless
1090         many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses
1091         Basic should be switched to Digest as soon as practical.
1092      </p>
1093      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Limited Use Nonce Values
1094      </h2>
1095      <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;</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,
1096         for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against,
1097         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
1098         implications. For example, a server may choose to allow each nonce value to be used only once by maintaining a record of whether
1099         or not each recently issued nonce has been returned and sending a next-nonce directive in the Authentication-Info header field
1100         of every response. This protects against even an immediate replay attack, but has a high cost checking nonce values, and perhaps
1101         more important will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication).
1102         Similarly, incorporating a request-specific element such as the Etag value for a resource limits the use of the nonce to that
1103         version of the resource and also defeats pipelining. Thus it may be useful to do so for methods with side effects but have
1104         unacceptable performance for those that do not.
1105      </p>
1106      <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;Comparison of Digest with Basic Authentication
1107      </h2>
1108      <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
1109         the two points out the utility, even necessity, of replacing Basic by Digest.
1110      </p>
1111      <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
1112         might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication
1113         an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often
1114         worse, will permit access to anything else the user protects with the same password.
1115      </p>
1116      <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
1117         password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document,
1118         and even that may be limited by the server's choice of nonce.
1119      </p>
1120      <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>
1121      <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
1122         already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested
1123         in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper
1124         has the user's password, any document protected by that password is open to him.
1125      </p>
1126      <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
1127         ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp,
1128         the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince
1129         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
1130         different from the address to which it believes it is sending the document. An attack can only succeed in the period before
1131         the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain
1132         state between transactions.
1133      </p>
1134      <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
1135         not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until
1136         the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.
1137      </p>
1138      <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
1139         server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int,
1140         an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even
1141         with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking
1142         provides some protection against replay of previously used valid credentials, but see 4.8.
1143      </p>
1144      <h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;Weakness Created by Multiple Authentication Schemes
1145      </h2>
1146      <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
1147         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.
1148      </p>
1149      <p id="rfc.section.4.6.p.2"> </p>
1150      <ul class="empty">
1151         <li>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
1152            only include Basic if it is minimally acceptable.
1153         </li>
1154      </ul>
1155      <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
1156         authentication is only as good as that of the of the weakest of the authentication schemes. See <a href="" title="Man in the Middle">Section&nbsp;4.8</a> below for discussion of particular attack scenarios that exploit multiple authentication schemes.
1157      </p>
1158      <h2 id="rfc.section.4.7"><a href="#rfc.section.4.7">4.7</a>&nbsp;Online dictionary attacks
1159      </h2>
1160      <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
1161         list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password
1162         on the list is paid once for each challenge.
1163      </p>
1164      <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>
1165      <h2 id="rfc.section.4.8"><a href="#rfc.section.4.8">4.8</a>&nbsp;<a id="" href="">Man in the Middle</a></h2>
1166      <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
1167         compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities
1168         to the attacker.
1169      </p>
1170      <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
1171         will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest
1172         scheme that it understands from the choices offered.
1173      </p>
1174      <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
1175         authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using
1176         the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy
1177         caching service to gullible users.
1178      </p>
1179      <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
1180         authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce
1181         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
1182         authentication in general, or from specific sites.
1183      </p>
1184      <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
1185         course, this is still much harder than a comparable attack against Basic Authentication.
1186      </p>
1187      <h2 id="rfc.section.4.9"><a href="#rfc.section.4.9">4.9</a>&nbsp;Chosen plaintext attacks
1188      </h2>
1189      <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
1190         the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much
1191         easier <a href="#ref8" id="rfc.xref.ref8.1"><cite title="Message Authentication with MD5">[8]</cite></a>.
1192      </p>
1193      <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>
1194      <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;
1195         this allows the client to vary the input to the hash in a way not chosen by the attacker.
1196      </p>
1197      <h2 id="rfc.section.4.10"><a href="#rfc.section.4.10">4.10</a>&nbsp;Precomputed dictionary attacks
1198      </h2>
1199      <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
1200         for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation
1201         can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding
1202         to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some
1203         might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list
1204         can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes
1205         of disk storage.
1206      </p>
1207      <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>
1208      <h2 id="rfc.section.4.11"><a href="#rfc.section.4.11">4.11</a>&nbsp;Batch brute force attacks
1209      </h2>
1210      <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
1211         same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response
1212         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
1213         of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even
1214         a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with
1215         six or fewer letters in a few hours.
1216      </p>
1217      <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>
1218      <h2 id="rfc.section.4.12"><a href="#rfc.section.4.12">4.12</a>&nbsp;Spoofing by Counterfeit Servers
1219      </h2>
1220      <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
1221         to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then
1222         the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult
1223         with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of
1224         the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack
1225         by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of
1226         each scheme.
1227      </p>
1228      <h2 id="rfc.section.4.13"><a href="#rfc.section.4.13">4.13</a>&nbsp;Storing passwords
1229      </h2>
1230      <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
1231         name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username
1232         and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.
1233      </p>
1234      <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
1235         documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted
1236         in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely
1237         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
1238         data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically
1239         compromise others with the same username and password (though it does expose them to brute force attack).
1240      </p>
1241      <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
1242         passwords, because for the purpose of accessing documents in its realm, it effectively does.
1243      </p>
1244      <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
1245         use. In particular a realm string should include the name of the host doing the authentication. The inability of the client
1246         to authenticate the server is a weakness of Digest Authentication.
1247      </p>
1248      <h2 id="rfc.section.4.14"><a href="#rfc.section.4.14">4.14</a>&nbsp;Summary
1249      </h2>
1250      <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
1251         for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending
1252         on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect
1253         the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may
1254         be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may
1255         satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited
1256         lifetime.
1257      </p>
1258      <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
1259         implementation will be far superior to Basic Authentication.
1260      </p>
1261      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Sample implementation
1262      </h1>
1263      <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
1264         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.
1265      </p>
1266      <div id="rfc.figure.u.28"></div>
1267      <p>File "digcalc.h":</p><pre class="text">#define HASHLEN 16
1268typedef char HASH[HASHLEN];
1269#define HASHHEXLEN 32
1270typedef char HASHHEX[HASHHEXLEN+1];
1271#define IN
1272#define OUT
1274/* calculate H(A1) as per HTTP Digest spec */
1275void DigestCalcHA1(
1276    IN char * pszAlg,
1277    IN char * pszUserName,
1278    IN char * pszRealm,
1279    IN char * pszPassword,
1280    IN char * pszNonce,
1281    IN char * pszCNonce,
1282    OUT HASHHEX SessionKey
1283    );
1285/* calculate request-digest/response-digest as per HTTP Digest spec */
1286void DigestCalcResponse(
1287    IN HASHHEX HA1,           /* H(A1) */
1288    IN char * pszNonce,       /* nonce from server */
1289    IN char * pszNonceCount,  /* 8 hex digits */
1290    IN char * pszCNonce,      /* client nonce */
1291    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1292    IN char * pszMethod,      /* method from the request */
1293    IN char * pszDigestUri,   /* requested URL */
1294    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1295    OUT HASHHEX Response      /* request-digest or response-digest */
1296    );
1297</pre><div id="rfc.figure.u.29"></div>
1298      <p>File "digcalc.c":</p><pre class="text">#include &lt;global.h&gt;
1299#include &lt;md5.h&gt;
1300#include &lt;string.h&gt;
1301#include "digcalc.h"
1303void CvtHex(
1304    IN HASH Bin,
1305    OUT HASHHEX Hex
1306    )
1308    unsigned short i;
1309    unsigned char j;
1311    for (i = 0; i &lt; HASHLEN; i++) {
1312        j = (Bin[i] &gt;&gt; 4) &amp; 0xf;
1313        if (j &lt;= 9)
1314            Hex[i*2] = (j + '0');
1315         else
1316            Hex[i*2] = (j + 'a' - 10);
1317        j = Bin[i] &amp; 0xf;
1318        if (j &lt;= 9)
1319            Hex[i*2+1] = (j + '0');
1320         else
1321            Hex[i*2+1] = (j + 'a' - 10);
1322    };
1323    Hex[HASHHEXLEN] = '\0';
1326/* calculate H(A1) as per spec */
1327void DigestCalcHA1(
1328    IN char * pszAlg,
1329    IN char * pszUserName,
1330    IN char * pszRealm,
1331    IN char * pszPassword,
1332    IN char * pszNonce,
1333    IN char * pszCNonce,
1334    OUT HASHHEX SessionKey
1335    )
1337      MD5_CTX Md5Ctx;
1338      HASH HA1;
1340      MD5Init(&amp;Md5Ctx);
1341      MD5Update(&amp;Md5Ctx, pszUserName, strlen(pszUserName));
1342      MD5Update(&amp;Md5Ctx, ":", 1);
1343      MD5Update(&amp;Md5Ctx, pszRealm, strlen(pszRealm));
1344      MD5Update(&amp;Md5Ctx, ":", 1);
1345      MD5Update(&amp;Md5Ctx, pszPassword, strlen(pszPassword));
1346      MD5Final(HA1, &amp;Md5Ctx);
1347      if (stricmp(pszAlg, "md5-sess") == 0) {
1348            MD5Init(&amp;Md5Ctx);
1349            MD5Update(&amp;Md5Ctx, HA1, HASHLEN);
1350            MD5Update(&amp;Md5Ctx, ":", 1);
1351            MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1352            MD5Update(&amp;Md5Ctx, ":", 1);
1353            MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1354            MD5Final(HA1, &amp;Md5Ctx);
1355      };
1356      CvtHex(HA1, SessionKey);
1359/* calculate request-digest/response-digest as per HTTP Digest spec */
1360void DigestCalcResponse(
1361    IN HASHHEX HA1,           /* H(A1) */
1362    IN char * pszNonce,       /* nonce from server */
1363    IN char * pszNonceCount,  /* 8 hex digits */
1364    IN char * pszCNonce,      /* client nonce */
1365    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1366    IN char * pszMethod,      /* method from the request */
1367    IN char * pszDigestUri,   /* requested URL */
1368    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1369    OUT HASHHEX Response      /* request-digest or response-digest */
1370    )
1372      MD5_CTX Md5Ctx;
1373      HASH HA2;
1374      HASH RespHash;
1375       HASHHEX HA2Hex;
1377      // calculate H(A2)
1378      MD5Init(&amp;Md5Ctx);
1379      MD5Update(&amp;Md5Ctx, pszMethod, strlen(pszMethod));
1380      MD5Update(&amp;Md5Ctx, ":", 1);
1381      MD5Update(&amp;Md5Ctx, pszDigestUri, strlen(pszDigestUri));
1382      if (stricmp(pszQop, "auth-int") == 0) {
1383            MD5Update(&amp;Md5Ctx, ":", 1);
1384            MD5Update(&amp;Md5Ctx, HEntity, HASHHEXLEN);
1385      };
1386      MD5Final(HA2, &amp;Md5Ctx);
1387       CvtHex(HA2, HA2Hex);
1389      // calculate response
1390      MD5Init(&amp;Md5Ctx);
1391      MD5Update(&amp;Md5Ctx, HA1, HASHHEXLEN);
1392      MD5Update(&amp;Md5Ctx, ":", 1);
1393      MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1394      MD5Update(&amp;Md5Ctx, ":", 1);
1395      if (*pszQop) {
1396          MD5Update(&amp;Md5Ctx, pszNonceCount, strlen(pszNonceCount));
1397          MD5Update(&amp;Md5Ctx, ":", 1);
1398          MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1399          MD5Update(&amp;Md5Ctx, ":", 1);
1400          MD5Update(&amp;Md5Ctx, pszQop, strlen(pszQop));
1401          MD5Update(&amp;Md5Ctx, ":", 1);
1402      };
1403      MD5Update(&amp;Md5Ctx, HA2Hex, HASHHEXLEN);
1404      MD5Final(RespHash, &amp;Md5Ctx);
1405      CvtHex(RespHash, Response);
1407</pre><div id="rfc.figure.u.30"></div>
1408      <p>File "digtest.c":</p><pre class="text">#include &lt;stdio.h&gt;
1409#include "digcalc.h"
1411void main(int argc, char ** argv) {
1413      char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
1414      char * pszCNonce = "0a4f113b";
1415      char * pszUser = "Mufasa";
1416      char * pszRealm = "";
1417      char * pszPass = "Circle Of Life";
1418      char * pszAlg = "md5";
1419      char szNonceCount[9] = "00000001";
1420      char * pszMethod = "GET";
1421      char * pszQop = "auth";
1422      char * pszURI = "/dir/index.html";
1423      HASHHEX HA1;
1424      HASHHEX HA2 = "";
1425      HASHHEX Response;
1427      DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass, pszNonce,
1428pszCNonce, HA1);
1429      DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
1430       pszMethod, pszURI, HA2, Response);
1431      printf("Response = %s\n", Response);
1433</pre><h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Acknowledgments
1434      </h1>
1435      <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>
1436      <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,
1437         Ned Freed, and David M. Kristol.
1438      </p>
1439      <p id="rfc.section.6.p.3">Jim Gettys and Larry Masinter edited this document for update.</p>
1440      <h1 id="rfc.references"><a href="#rfc.section.7" id="rfc.section.7">7.</a> References
1441      </h1>
1442      <table>
1443         <tr>
1444            <td class="reference"><b id="RFC1945">[1]</b></td>
1445            <td class="top"><a href="" title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a href="" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.</a>, and <a href="" title="W3 Consortium, MIT Laboratory for Computer Science">H. Nielsen</a>, “<a href="">Hypertext Transfer Protocol -- HTTP/1.0</a>”, RFC&nbsp;1945, May&nbsp;1996.
1446            </td>
1447         </tr> 
1448         <tr>
1449            <td class="reference"><b id="RFC2616">[2]</b></td>
1450            <td class="top"><a href="" title="University of California, Irvine, Information and Computer Science">Fielding, R.</a>, <a href="" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="" title="Compaq Computer Corporation, Western Research Laboratory">Mogul, J.</a>, <a href="" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Nielsen, H.</a>, <a href="" title="Xerox Corporation">Masinter, L.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, and <a href="" title="World Wide Web Consortium, MIT Laboratory for Computer Science">T. Berners-Lee</a>, “<a href="">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1451            </td>
1452         </tr> 
1453         <tr>
1454            <td class="reference"><b id="RFC1321">[3]</b></td>
1455            <td class="top"><a href="" title="Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science">Rivest, R.</a>, “<a href="">The MD5 Message-Digest Algorithm</a>”, RFC&nbsp;1321, April&nbsp;1992.
1456            </td>
1457         </tr> 
1458         <tr>
1459            <td class="reference"><b id="RFC2045">[4]</b></td>
1460            <td class="top"><a href="" title="Innosoft International, Inc.">Freed, N.</a> and <a href="" title="First Virtual Holdings">N. Borenstein</a>, “<a href="">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</a>”, RFC&nbsp;2045, November&nbsp;1996.
1461            </td>
1462         </tr> 
1463         <tr>
1464            <td class="reference"><b id="RFC2246">[5]</b></td>
1465            <td class="top"><a href="" title="Certicom">Dierks, T.</a> and <a href="" title="Certicom">C. Allen</a>, “<a href="">The TLS Protocol Version 1.0</a>”, RFC&nbsp;2246, January&nbsp;1999.
1466            </td>
1467         </tr> 
1468         <tr>
1469            <td class="reference"><b id="RFC2069">[6]</b></td>
1470            <td class="top"><a href="" title="Northwestern University,  Department of Mathematics">Franks, J.</a>, <a href="" title="CERN">Hallam-Baker, P.</a>, <a href="" title="Spyglass, Inc.">Hostetler, J.</a>, <a href="" title="Microsoft Corporation">Leach, P.</a>, <a href="" title="Netscape Communications Corporation">Luotonen, A.</a>, <a href="" title="Spyglass, Inc.">Sink, E.</a>, and <a href="" title="Open Market, Inc.">L. Stewart</a>, “<a href="">An Extension to HTTP : Digest Access Authentication</a>”, RFC&nbsp;2069, January&nbsp;1997.
1471            </td>
1472         </tr> 
1473         <tr>
1474            <td class="reference"><b id="RFC2396">[7]</b></td>
1475            <td class="top"><a href="" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="" title="Department of Information and Computer Science">Fielding, R.</a>, and <a href="" title="Xerox PARC">L. Masinter</a>, “<a href="">Uniform Resource Identifiers (URI): Generic Syntax</a>”, RFC&nbsp;2396, August&nbsp;1998.
1476            </td>
1477         </tr> 
1478         <tr>
1479            <td class="reference"><b id="ref8">[8]</b></td>
1480            <td class="top">Kaliski, B. and M. Robshaw, “<a href="">Message Authentication with MD5</a>”, 1995, &lt;<a href=""></a>&gt;.<br>CryptoBytes, Spring 1995
1481            </td>
1482         </tr> 
1483         <tr>
1484            <td class="reference"><b id="RFC2195">[9]</b></td>
1485            <td class="top"><a href="" title="MCI">Klensin, J.</a>, <a href="" title="MCI">Catoe, R.</a>, and <a href="" title="MCI">P. Krumviede</a>, “<a href="">IMAP/POP AUTHorize Extension for Simple Challenge/Response</a>”, RFC&nbsp;2195, September&nbsp;1997.
1486            </td>
1487         </tr> 
1488         <tr>
1489            <td class="reference"><b id="ref10">[10]</b></td>
1490            <td class="top">Morgan, B., Alvestrand, H., Hodges, J., and M. Wahl, “Authentication Methods for LDAP”.<br>Work in progress.
1491            </td>
1492         </tr>
1493      </table>
1494      <div class="avoidbreak">
1495         <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1496         <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=""><span class="email"></span></a></span></address>
1497         <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=""><span class="email"></span></a></span></address>
1498         <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=""><span class="email"></span></a></span></address>
1499         <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=""><span class="email"></span></a></span></address>
1500         <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=""><span class="email"></span></a></span></address>
1501         <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>
1502         <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=""><span class="email"></span></a></span></address>
1503      </div>
1504      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1505      <p class="noprint"><a href="#rfc.index.A">A</a> <a href="#rfc.index.B">B</a> <a href="#rfc.index.C">C</a> <a href="#rfc.index.D">D</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.N">N</a> <a href="#rfc.index.O">O</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.Q">Q</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.U">U</a> <a href="#rfc.index.W">W</a>
1506      </p>
1507      <div class="print2col">
1508         <ul class="ind">
1509            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
1510                  <li class="indline1"><tt>algorithm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.3"><b>3.2.1</b></a></li>
1511                  <li class="indline1"><tt>auth-info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.7"><b>3.2.3</b></a></li>
1512                  <li class="indline1"><tt>auth-param</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>1.2</b></a></li>
1513                  <li class="indline1"><tt>auth-scheme</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1"><b>1.2</b></a></li>
1514                  <li class="indline1"><tt>Authentication-Info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.6"><b>3.2.3</b></a></li>
1515                  <li class="indline1">Authentication-Info header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.5"><b>3.2.3</b></a></li>
1516                  <li class="indline1">Authorization header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.4"><b>3.2.2</b></a></li>
1517               </ul>
1518            </li>
1519            <li class="indline0"><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul class="ind">
1520                  <li class="indline1"><tt>base64-user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.2"><b>2</b></a></li>
1521                  <li class="indline1"><tt>basic-credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.1"><b>2</b></a></li>
1522               </ul>
1523            </li>
1524            <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
1525                  <li class="indline1"><tt>challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.1"><b>1.2</b></a>, <a class="iref" href="#rfc.iref.c.3">2</a>, <a class="iref" href="#rfc.iref.c.5">3.2.1</a></li>
1526                  <li class="indline1"><tt>cnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.2</b></a></li>
1527                  <li class="indline1"><tt>cnonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.2</b></a></li>
1528                  <li class="indline1"><tt>credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.2"><b>1.2</b></a>, <a class="iref" href="#rfc.iref.c.4">2</a>, <a class="iref" href="#rfc.iref.c.6">3.2.2</a></li>
1529               </ul>
1530            </li>
1531            <li class="indline0"><a id="rfc.index.D" href="#rfc.index.D"><b>D</b></a><ul class="ind">
1532                  <li class="indline1"><tt>digest-challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.1"><b>3.2.1</b></a></li>
1533                  <li class="indline1"><tt>digest-response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.3"><b>3.2.2</b></a></li>
1534                  <li class="indline1"><tt>digest-uri</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.4"><b>3.2.2</b></a></li>
1535                  <li class="indline1"><tt>digest-uri-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.5"><b>3.2.2</b></a></li>
1536                  <li class="indline1"><tt>domain</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.2"><b>3.2.1</b></a></li>
1537               </ul>
1538            </li>
1539            <li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind">
1540                  <li class="indline1">Headers&nbsp;&nbsp;
1541                     <ul class="ind">
1542                        <li class="indline1">Authentication-Info&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.3"><b>3.2.3</b></a></li>
1543                        <li class="indline1">Authorization&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>3.2.2</b></a></li>
1544                        <li class="indline1">WWW-Authenticate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.1"><b>3.2.1</b></a></li>
1545                     </ul>
1546                  </li>
1547               </ul>
1548            </li>
1549            <li class="indline0"><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul class="ind">
1550                  <li class="indline1"><tt>LHEX</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.l.1"><b>3.2.2</b></a></li>
1551               </ul>
1552            </li>
1553            <li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind">
1554                  <li class="indline1"><tt>message-qop</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.2</b></a></li>
1555               </ul>
1556            </li>
1557            <li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind">
1558                  <li class="indline1"><tt>nc-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
1559                  <li class="indline1"><tt>nextnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.5"><b>3.2.3</b></a></li>
1560                  <li class="indline1"><tt>nonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a></li>
1561                  <li class="indline1"><tt>nonce-count</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.2</b></a></li>
1562                  <li class="indline1"><tt>nonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a></li>
1563               </ul>
1564            </li>
1565            <li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind">
1566                  <li class="indline1"><tt>opaque</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
1567               </ul>
1568            </li>
1569            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
1570                  <li class="indline1"><tt>password</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>2</b></a></li>
1571               </ul>
1572            </li>
1573            <li class="indline0"><a id="rfc.index.Q" href="#rfc.index.Q"><b>Q</b></a><ul class="ind">
1574                  <li class="indline1"><tt>qop-options</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.1"><b>3.2.1</b></a></li>
1575                  <li class="indline1"><tt>qop-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.2"><b>3.2.1</b></a></li>
1576               </ul>
1577            </li>
1578            <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
1579                  <li class="indline1"><tt>realm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.1"><b>1.2</b></a></li>
1580                  <li class="indline1"><tt>realm-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.2"><b>1.2</b></a></li>
1581                  <li class="indline1"><em>ref10</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ref10.1">4.2</a>, <a class="iref" href="#ref10"><b>7</b></a></li>
1582                  <li class="indline1"><em>ref8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.ref8.1">4.9</a>, <a class="iref" href="#ref8"><b>7</b></a></li>
1583                  <li class="indline1"><tt>request-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.4"><b>3.2.2</b></a></li>
1584                  <li class="indline1"><tt>response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.3"><b>3.2.2</b></a></li>
1585                  <li class="indline1"><tt>response-auth</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.5"><b>3.2.3</b></a></li>
1586                  <li class="indline1"><tt>response-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.6"><b>3.2.3</b></a></li>
1587                  <li class="indline1"><em>RFC1321</em>&nbsp;&nbsp;<a class="iref" href="#RFC1321"><b>7</b></a></li>
1588                  <li class="indline1"><em>RFC1945</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1945.1">3.1.1</a>, <a class="iref" href="#RFC1945"><b>7</b></a></li>
1589                  <li class="indline1"><em>RFC2045</em>&nbsp;&nbsp;<a class="iref" href="#RFC2045"><b>7</b></a></li>
1590                  <li class="indline1"><em>RFC2069</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2069.1">§</a>, <a class="iref" href="#rfc.xref.RFC2069.2">3.2.1</a>, <a class="iref" href="#rfc.xref.RFC2069.3">3.2.2</a>, <a class="iref" href="#RFC2069"><b>7</b></a></li>
1591                  <li class="indline1"><em>RFC2195</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2195.1">4.2</a>, <a class="iref" href="#RFC2195"><b>7</b></a></li>
1592                  <li class="indline1"><em>RFC2246</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2246.1">§</a>, <a class="iref" href="#RFC2246"><b>7</b></a></li>
1593                  <li class="indline1"><em>RFC2396</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2396.1">2</a>, <a class="iref" href="#rfc.xref.RFC2396.2">3.2.1</a>, <a class="iref" href="#RFC2396"><b>7</b></a></li>
1594                  <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.2">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.4">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.5">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.6">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.7">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.8">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.9"></a>, <a class="iref" href="#rfc.xref.RFC2616.10"></a>, <a class="iref" href="#rfc.xref.RFC2616.11"></a>, <a class="iref" href="#rfc.xref.RFC2616.12"></a>, <a class="iref" href="#rfc.xref.RFC2616.13"></a>, <a class="iref" href="#rfc.xref.RFC2616.14"></a>, <a class="iref" href="#rfc.xref.RFC2616.15"></a>, <a class="iref" href="#rfc.xref.RFC2616.16"></a>, <a class="iref" href="#rfc.xref.RFC2616.17"></a>, <a class="iref" href="#rfc.xref.RFC2616.18"></a>, <a class="iref" href="#rfc.xref.RFC2616.19">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.20">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.21">3.6</a>, <a class="iref" href="#RFC2616"><b>7</b></a><ul class="ind">
1595                        <li class="indline1"><em>Section 10.33</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.19">3.6</a></li>
1596                        <li class="indline1"><em>Section 10.34</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.20">3.6</a></li>
1597                     </ul>
1598                     <ul class="ind">
1599                        <li class="indline1"><em>Section 2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.2">1.1</a></li>
1600                        <li class="indline1"><em>Section 5.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.9"></a></li>
1601                        <li class="indline1"><em>Section 5.1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.11"></a>, <a class="iref" href="#rfc.xref.RFC2616.13"></a></li>
1602                        <li class="indline1"><em>Section 13.5.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.7">1.2</a></li>
1603                        <li class="indline1"><em>Section 13.7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.15"></a></li>
1604                        <li class="indline1"><em>Section 14.8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.5">1.2</a></li>
1605                        <li class="indline1"><em>Section 14.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.17"></a></li>
1606                     </ul>
1607                  </li>
1608               </ul>
1609            </li>
1610            <li class="indline0"><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul class="ind">
1611                  <li class="indline1"><tt>stale</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1"><b>3.2.1</b></a></li>
1612               </ul>
1613            </li>
1614            <li class="indline0"><a id="rfc.index.U" href="#rfc.index.U"><b>U</b></a><ul class="ind">
1615                  <li class="indline1"><tt>URI</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.3"><b>3.2.1</b></a></li>
1616                  <li class="indline1"><tt>user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.1"><b>2</b></a></li>
1617                  <li class="indline1"><tt>userid</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.2"><b>2</b></a></li>
1618                  <li class="indline1"><tt>username</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.4"><b>3.2.2</b></a></li>
1619                  <li class="indline1"><tt>username-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.5"><b>3.2.2</b></a></li>
1620               </ul>
1621            </li>
1622            <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
1623                  <li class="indline1">WWW-Authenticate header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.w.1"><b>3.2.1</b></a></li>
1624               </ul>
1625            </li>
1626         </ul>
1627      </div>
1628      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
1629      <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
1630      <p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise
1631         explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without
1632         restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative
1633         works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references
1634         to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards
1635         in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to
1636         translate it into languages other than English.
1637      </p>
1638      <p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.</p>
1639      <p>This document and the information contained herein is provided on an “AS IS” basis and THE INTERNET SOCIETY AND THE INTERNET
1642         PURPOSE.
1643      </p>
1644      <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
1645      <p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed
1646         to pertain to the implementation or use of the technology described in this document or the extent to which any license under
1647         such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights.
1648         Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be
1649         found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available,
1650         or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors
1651         or users of this specification can be obtained from the IETF Secretariat.
1652      </p>
1653      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
1654         rights which may cover technology that may be required to practice this standard. Please address the information to the IETF
1655         Executive Director.
1656      </p>
1657      <h1>Acknowledgment</h1>
1658      <p>Funding for the RFC Editor function is currently provided by the Internet Society.</p>
1659   </body>
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