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287       content: "RFC 2617"; 
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290       content: "June 1999"; 
291  } 
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293       content: "HTTP Authentication"; 
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296       content: "Franks, et al."; 
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317</style><link rel="Contents" href="#rfc.toc">
318      <link rel="Author" href="#rfc.authors">
319      <link rel="Copyright" href="#rfc.copyright">
320      <link rel="Index" href="#rfc.index">
321      <link rel="Chapter" title="1 Access Authentication" href="#rfc.section.1">
322      <link rel="Chapter" title="2 Basic Authentication Scheme" href="#rfc.section.2">
323      <link rel="Chapter" title="3 Digest Access Authentication Scheme" href="#rfc.section.3">
324      <link rel="Chapter" title="4 Security Considerations" href="#rfc.section.4">
325      <link rel="Chapter" title="5 Sample implementation" href="#rfc.section.5">
326      <link rel="Chapter" title="6 Acknowledgments" href="#rfc.section.6">
327      <link rel="Chapter" href="#rfc.section.7" title="7 References">
328      <link rel="Alternate" title="Authorative ASCII Version" href="http://www.ietf.org/rfc/rfc2617.txt">
329      <link rel="Help" title="Additional Information on tools.ietf.org" href="http://tools.ietf.org/html/rfc2617">
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331      <link rel="schema.DC" href="http://purl.org/dc/elements/1.1/">
332      <meta name="DC.Creator" content="Franks, J.">
333      <meta name="DC.Creator" content="Hallam-Baker, P.M.">
334      <meta name="DC.Creator" content="Hostetler, J.L.">
335      <meta name="DC.Creator" content="Lawrence, S.D.">
336      <meta name="DC.Creator" content="Leach, P.J.">
337      <meta name="DC.Creator" content="Luotonen, A.">
338      <meta name="DC.Creator" content="Stewart, L.">
339      <meta name="DC.Identifier" content="urn:ietf:rfc:2617">
340      <meta name="DC.Date.Issued" scheme="ISO8601" content="1999-06">
341      <meta name="DC.Relation.Replaces" content="urn:ietf:rfc:2069">
342      <meta name="DC.Description.Abstract" content="&#34;HTTP/1.0&#34;, includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure method of user authentication (unless used in conjunction with some external secure system such as SSL ), as the user name and password are passed over the network as cleartext. This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme and a scheme based on cryptographic hashes, referred to as &#34;Digest Access Authentication&#34;. It is therefore also intended to serve as a replacement for RFC 2069 . Some optional elements specified by RFC 2069 have been removed from this specification due to problems found since its publication; other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended. Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password); unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness. As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself but in policies and procedures surrounding its use.">
343      <meta name="DC.isPartOf" content="urn:ISSN:2070-1721">
344   </head>
345   <body>
346      <table summary="header information" class="header" border="0" cellpadding="1" cellspacing="1">
347         <tr>
348            <td class="header left">Network Working Group</td>
349            <td class="header right">J. Franks</td>
350         </tr>
351         <tr>
352            <td class="header left">Request for Comments: 2617</td>
353            <td class="header right">Northwestern University, Department of Mathematics</td>
354         </tr>
355         <tr>
356            <td class="header left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2069">2069</a></td>
357            <td class="header right">P.M. Hallam-Baker</td>
358         </tr>
359         <tr>
360            <td class="header left">Category: Standards Track</td>
361            <td class="header right">Verisign Inc.</td>
362         </tr>
363         <tr>
364            <td class="header left"></td>
365            <td class="header right">J.L. Hostetler</td>
366         </tr>
367         <tr>
368            <td class="header left"></td>
369            <td class="header right">AbiSource, Inc.</td>
370         </tr>
371         <tr>
372            <td class="header left"></td>
373            <td class="header right">S.D. Lawrence</td>
374         </tr>
375         <tr>
376            <td class="header left"></td>
377            <td class="header right">Agranat Systems, Inc.</td>
378         </tr>
379         <tr>
380            <td class="header left"></td>
381            <td class="header right">P.J. Leach</td>
382         </tr>
383         <tr>
384            <td class="header left"></td>
385            <td class="header right">Microsoft Corporation</td>
386         </tr>
387         <tr>
388            <td class="header left"></td>
389            <td class="header right">A. Luotonen</td>
390         </tr>
391         <tr>
392            <td class="header left"></td>
393            <td class="header right">Netscape Communications Corporation</td>
394         </tr>
395         <tr>
396            <td class="header left"></td>
397            <td class="header right">L. Stewart</td>
398         </tr>
399         <tr>
400            <td class="header left"></td>
401            <td class="header right">Open Market, Inc.</td>
402         </tr>
403         <tr>
404            <td class="header left"></td>
405            <td class="header right">June 1999</td>
406         </tr>
407      </table>
408      <p class="title">HTTP Authentication: Basic and Digest Access Authentication</p>
409      <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1>
410      <p>This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions
411         for improvements. Please refer to the current edition of the “Internet Official Protocol Standards” (STD 1) for the standardization
412         state and status of this protocol. Distribution of this memo is unlimited.
413      </p>
414      <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
415      <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
416      <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 
417      <p>"HTTP/1.0", includes the specification for a Basic Access Authentication scheme. This scheme is not considered to be a secure
418         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.
419      </p>
420      <p>This document also provides the specification for HTTP's authentication framework, the original Basic authentication scheme
421         and a scheme based on cryptographic hashes, referred to as "Digest Access Authentication". It is therefore also intended to
422         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;
423         other new elements have been added for compatibility, those new elements have been made optional, but are strongly recommended.
424      </p>
425      <p>Like Basic, Digest access authentication verifies that both parties to a communication know a shared secret (a password);
426         unlike Basic, this verification can be done without sending the password in the clear, which is Basic's biggest weakness.
427         As with most other authentication protocols, the greatest sources of risks are usually found not in the core protocol itself
428         but in policies and procedures surrounding its use.
429      </p> 
430      <hr class="noprint">
431      <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
432      <ul class="toc">
433         <li class="tocline0">1.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1">Access Authentication</a><ul class="toc">
434               <li class="tocline1">1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.1.1">Reliance on the HTTP/1.1 Specification</a></li>
435               <li class="tocline1">1.2&nbsp;&nbsp;&nbsp;<a href="#access.authentication.framework">Access Authentication Framework</a></li>
436            </ul>
437         </li>
438         <li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.2">Basic Authentication Scheme</a></li>
439         <li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3">Digest Access Authentication Scheme</a><ul class="toc">
440               <li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1">Introduction</a><ul class="toc">
441                     <li class="tocline1">3.1.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.1">Purpose</a></li>
442                     <li class="tocline1">3.1.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.2">Overall Operation</a></li>
443                     <li class="tocline1">3.1.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.3">Representation of digest values</a></li>
444                     <li class="tocline1">3.1.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.1.4">Limitations</a></li>
445                  </ul>
446               </li>
447               <li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers">Specification of Digest Headers</a><ul class="toc">
448                     <li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#the.www-authenticate.response.header">The WWW-Authenticate Response Header</a></li>
449                     <li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#the.authorization.request.header">The Authorization Request Header</a><ul class="toc">
450                           <li class="tocline1">3.2.2.1&nbsp;&nbsp;&nbsp;<a href="#request-digest">Request-Digest</a></li>
451                           <li class="tocline1">3.2.2.2&nbsp;&nbsp;&nbsp;<a href="#A1">A1</a></li>
452                           <li class="tocline1">3.2.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.3">A2</a></li>
453                           <li class="tocline1">3.2.2.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.4">Directive values and quoted-string</a></li>
454                           <li class="tocline1">3.2.2.5&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.2.5">Various considerations</a></li>
455                        </ul>
456                     </li>
457                     <li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.2.3">The Authentication-Info Header</a></li>
458                  </ul>
459               </li>
460               <li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#digest.operation">Digest Operation</a></li>
461               <li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.3.4">Security Protocol Negotiation</a></li>
462               <li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#specification.of.digest.headers.example">Example</a></li>
463               <li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#proxy-authentication.and.proxy-authorization">Proxy-Authentication and Proxy-Authorization</a></li>
464            </ul>
465         </li>
466         <li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a><ul class="toc">
467               <li class="tocline1">4.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.1">Authentication of Clients using Basic Authentication</a></li>
468               <li class="tocline1">4.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.2">Authentication of Clients using Digest Authentication</a></li>
469               <li class="tocline1">4.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.3">Limited Use Nonce Values</a></li>
470               <li class="tocline1">4.4&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.4">Comparison of Digest with Basic Authentication</a></li>
471               <li class="tocline1">4.5&nbsp;&nbsp;&nbsp;<a href="#replay.attacks">Replay Attacks</a></li>
472               <li class="tocline1">4.6&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.6">Weakness Created by Multiple Authentication Schemes</a></li>
473               <li class="tocline1">4.7&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.7">Online dictionary attacks</a></li>
474               <li class="tocline1">4.8&nbsp;&nbsp;&nbsp;<a href="#man.in.the.middle">Man in the Middle</a></li>
475               <li class="tocline1">4.9&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.9">Chosen plaintext attacks</a></li>
476               <li class="tocline1">4.10&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.10">Precomputed dictionary attacks</a></li>
477               <li class="tocline1">4.11&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.11">Batch brute force attacks</a></li>
478               <li class="tocline1">4.12&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.12">Spoofing by Counterfeit Servers</a></li>
479               <li class="tocline1">4.13&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.13">Storing passwords</a></li>
480               <li class="tocline1">4.14&nbsp;&nbsp;&nbsp;<a href="#rfc.section.4.14">Summary</a></li>
481            </ul>
482         </li>
483         <li class="tocline0">5.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.5">Sample implementation</a></li>
484         <li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#rfc.section.6">Acknowledgments</a></li>
485         <li class="tocline0">7.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a></li>
486         <li class="tocline0"><a href="#rfc.authors">Authors' Addresses</a></li>
487         <li class="tocline0"><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li>
488         <li class="tocline0"><a href="#rfc.index">Index</a></li>
489      </ul>
490      <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a>&nbsp;Access Authentication
491      </h1>
492      <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a>&nbsp;Reliance on the HTTP/1.1 Specification
493      </h2>
494      <p id="rfc.section.1.1.p.1">This specification is a companion to the HTTP/1.1 specification <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. It uses the augmented BNF section <a href="http://tools.ietf.org/html/rfc2616#section-2.1" title="Augmented BNF" id="rfc.xref.RFC2616.2">2.1</a> of that document, and relies on both the non-terminals defined in that document and other aspects of the HTTP/1.1 specification.
495      </p>
496      <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>
497      <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,
498         case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs
499         which carry the parameters necessary for achieving authentication via that scheme.
500      </p>
501      <div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.a.1"></span>      auth-scheme    = token
502<span id="rfc.iref.a.2"></span>      auth-param     = token "=" ( token | quoted-string )
503</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
504         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.
505      </p>
506      <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
507</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
508         if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents
509         of a challenge may itself contain a comma-separated list of authentication parameters.
510      </p>
511      <p id="rfc.section.1.2.p.6">The authentication parameter realm is defined for all authentication schemes:</p>
512      <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.r.1"></span>      realm       = "realm" "=" realm-value
513<span id="rfc.iref.r.2"></span>      realm-value = quoted-string
514</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
515         (case-sensitive), in combination with the canonical root URL (the absoluteURI for the server whose abs_path is empty; see
516         section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" title="Request-URI" id="rfc.xref.RFC2616.3">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.4"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be
517         partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm
518         value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication
519         scheme. Note that there may be multiple challenges with the same auth-scheme but different realms.
520      </p>
521      <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
522         (Unauthorized)--MAY do so by including an Authorization header field with the request. A client that wishes to authenticate
523         itself with a proxy--usually, but not necessarily, after receiving a 407 (Proxy Authentication Required)--MAY do so by including
524         a Proxy-Authorization header field with the request. Both the Authorization field value and the Proxy-Authorization field
525         value consist of credentials containing the authentication information of the client for the realm of the resource being requested.
526         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
527         upon that challenge.
528      </p>
529      <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.c.2"></span>   credentials = auth-scheme #auth-param
530</pre><p id="rfc.section.1.2.p.11"> </p>
531      <dl class="empty">
532         <dd>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
533            only include Basic if it is minimally acceptable.
534         </dd>
535      </dl>
536      <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
537         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,
538         parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot
539         extend outside the scope of its server.
540      </p>
541      <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.
542         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
543         resource.
544      </p>
545      <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
546         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
547         authentication information. However, these additional mechanisms are not defined by this specification.
548      </p>
549      <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
550         and Authorization headers untouched, and follow the rules found in section <a href="http://tools.ietf.org/html/rfc2616#section-14.8" title="Authorization" id="rfc.xref.RFC2616.5">14.8</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.6"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. Both the Proxy-Authenticate and the Proxy-Authorization header fields are hop-by-hop headers (see section <a href="http://tools.ietf.org/html/rfc2616#section-13.5.1" title="End-to-end and Hop-by-hop Headers" id="rfc.xref.RFC2616.7">13.5.1</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.8"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>).
551      </p>
552      <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;Basic Authentication Scheme
553      </h1>
554      <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
555         for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms
556         on that server. The server will service the request only if it can validate the user-ID and password for the protection space
557         of the Request-URI. There are no optional authentication parameters.
558      </p>
559      <p id="rfc.section.2.p.2">For Basic, the framework above is utilized as follows:</p>
560      <div id="rfc.figure.u.5"></div><pre class="inline"><span id="rfc.iref.c.3"></span>      challenge   = "Basic" realm
561<span id="rfc.iref.c.4"></span>      credentials = "Basic" basic-credentials
562</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:
563      </p>
564      <div id="rfc.figure.u.6"></div><pre class="text">      WWW-Authenticate: Basic realm="WallyWorld"
565</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
566         with the same challenge using the Proxy-Authenticate header field.
567      </p>
568      <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
569         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.
570      </p>
571      <div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.b.1"></span>      basic-credentials = base64-user-pass
572<span id="rfc.iref.b.2"></span>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
573                       except not limited to 76 char/line&gt;
574<span id="rfc.iref.u.1"></span>      user-pass   = userid ":" password
575<span id="rfc.iref.u.2"></span>      userid      = *&lt;TEXT excluding ":"&gt;
576<span id="rfc.iref.p.1"></span>      password    = *TEXT
577</pre><p id="rfc.section.2.p.9">Userids might be case sensitive.</p>
578      <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>
579      <div id="rfc.figure.u.8"></div><pre class="text">      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
580</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
581         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
582         challenge from the server. Similarly, when a client sends a request to a proxy, it may reuse a userid and password in the
583         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.
584      </p>
585      <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;Digest Access Authentication Scheme
586      </h1>
587      <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;Introduction
588      </h2>
589      <h3 id="rfc.section.3.1.1"><a href="#rfc.section.3.1.1">3.1.1</a>&nbsp;Purpose
590      </h3>
591      <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
592         the network in an unencrypted form. This section provides the specification for a scheme that does not send the password in
593         cleartext, referred to as "Digest Access Authentication".
594      </p>
595      <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
596         Web. This scheme provides no encryption of message content. The intent is simply to create an access authentication method
597         that avoids the most serious flaws of Basic authentication.
598      </p>
599      <h3 id="rfc.section.3.1.2"><a href="#rfc.section.3.1.2">3.1.2</a>&nbsp;Overall Operation
600      </h3>
601      <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
602         using a nonce value. A valid response contains a checksum (by default, the MD5 checksum) of the username, the password, the
603         given nonce value, the HTTP method, and the requested URI. In this way, the password is never sent in the clear. Just as with
604         the Basic scheme, the username and password must be prearranged in some fashion not addressed by this document.
605      </p>
606      <h3 id="rfc.section.3.1.3"><a href="#rfc.section.3.1.3">3.1.3</a>&nbsp;Representation of digest values
607      </h3>
608      <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
609         is used and that is the only algorithm described in this document.
610      </p>
611      <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
612         the 128 bit digest are converted from most significant to least significant bit, four bits at a time to their ASCII presentation
613         as follows. Each four bits is represented by its familiar hexadecimal notation from the characters 0123456789abcdef. That
614         is, binary 0000 gets represented by the character '0', 0001, by '1', and so on up to the representation of 1111 as 'f'.
615      </p>
616      <h3 id="rfc.section.3.1.4"><a href="#rfc.section.3.1.4">3.1.4</a>&nbsp;Limitations
617      </h3>
618      <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
619         for Basic authentication and nothing more. It is a password-based system and (on the server side) suffers from all the same
620         problems of any password system. In particular, no provision is made in this protocol for the initial secure arrangement between
621         user and server to establish the user's password.
622      </p>
623      <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
624         private-key scheme. Nevertheless it is better than nothing, better than what is commonly used with telnet and ftp, and better
625         than Basic authentication.
626      </p>
627      <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>
628      <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
629         header line and the Authorization header line are specified below. In addition, a new header, Authentication-Info, is specified.
630      </p>
631      <div id="rfc.iref.h.1"></div>
632      <div id="rfc.iref.w.1"></div>
633      <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>
634      <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
635         responds with a "401 Unauthorized" status code, and a WWW-Authenticate header as per the framework defined above, which for
636         the digest scheme is utilized as follows:
637      </p>
638      <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.c.5"></span>      challenge        =  "Digest" digest-challenge
639
640<span id="rfc.iref.d.1"></span>      digest-challenge  = 1#( realm | [ domain ] | nonce |
641                          [ opaque ] |[ stale ] | [ algorithm ] |
642                          [ qop-options ] | [auth-param] )
643
644
645<span id="rfc.iref.d.2"></span>      domain            = "domain" "=" &lt;"&gt; URI ( 1*SP URI ) &lt;"&gt;
646<span id="rfc.iref.u.3"></span>      URI               = absoluteURI | abs_path
647<span id="rfc.iref.n.1"></span>      nonce             = "nonce" "=" nonce-value
648<span id="rfc.iref.n.2"></span>      nonce-value       = quoted-string
649<span id="rfc.iref.o.1"></span>      opaque            = "opaque" "=" quoted-string
650<span id="rfc.iref.s.1"></span>      stale             = "stale" "=" ( "true" | "false" )
651<span id="rfc.iref.a.3"></span>      algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
652                           token )
653<span id="rfc.iref.q.1"></span>      qop-options       = "qop" "=" &lt;"&gt; 1#qop-value &lt;"&gt;
654<span id="rfc.iref.q.2"></span>      qop-value         = "auth" | "auth-int" | token
655</pre><p id="rfc.section.3.2.1.p.3">The meanings of the values of the directives used above are as follows:</p>
656      <p id="rfc.section.3.2.1.p.4">realm </p>
657      <dl class="empty">
658         <dd>A string to be displayed to users so they know which username and password to use. This string should contain at least the
659            name of the host performing the authentication and might additionally indicate the collection of users who might have access.
660            An example might be "registered_users@gotham.news.com".
661         </dd>
662      </dl>
663      <p id="rfc.section.3.2.1.p.5">domain </p>
664      <dl class="empty">
665         <dd>A quoted, space-separated list of URIs, as specified in RFC XURI <a href="#RFC2396" id="rfc.xref.RFC2396.2"><cite title="Uniform Resource Identifiers (URI): Generic Syntax">[7]</cite></a>, that define the protection space. If a URI is an abs_path, it is relative to the canonical root URL (see <a href="#access.authentication.framework" title="Access Authentication Framework">Section&nbsp;1.2</a> above) of the server being accessed. An absoluteURI in this list may refer to a different server than the one being accessed.
666            The client can use this list to determine the set of URIs for which the same authentication information may be sent: any URI
667            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.
668            If this directive is omitted or its value is empty, the client should assume that the protection space consists of all URIs
669            on the responding server. This directive is not meaningful in Proxy-Authenticate headers, for which the protection space is
670            always the entire proxy; if present it should be ignored.
671         </dd>
672      </dl>
673      <p id="rfc.section.3.2.1.p.6">nonce </p>
674      <dl class="empty">
675         <dd>A server-specified data string which should be uniquely generated each time a 401 response is made. It is recommended that
676            this string be base64 or hexadecimal data. Specifically, since the string is passed in the header lines as a quoted string,
677            the double-quote character is not allowed.
678         </dd>
679         <dd>The contents of the nonce are implementation dependent. The quality of the implementation depends on a good choice. A nonce
680            might, for example, be constructed as the base 64 encoding of
681         </dd>
682         <dd>
683            <div id="rfc.figure.u.10"></div><pre class="text">         time-stamp H(time-stamp ":" ETag ":" private-key)
684  </pre></dd>
685         <dd>where time-stamp is a server-generated time or other non-repeating value, ETag is the value of the HTTP ETag header associated
686            with the requested entity, and private-key is data known only to the server. With a nonce of this form a server would recalculate
687            the hash portion after receiving the client authentication header and reject the request if it did not match the nonce from
688            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.
689            The inclusion of the ETag prevents a replay request for an updated version of the resource. (Note: including the IP address
690            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
691            that originally got it. However, that would break proxy farms, where requests from a single user often go through different
692            proxies in the farm. Also, IP address spoofing is not that hard.)
693         </dd>
694         <dd>An implementation might choose not to accept a previously used nonce or a previously used digest, in order to protect against
695            a replay attack. Or, an implementation might choose to use one-time nonces or digests for POST or PUT requests and a time-stamp
696            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.
697         </dd>
698         <dd>The nonce is opaque to the client.</dd>
699      </dl>
700      <p id="rfc.section.3.2.1.p.7">opaque </p>
701      <dl class="empty">
702         <dd>A string of data, specified by the server, which should be returned by the client unchanged in the Authorization header of
703            subsequent requests with URIs in the same protection space. It is recommended that this string be base64 or hexadecimal data.
704         </dd>
705      </dl>
706      <p id="rfc.section.3.2.1.p.8">stale </p>
707      <dl class="empty">
708         <dd>A flag, indicating that the previous request from the client was rejected because the nonce value was stale. If stale is TRUE
709            (case-insensitive), the client may wish to simply retry the request with a new encrypted response, without reprompting the
710            user for a new username and password. The server should only set stale to TRUE if it receives a request for which the nonce
711            is invalid but with a valid digest for that nonce (indicating that the client knows the correct username/password). If stale
712            is FALSE, or anything other than TRUE, or the stale directive is not present, the username and/or password are invalid, and
713            new values must be obtained.
714         </dd>
715      </dl>
716      <p id="rfc.section.3.2.1.p.9">algorithm </p>
717      <dl class="empty">
718         <dd>A string indicating a pair of algorithms used to produce the digest and a checksum. If this is not present it is assumed to
719            be "MD5". If the algorithm is not understood, the challenge should be ignored (and a different one used, if there is more
720            than one).
721         </dd>
722         <dd>In this document the string obtained by applying the digest algorithm to the data "data" with secret "secret" will be denoted
723            by KD(secret, data), and the string obtained by applying the checksum algorithm to the data "data" will be denoted H(data).
724            The notation unq(X) means the value of the quoted-string X without the surrounding quotes.
725         </dd>
726         <dd>For the "MD5" and "MD5-sess" algorithms</dd>
727         <dd>
728            <div id="rfc.figure.u.11"></div><pre class="text">         H(data) = MD5(data)
729    </pre></dd>
730         <dd>and</dd>
731         <dd>
732            <div id="rfc.figure.u.12"></div><pre class="text">         KD(secret, data) = H(concat(secret, ":", data))
733    </pre></dd>
734         <dd>i.e., the digest is the MD5 of the secret concatenated with a colon concatenated with the data. The "MD5-sess" algorithm is
735            intended to allow efficient 3rd party authentication servers; for the difference in usage, see the description in <a href="#A1" title="A1">Section&nbsp;3.2.2.2</a>.
736         </dd>
737      </dl>
738      <p id="rfc.section.3.2.1.p.10">qop-options </p>
739      <dl class="empty">
740         <dd>This directive is optional, but is made so only for backward compatibility with RFC 2069 <a href="#RFC2069" id="rfc.xref.RFC2069.2"><cite title="An Extension to HTTP : Digest Access Authentication">[6]</cite></a>; it <em class="bcp14">SHOULD</em> be used by all implementations compliant with this version of the Digest scheme. If present, it is a quoted string of one
741            or more tokens indicating the "quality of protection" values supported by the server. The value "auth" indicates authentication;
742            the value "auth-int" indicates authentication with integrity protection; see the descriptions below for calculating the response
743            directive value for the application of this choice. Unrecognized options <em class="bcp14">MUST</em> be ignored.
744         </dd>
745      </dl>
746      <p id="rfc.section.3.2.1.p.11">auth-param </p>
747      <dl class="empty">
748         <dd>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
749         </dd>
750      </dl>
751      <div id="rfc.iref.h.2"></div>
752      <div id="rfc.iref.a.4"></div>
753      <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>
754      <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
755         above, utilized as follows.
756      </p>
757      <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.c.6"></span>       credentials      = "Digest" digest-response
758<span id="rfc.iref.d.3"></span>       digest-response  = 1#( username | realm | nonce | digest-uri
759                       | response | [ algorithm ] | [cnonce] |
760                       [opaque] | [message-qop] |
761                           [nonce-count]  | [auth-param] )
762
763<span id="rfc.iref.u.4"></span>       username         = "username" "=" username-value
764<span id="rfc.iref.u.5"></span>       username-value   = quoted-string
765<span id="rfc.iref.d.4"></span>       digest-uri       = "uri" "=" digest-uri-value
766<span id="rfc.iref.d.5"></span>       digest-uri-value = request-uri   ; As specified by HTTP/1.1
767<span id="rfc.iref.m.1"></span>       message-qop      = "qop" "=" qop-value
768<span id="rfc.iref.c.7"></span>       cnonce           = "cnonce" "=" cnonce-value
769<span id="rfc.iref.c.8"></span>       cnonce-value     = nonce-value
770<span id="rfc.iref.n.3"></span>       nonce-count      = "nc" "=" nc-value
771<span id="rfc.iref.n.4"></span>       nc-value         = 8LHEX
772<span id="rfc.iref.r.3"></span>       response         = "response" "=" request-digest
773<span id="rfc.iref.r.4"></span>       request-digest = &lt;"&gt; 32LHEX &lt;"&gt;
774<span id="rfc.iref.l.1"></span>       LHEX             =  "0" | "1" | "2" | "3" |
775                           "4" | "5" | "6" | "7" |
776                           "8" | "9" | "a" | "b" |
777                           "c" | "d" | "e" | "f"
778</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
779         being requested.
780      </p>
781      <p id="rfc.section.3.2.2.p.4">response </p>
782      <dl class="empty">
783         <dd>A string of 32 hex digits computed as defined below, which proves that the user knows a password</dd>
784      </dl>
785      <p id="rfc.section.3.2.2.p.5">username </p>
786      <dl class="empty">
787         <dd>The user's name in the specified realm.</dd>
788      </dl>
789      <p id="rfc.section.3.2.2.p.6">digest-uri </p>
790      <dl class="empty">
791         <dd>The URI from Request-URI of the Request-Line; duplicated here because proxies are allowed to change the Request-Line in transit.</dd>
792      </dl>
793      <p id="rfc.section.3.2.2.p.7">qop </p>
794      <dl class="empty">
795         <dd>Indicates what "quality of protection" the client has applied to the message. If present, its value <em class="bcp14">MUST</em> be one of the alternatives the server indicated it supports in the WWW-Authenticate header. These values affect the computation
796            of the request-digest. Note that this is a single token, not a quoted list of alternatives as in WWW-Authenticate. This directive
797            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.
798         </dd>
799      </dl>
800      <p id="rfc.section.3.2.2.p.8">cnonce </p>
801      <dl class="empty">
802         <dd>This <em class="bcp14">MUST</em> be specified if a qop directive is sent (see above), and <em class="bcp14">MUST NOT</em> be specified if the server did not send a qop directive in the WWW-Authenticate header field. The cnonce-value is an opaque
803            quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, to provide
804            mutual authentication, and to provide some message integrity protection. See the descriptions below of the calculation of
805            the response-digest and request-digest values.
806         </dd>
807      </dl>
808      <p id="rfc.section.3.2.2.p.9">nonce-count </p>
809      <dl class="empty">
810         <dd>This <em class="bcp14">MUST</em> be specified if a qop directive is sent (see above), and <em class="bcp14">MUST NOT</em> be specified if the server did not send a qop directive in the WWW-Authenticate header field. The nc-value is the hexadecimal
811            count of the number of requests (including the current request) that the client has sent with the nonce value in this request.
812            For example, in the first request sent in response to a given nonce value, the client sends "nc=00000001". The purpose of
813            this directive is to allow the server to detect request replays by maintaining its own copy of this count - if the same nc-value
814            is seen twice, then the request is a replay. See the description below of the construction of the request-digest value.
815         </dd>
816      </dl>
817      <p id="rfc.section.3.2.2.p.10">auth-param </p>
818      <dl class="empty">
819         <dd>This directive allows for future extensions. Any unrecognized directive <em class="bcp14">MUST</em> be ignored.
820         </dd>
821      </dl>
822      <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
823         request-digest is invalid, then a login failure should be logged, since repeated login failures from a single client may indicate
824         an attacker attempting to guess passwords.
825      </p>
826      <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
827         is computed.
828      </p>
829      <h4 id="rfc.section.3.2.2.1"><a href="#rfc.section.3.2.2.1">3.2.2.1</a>&nbsp;<a id="request-digest" href="#request-digest">Request-Digest</a></h4>
830      <p id="rfc.section.3.2.2.1.p.1">If the "qop" value is "auth" or "auth-int":</p>
831      <div id="rfc.figure.u.14"></div><pre class="inline">      request-digest  = &lt;"&gt; &lt; KD ( H(A1),     unq(nonce-value)
832                                          ":" nc-value
833                                          ":" unq(cnonce-value)
834                                          ":" unq(qop-value)
835                                          ":" H(A2)
836                                  ) &lt;"&gt;
837</pre><p id="rfc.section.3.2.2.1.p.3">If the "qop" directive is not present (this construction is for compatibility with RFC 2069):</p>
838      <div id="rfc.figure.u.15"></div><pre class="inline">      request-digest  =
839                 &lt;"&gt; &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) &gt;
840   &lt;"&gt;
841</pre><p id="rfc.section.3.2.2.1.p.5">See below for the definitions for A1 and A2.</p>
842      <h4 id="rfc.section.3.2.2.2"><a href="#rfc.section.3.2.2.2">3.2.2.2</a>&nbsp;<a id="A1" href="#A1">A1</a></h4>
843      <p id="rfc.section.3.2.2.2.p.1">If the "algorithm" directive's value is "MD5" or is unspecified, then A1 is:</p>
844      <div id="rfc.figure.u.16"></div><pre class="inline">      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
845</pre><p id="rfc.section.3.2.2.2.p.3">where</p>
846      <div id="rfc.figure.u.17"></div><pre class="inline">      passwd   = &lt; user's password &gt;
847</pre><p id="rfc.section.3.2.2.2.p.5">If the "algorithm" directive's value is "MD5-sess", then A1 is calculated only once - on the first request by the client following
848         receipt of a WWW-Authenticate challenge from the server. It uses the server nonce from that challenge, and the first client
849         nonce value to construct A1 as follows:
850      </p>
851      <div id="rfc.figure.u.18"></div><pre class="inline">      A1       = H( unq(username-value) ":" unq(realm-value)
852                     ":" passwd )
853                     ":" unq(nonce-value) ":" unq(cnonce-value)
854</pre><p id="rfc.section.3.2.2.2.p.7">This creates a 'session key' for the authentication of subsequent requests and responses which is different for each "authentication
855         session", thus limiting the amount of material hashed with any one key. (Note: see further discussion of the authentication
856         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
857         be used in conjunction with a third party authentication service so that the web server would not need the actual password
858         value. The specification of such a protocol is beyond the scope of this specification.
859      </p>
860      <h4 id="rfc.section.3.2.2.3"><a href="#rfc.section.3.2.2.3">3.2.2.3</a>&nbsp;A2
861      </h4>
862      <p id="rfc.section.3.2.2.3.p.1">If the "qop" directive's value is "auth" or is unspecified, then A2 is:</p>
863      <div id="rfc.figure.u.19"></div><pre class="inline">      A2       = Method ":" digest-uri-value
864</pre><p id="rfc.section.3.2.2.3.p.3">If the "qop" value is "auth-int", then A2 is:</p>
865      <div id="rfc.figure.u.20"></div><pre class="inline">      A2       = Method ":" digest-uri-value ":" H(entity-body)
866</pre><h4 id="rfc.section.3.2.2.4"><a href="#rfc.section.3.2.2.4">3.2.2.4</a>&nbsp;Directive values and quoted-string
867      </h4>
868      <p id="rfc.section.3.2.2.4.p.1">Note that the value of many of the directives, such as "username-value", are defined as a "quoted-string". However, the "unq"
869         notation indicates that surrounding quotation marks are removed in forming the string A1. Thus if the Authorization header
870         includes the fields
871      </p>
872      <div id="rfc.figure.u.21"></div><pre class="text">     username="Mufasa", realm=myhost@testrealm.com
873</pre><p id="rfc.section.3.2.2.4.p.3">and the user Mufasa has password "Circle Of Life" then H(A1) would be H(Mufasa:myhost@testrealm.com:Circle Of Life) with no
874         quotation marks in the digested string.
875      </p>
876      <p id="rfc.section.3.2.2.4.p.4">No white space is allowed in any of the strings to which the digest function H() is applied unless that white space exists
877         in the quoted strings or entity body whose contents make up the string to be digested. For example, the string A1 illustrated
878         above must be
879      </p>
880      <div id="rfc.figure.u.22"></div><pre class="text">        Mufasa:myhost@testrealm.com:Circle Of Life
881</pre><p id="rfc.section.3.2.2.4.p.6">with no white space on either side of the colons, but with the white space between the words used in the password value. Likewise,
882         the other strings digested by H() must not have white space on either side of the colons which delimit their fields unless
883         that white space was in the quoted strings or entity body being digested.
884      </p>
885      <p id="rfc.section.3.2.2.4.p.7">Also note that if integrity protection is applied (qop=auth-int), the H(entity-body) is the hash of the entity body, not the
886         message body - it is computed before any transfer encoding is applied by the sender and after it has been removed by the recipient.
887         Note that this includes multipart boundaries and embedded headers in each part of any multipart content-type.
888      </p>
889      <h4 id="rfc.section.3.2.2.5"><a href="#rfc.section.3.2.2.5">3.2.2.5</a>&nbsp;Various considerations
890      </h4>
891      <p id="rfc.section.3.2.2.5.p.1">The "Method" value is the HTTP request method as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.1" title="Method" id="rfc.xref.RFC2616.9">5.1.1</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.10"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. The "request-uri" value is the Request-URI from the request line as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" title="Request-URI" id="rfc.xref.RFC2616.11">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.12"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>. This may be "*", an "absoluteURL" or an "abs_path" as specified in section <a href="http://tools.ietf.org/html/rfc2616#section-5.1.2" title="Request-URI" id="rfc.xref.RFC2616.13">5.1.2</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.14"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>, but it <em class="bcp14">MUST</em> agree with the Request-URI. In particular, it <em class="bcp14">MUST</em> be an "absoluteURL" if the Request-URI is an "absoluteURL". The "cnonce-value" is an optional client-chosen value whose purpose
892         is to foil chosen plaintext attacks.
893      </p>
894      <p id="rfc.section.3.2.2.5.p.2">The authenticating server must assure that the resource designated by the "uri" directive is the same as the resource specified
895         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
896         such errors.) The purpose of duplicating information from the request URL in this field is to deal with the possibility that
897         an intermediate proxy may alter the client's Request-Line. This altered (but presumably semantically equivalent) request would
898         not result in the same digest as that calculated by the client.
899      </p>
900      <p id="rfc.section.3.2.2.5.p.3">Implementers should be aware of how authenticated transactions interact with shared caches. The HTTP/1.1 protocol specifies
901         that when a shared cache (see section <a href="http://tools.ietf.org/html/rfc2616#section-13.7" title="Shared and Non-Shared Caches" id="rfc.xref.RFC2616.15">13.7</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.16"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) has received a request containing an Authorization header and a response from relaying that request, it <em class="bcp14">MUST NOT</em> return that response as a reply to any other request, unless one of two Cache-Control (see section <a href="http://tools.ietf.org/html/rfc2616#section-14.9" title="Cache-Control" id="rfc.xref.RFC2616.17">14.9</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.18"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a>) directives was present in the response. If the original response included the "must-revalidate" Cache-Control directive,
902         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
903         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.
904      </p>
905      <div id="rfc.iref.h.3"></div>
906      <div id="rfc.iref.a.5"></div>
907      <h3 id="rfc.section.3.2.3"><a href="#rfc.section.3.2.3">3.2.3</a>&nbsp;The Authentication-Info Header
908      </h3>
909      <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
910         in the response.
911      </p>
912      <div id="rfc.figure.u.23"></div><pre class="inline"><span id="rfc.iref.a.6"></span>        AuthenticationInfo = "Authentication-Info" ":" auth-info
913<span id="rfc.iref.a.7"></span>        auth-info          = 1#(nextnonce | [ message-qop ]
914                               | [ response-auth ] | [ cnonce ]
915                               | [nonce-count] )
916<span id="rfc.iref.n.5"></span>        nextnonce          = "nextnonce" "=" nonce-value
917<span id="rfc.iref.r.5"></span>        response-auth      = "rspauth" "=" response-digest
918<span id="rfc.iref.r.6"></span>        response-digest    = &lt;"&gt; *LHEX &lt;"&gt;
919</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.
920         The server may send the Authentication-Info header with a nextnonce field as a means of implementing one-time or otherwise
921         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
922         to re-authenticate from the server with the "stale=TRUE".
923      </p>
924      <p id="rfc.section.3.2.3.p.4"> </p>
925      <dl class="empty">
926         <dd>Server implementations should carefully consider the performance implications of the use of this mechanism; pipelined requests
927            will not be possible if every response includes a nextnonce directive that must be used on the next request received by the
928            server. Consideration should be given to the performance vs. security tradeoffs of allowing an old nonce value to be used
929            for a limited time to permit request pipelining. Use of the nonce-count can retain most of the security advantages of a new
930            server nonce without the deleterious affects on pipelining.
931         </dd>
932      </dl>
933      <p id="rfc.section.3.2.3.p.5">message-qop</p>
934      <dl class="empty">
935         <dd>Indicates the "quality of protection" options applied to the response by the server. The value "auth" indicates authentication;
936            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.
937         </dd>
938      </dl>
939      <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
940         knows the user's secret, and with qop=auth-int also provides limited integrity protection of the response. The "response-digest"
941         value is calculated as for the "request-digest" in the Authorization header, except that if "qop=auth" or is not specified
942         in the Authorization header for the request, A2 is
943      </p>
944      <div id="rfc.figure.u.24"></div><pre class="inline">      A2       = ":" digest-uri-value
945</pre><p id="rfc.section.3.2.3.p.9">and if "qop=auth-int", then A2 is</p>
946      <div id="rfc.figure.u.25"></div><pre class="inline">      A2       = ":" digest-uri-value ":" H(entity-body)
947</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"
948         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"
949         directives <em class="bcp14">MUST</em> BE present if "qop=auth" or "qop=auth-int" is specified.
950      </p>
951      <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>
952      <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>
953      <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
954         the submitted username. Then, the server must perform the same digest operation (e.g., MD5) performed by the client, and compare
955         the result to the given request-digest value.
956      </p>
957      <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
958         server, the validity of an Authorization header may be verified.
959      </p>
960      <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
961         space. The authentication session lasts until the client receives another WWW-Authenticate challenge from any server in the
962         protection space. A client should remember the username, password, nonce, nonce count and opaque values associated with an
963         authentication session to use to construct the Authorization header in future requests within that protection space. The Authorization
964         header may be included preemptively; doing so improves server efficiency and avoids extra round trips for authentication challenges.
965         The server may choose to accept the old Authorization header information, even though the nonce value included might not be
966         fresh. Alternatively, the server may return a 401 response with a new nonce value, causing the client to retry the request;
967         by specifying stale=TRUE with this response, the server tells the client to retry with the new nonce, but without prompting
968         for a new username and password.
969      </p>
970      <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
971         session, the opaque data may be used to transport authentication session state information. (Note that any such use can also
972         be accomplished more easily and safely by including the state in the nonce.) For example, a server could be responsible for
973         authenticating content that actually sits on another server. It would achieve this by having the first 401 response include
974         a domain directive whose value includes a URI on the second server, and an opaque directive whose value contains the state
975         information. The client will retry the request, at which time the server might respond with a 301/302 redirection, pointing
976         to the URI on the second server. The client will follow the redirection, and pass an Authorization header , including the
977         &lt;opaque&gt; data.
978      </p>
979      <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
980         must forward the WWW-Authenticate, Authentication-Info and Authorization headers untouched. If a proxy wants to authenticate
981         a client before a request is forwarded to the server, it can be done using the Proxy-Authenticate and Proxy-Authorization
982         headers described in <a href="#proxy-authentication.and.proxy-authorization" title="Proxy-Authentication and Proxy-Authorization">Section&nbsp;3.6</a> below.
983      </p>
984      <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a>&nbsp;Security Protocol Negotiation
985      </h2>
986      <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>
987      <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
988         the client supports it. A client is encouraged to fail gracefully if the server specifies only authentication schemes it cannot
989         handle.
990      </p>
991      <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>
992      <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
993         URI of the document is "http://www.nowhere.org/dir/index.html". Both client and server know that the username for this document
994         is "Mufasa", and the password is "Circle Of Life" (with one space between each of the three words).
995      </p>
996      <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>
997      <div id="rfc.figure.u.26"></div><pre class="text">         HTTP/1.1 401 Unauthorized
998         WWW-Authenticate: Digest
999                 realm="testrealm@host.com",
1000                 qop="auth,auth-int",
1001                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1002                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1003</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
1004         following Authorization header:
1005      </p>
1006      <div id="rfc.figure.u.27"></div><pre class="text">         Authorization: Digest username="Mufasa",
1007                 realm="testrealm@host.com",
1008                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1009                 uri="/dir/index.html",
1010                 qop=auth,
1011                 nc=00000001,
1012                 cnonce="0a4f113b",
1013                 response="6629fae49393a05397450978507c4ef1",
1014                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1015</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>
1016      <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
1017         servers by use of the Proxy-Authenticate and Proxy-Authorization headers. These headers are instances of the Proxy-Authenticate
1018         and Proxy-Authorization headers specified in sections <a href="http://tools.ietf.org/html/rfc2616#section-10.33" id="rfc.xref.RFC2616.19">10.33</a> and <a href="http://tools.ietf.org/html/rfc2616#section-10.34" id="rfc.xref.RFC2616.20">10.34</a> of the HTTP/1.1 specification <a href="#RFC2616" id="rfc.xref.RFC2616.21"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[2]</cite></a> and their behavior is subject to restrictions described there. The transactions for proxy authentication are very similar
1019         to those already described. Upon receiving a request which requires authentication, the proxy/server must issue the "407 Proxy
1020         Authentication Required" response with a "Proxy-Authenticate" header. The digest-challenge used in the Proxy-Authenticate
1021         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>.
1022      </p>
1023      <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
1024         header in <a href="#the.authorization.request.header" title="The Authorization Request Header">Section&nbsp;3.2.2</a> above.
1025      </p>
1026      <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
1027         header field.
1028      </p>
1029      <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
1030         same response.
1031      </p>
1032      <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
1033      <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;Authentication of Clients using Basic Authentication
1034      </h2>
1035      <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,
1036         which is transmitted in cleartext across the physical network used as the carrier. HTTP does not prevent additional authentication
1037         schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes
1038         to use one-time passwords) to Basic authentication.
1039      </p>
1040      <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
1041         over the physical network. It is this problem which Digest Authentication attempts to address.
1042      </p>
1043      <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.
1044      </p>
1045      <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
1046         as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this
1047         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
1048         concern. This is only correct if the server issues both user name and password to the users and in particular does not allow
1049         the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid
1050         the task of maintaining multiple passwords.
1051      </p>
1052      <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
1053         server but also unauthorized access to any other resources on other systems that the user protects with the same password.
1054         Furthermore, in the server's password database, many of the passwords may also be users' passwords for other sites. The owner
1055         or administrator of such a system could therefore expose all users of the system to the risk of unauthorized access to all
1056         those sites if this information is not maintained in a secure fashion.
1057      </p>
1058      <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
1059         to a host containing information protected by Basic authentication when, in fact, he is connecting to a hostile server or
1060         gateway, then the attacker can request a password, store it for later use, and feign an error. This type of attack is not
1061         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
1062         for a server to simply turn over a connection to a gateway. That gateway can then use the persistent connection mechanism
1063         to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable
1064         by the client.
1065      </p>
1066      <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;Authentication of Clients using Digest Authentication
1067      </h2>
1068      <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
1069         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.
1070      </p>
1071      <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
1072         and response are available to an eavesdropper.
1073      </p>
1074      <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
1075         is used, those parts of the message used in the calculation of the WWW-Authenticate and Authorization header field response
1076         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.
1077      </p>
1078      <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
1079         protocols. In particular Digest authentication cannot be used for any transaction requiring confidentiality protection. Nevertheless
1080         many functions remain for which Digest authentication is both useful and appropriate. Any service in present use that uses
1081         Basic should be switched to Digest as soon as practical.
1082      </p>
1083      <h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;Limited Use Nonce Values
1084      </h2>
1085      <p id="rfc.section.4.3.p.1">The Digest scheme uses a server-specified nonce to seed the generation of the request-digest value (as specified in <a href="#request-digest" title="Request-Digest">Section&nbsp;3.2.2.1</a> above). As shown in the example nonce in <a href="#the.www-authenticate.response.header" title="The WWW-Authenticate Response Header">Section&nbsp;3.2.1</a>, the server is free to construct the nonce such that it may only be used from a particular client, for a particular resource,
1086         for a limited period of time or number of uses, or any other restrictions. Doing so strengthens the protection provided against,
1087         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
1088         implications. For example, a server may choose to allow each nonce value to be used only once by maintaining a record of whether
1089         or not each recently issued nonce has been returned and sending a next-nonce directive in the Authentication-Info header field
1090         of every response. This protects against even an immediate replay attack, but has a high cost checking nonce values, and perhaps
1091         more important will cause authentication failures for any pipelined requests (presumably returning a stale nonce indication).
1092         Similarly, incorporating a request-specific element such as the Etag value for a resource limits the use of the nonce to that
1093         version of the resource and also defeats pipelining. Thus it may be useful to do so for methods with side effects but have
1094         unacceptable performance for those that do not.
1095      </p>
1096      <h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;Comparison of Digest with Basic Authentication
1097      </h2>
1098      <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
1099         the two points out the utility, even necessity, of replacing Basic by Digest.
1100      </p>
1101      <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
1102         might involve, for example, online access to a database whose use is restricted to paying subscribers. With Basic authentication
1103         an eavesdropper can obtain the password of the user. This not only permits him to access anything in the database, but, often
1104         worse, will permit access to anything else the user protects with the same password.
1105      </p>
1106      <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
1107         password. The information gained by the eavesdropper would permit a replay attack, but only with a request for the same document,
1108         and even that may be limited by the server's choice of nonce.
1109      </p>
1110      <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>
1111      <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
1112         already have seen the only document he could obtain with a replay. This is because the URI of the requested document is digested
1113         in the client request and the server will only deliver that document. By contrast under Basic Authentication once the eavesdropper
1114         has the user's password, any document protected by that password is open to him.
1115      </p>
1116      <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
1117         ways. The server created "nonce" value is implementation dependent, but if it contains a digest of the client IP, a time-stamp,
1118         the resource ETag, and a private server key (as recommended above) then a replay attack is not simple. An attacker must convince
1119         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
1120         different from the address to which it believes it is sending the document. An attack can only succeed in the period before
1121         the time-stamp expires. Digesting the client IP and time-stamp in the nonce permits an implementation which does not maintain
1122         state between transactions.
1123      </p>
1124      <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
1125         not be honored for a second use. This requires the overhead of the server remembering which nonce values have been used until
1126         the nonce time-stamp (and hence the digest built with it) has expired, but it effectively protects against replay attacks.
1127      </p>
1128      <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
1129         server employs one-time or otherwise limited-use nonces and/or insists on the use of the integrity protection of qop=auth-int,
1130         an attacker could replay valid credentials from a successful request with counterfeit form data or other message body. Even
1131         with the use of integrity protection most metadata in header fields is not protected. Proper nonce generation and checking
1132         provides some protection against replay of previously used valid credentials, but see 4.8.
1133      </p>
1134      <h2 id="rfc.section.4.6"><a href="#rfc.section.4.6">4.6</a>&nbsp;Weakness Created by Multiple Authentication Schemes
1135      </h2>
1136      <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
1137         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.
1138      </p>
1139      <p id="rfc.section.4.6.p.2"> </p>
1140      <dl class="empty">
1141         <dd>Note that many browsers will only recognize Basic and will require that it be the first auth-scheme presented. Servers should
1142            only include Basic if it is minimally acceptable.
1143         </dd>
1144      </dl>
1145      <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
1146         authentication is only as good as that of the of the weakest of the authentication schemes. See <a href="#man.in.the.middle" title="Man in the Middle">Section&nbsp;4.8</a> below for discussion of particular attack scenarios that exploit multiple authentication schemes.
1147      </p>
1148      <h2 id="rfc.section.4.7"><a href="#rfc.section.4.7">4.7</a>&nbsp;Online dictionary attacks
1149      </h2>
1150      <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
1151         list is usually much smaller than the total number of possible passwords. The cost of computing the response for each password
1152         on the list is paid once for each challenge.
1153      </p>
1154      <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>
1155      <h2 id="rfc.section.4.8"><a href="#rfc.section.4.8">4.8</a>&nbsp;<a id="man.in.the.middle" href="#man.in.the.middle">Man in the Middle</a></h2>
1156      <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
1157         compromised proxy. Clearly, this would present all the problems of eavesdropping. But it also offers some additional opportunities
1158         to the attacker.
1159      </p>
1160      <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
1161         will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest
1162         scheme that it understands from the choices offered.
1163      </p>
1164      <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
1165         authentication, then uses the cleartext credentials from the Basic authentication to authenticate to the origin server using
1166         the stronger scheme it requested. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy
1167         caching service to gullible users.
1168      </p>
1169      <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
1170         authentication scheme is to be used, or remembering the strongest authentication scheme ever requested by a server and produce
1171         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
1172         authentication in general, or from specific sites.
1173      </p>
1174      <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
1175         course, this is still much harder than a comparable attack against Basic Authentication.
1176      </p>
1177      <h2 id="rfc.section.4.9"><a href="#rfc.section.4.9">4.9</a>&nbsp;Chosen plaintext attacks
1178      </h2>
1179      <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
1180         the response. This is called a "chosen plaintext" attack. The ability to choose the nonce is known to make cryptanalysis much
1181         easier <a href="#ref8" id="rfc.xref.ref8.1"><cite title="Message Authentication with MD5">[8]</cite></a>.
1182      </p>
1183      <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>
1184      <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;
1185         this allows the client to vary the input to the hash in a way not chosen by the attacker.
1186      </p>
1187      <h2 id="rfc.section.4.10"><a href="#rfc.section.4.10">4.10</a>&nbsp;Precomputed dictionary attacks
1188      </h2>
1189      <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
1190         for many common words to a nonce of its choice, and store a dictionary of (response, password) pairs. Such precomputation
1191         can often be done in parallel on many machines. It can then use the chosen plaintext attack to acquire a response corresponding
1192         to that challenge, and just look up the password in the dictionary. Even if most passwords are not in the dictionary, some
1193         might be. Since the attacker gets to pick the challenge, the cost of computing the response for each password on the list
1194         can be amortized over finding many passwords. A dictionary with 100 million password/response pairs would take about 3.2 gigabytes
1195         of disk storage.
1196      </p>
1197      <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>
1198      <h2 id="rfc.section.4.11"><a href="#rfc.section.4.11">4.11</a>&nbsp;Batch brute force attacks
1199      </h2>
1200      <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
1201         same nonce. It can then find all the passwords within any subset of password space that would generate one of the nonce/response
1202         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
1203         of nonce/response pairs gathered. This search of the password space can often be done in parallel on many machines, and even
1204         a single machine can search large subsets of the password space very quickly -- reports exist of searching all passwords with
1205         six or fewer letters in a few hours.
1206      </p>
1207      <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>
1208      <h2 id="rfc.section.4.12"><a href="#rfc.section.4.12">4.12</a>&nbsp;Spoofing by Counterfeit Servers
1209      </h2>
1210      <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
1211         to a host containing information protected by a password she knows, when in fact she is connecting to a hostile server, then
1212         the hostile server can request a password, store it away for later use, and feign an error. This type of attack is more difficult
1213         with Digest Authentication -- but the client must know to demand that Digest authentication be used, perhaps using some of
1214         the techniques described above to counter "man-in-the-middle" attacks. Again, the user can be helped in detecting this attack
1215         by a visual indication of the authentication mechanism in use with appropriate guidance in interpreting the implications of
1216         each scheme.
1217      </p>
1218      <h2 id="rfc.section.4.13"><a href="#rfc.section.4.13">4.13</a>&nbsp;Storing passwords
1219      </h2>
1220      <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
1221         name and password in a "password file" associated with a given realm. Normally this might contain pairs consisting of username
1222         and H(A1), where H(A1) is the digested value of the username, realm, and password as described above.
1223      </p>
1224      <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
1225         documents on the server using this realm. Unlike, say a standard UNIX password file, this information need not be decrypted
1226         in order to access documents in the server realm associated with this file. On the other hand, decryption, or more likely
1227         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
1228         data stored in the password file. It means that if one Digest authentication password file is compromised, it does not automatically
1229         compromise others with the same username and password (though it does expose them to brute force attack).
1230      </p>
1231      <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
1232         passwords, because for the purpose of accessing documents in its realm, it effectively does.
1233      </p>
1234      <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
1235         use. In particular a realm string should include the name of the host doing the authentication. The inability of the client
1236         to authenticate the server is a weakness of Digest Authentication.
1237      </p>
1238      <h2 id="rfc.section.4.14"><a href="#rfc.section.4.14">4.14</a>&nbsp;Summary
1239      </h2>
1240      <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
1241         for Basic Authentication. It remedies some, but not all, weaknesses of Basic Authentication. Its strength may vary depending
1242         on the implementation. In particular the structure of the nonce (which is dependent on the server implementation) may affect
1243         the ease of mounting a replay attack. A range of server options is appropriate since, for example, some implementations may
1244         be willing to accept the server overhead of one-time nonces or digests to eliminate the possibility of replay. Others may
1245         satisfied with a nonce like the one recommended above restricted to a single IP address and a single ETag or with a limited
1246         lifetime.
1247      </p>
1248      <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
1249         implementation will be far superior to Basic Authentication.
1250      </p>
1251      <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;Sample implementation
1252      </h1>
1253      <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
1254         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.
1255      </p>
1256      <div id="rfc.figure.u.28"></div>
1257      <p>File "digcalc.h":</p><pre class="text">#define HASHLEN 16
1258typedef char HASH[HASHLEN];
1259#define HASHHEXLEN 32
1260typedef char HASHHEX[HASHHEXLEN+1];
1261#define IN
1262#define OUT
1263
1264/* calculate H(A1) as per HTTP Digest spec */
1265void DigestCalcHA1(
1266    IN char * pszAlg,
1267    IN char * pszUserName,
1268    IN char * pszRealm,
1269    IN char * pszPassword,
1270    IN char * pszNonce,
1271    IN char * pszCNonce,
1272    OUT HASHHEX SessionKey
1273    );
1274
1275/* calculate request-digest/response-digest as per HTTP Digest spec */
1276void DigestCalcResponse(
1277    IN HASHHEX HA1,           /* H(A1) */
1278    IN char * pszNonce,       /* nonce from server */
1279    IN char * pszNonceCount,  /* 8 hex digits */
1280    IN char * pszCNonce,      /* client nonce */
1281    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1282    IN char * pszMethod,      /* method from the request */
1283    IN char * pszDigestUri,   /* requested URL */
1284    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1285    OUT HASHHEX Response      /* request-digest or response-digest */
1286    );
1287</pre><div id="rfc.figure.u.29"></div>
1288      <p>File "digcalc.c":</p><pre class="text">#include &lt;global.h&gt;
1289#include &lt;md5.h&gt;
1290#include &lt;string.h&gt;
1291#include "digcalc.h"
1292
1293void CvtHex(
1294    IN HASH Bin,
1295    OUT HASHHEX Hex
1296    )
1297{
1298    unsigned short i;
1299    unsigned char j;
1300
1301    for (i = 0; i &lt; HASHLEN; i++) {
1302        j = (Bin[i] &gt;&gt; 4) &amp; 0xf;
1303        if (j &lt;= 9)
1304            Hex[i*2] = (j + '0');
1305         else
1306            Hex[i*2] = (j + 'a' - 10);
1307        j = Bin[i] &amp; 0xf;
1308        if (j &lt;= 9)
1309            Hex[i*2+1] = (j + '0');
1310         else
1311            Hex[i*2+1] = (j + 'a' - 10);
1312    };
1313    Hex[HASHHEXLEN] = '\0';
1314};
1315
1316/* calculate H(A1) as per spec */
1317void DigestCalcHA1(
1318    IN char * pszAlg,
1319    IN char * pszUserName,
1320    IN char * pszRealm,
1321    IN char * pszPassword,
1322    IN char * pszNonce,
1323    IN char * pszCNonce,
1324    OUT HASHHEX SessionKey
1325    )
1326{
1327      MD5_CTX Md5Ctx;
1328      HASH HA1;
1329
1330      MD5Init(&amp;Md5Ctx);
1331      MD5Update(&amp;Md5Ctx, pszUserName, strlen(pszUserName));
1332      MD5Update(&amp;Md5Ctx, ":", 1);
1333      MD5Update(&amp;Md5Ctx, pszRealm, strlen(pszRealm));
1334      MD5Update(&amp;Md5Ctx, ":", 1);
1335      MD5Update(&amp;Md5Ctx, pszPassword, strlen(pszPassword));
1336      MD5Final(HA1, &amp;Md5Ctx);
1337      if (stricmp(pszAlg, "md5-sess") == 0) {
1338            MD5Init(&amp;Md5Ctx);
1339            MD5Update(&amp;Md5Ctx, HA1, HASHLEN);
1340            MD5Update(&amp;Md5Ctx, ":", 1);
1341            MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1342            MD5Update(&amp;Md5Ctx, ":", 1);
1343            MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1344            MD5Final(HA1, &amp;Md5Ctx);
1345      };
1346      CvtHex(HA1, SessionKey);
1347};
1348
1349/* calculate request-digest/response-digest as per HTTP Digest spec */
1350void DigestCalcResponse(
1351    IN HASHHEX HA1,           /* H(A1) */
1352    IN char * pszNonce,       /* nonce from server */
1353    IN char * pszNonceCount,  /* 8 hex digits */
1354    IN char * pszCNonce,      /* client nonce */
1355    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1356    IN char * pszMethod,      /* method from the request */
1357    IN char * pszDigestUri,   /* requested URL */
1358    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1359    OUT HASHHEX Response      /* request-digest or response-digest */
1360    )
1361{
1362      MD5_CTX Md5Ctx;
1363      HASH HA2;
1364      HASH RespHash;
1365       HASHHEX HA2Hex;
1366
1367      // calculate H(A2)
1368      MD5Init(&amp;Md5Ctx);
1369      MD5Update(&amp;Md5Ctx, pszMethod, strlen(pszMethod));
1370      MD5Update(&amp;Md5Ctx, ":", 1);
1371      MD5Update(&amp;Md5Ctx, pszDigestUri, strlen(pszDigestUri));
1372      if (stricmp(pszQop, "auth-int") == 0) {
1373            MD5Update(&amp;Md5Ctx, ":", 1);
1374            MD5Update(&amp;Md5Ctx, HEntity, HASHHEXLEN);
1375      };
1376      MD5Final(HA2, &amp;Md5Ctx);
1377       CvtHex(HA2, HA2Hex);
1378
1379      // calculate response
1380      MD5Init(&amp;Md5Ctx);
1381      MD5Update(&amp;Md5Ctx, HA1, HASHHEXLEN);
1382      MD5Update(&amp;Md5Ctx, ":", 1);
1383      MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1384      MD5Update(&amp;Md5Ctx, ":", 1);
1385      if (*pszQop) {
1386          MD5Update(&amp;Md5Ctx, pszNonceCount, strlen(pszNonceCount));
1387          MD5Update(&amp;Md5Ctx, ":", 1);
1388          MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1389          MD5Update(&amp;Md5Ctx, ":", 1);
1390          MD5Update(&amp;Md5Ctx, pszQop, strlen(pszQop));
1391          MD5Update(&amp;Md5Ctx, ":", 1);
1392      };
1393      MD5Update(&amp;Md5Ctx, HA2Hex, HASHHEXLEN);
1394      MD5Final(RespHash, &amp;Md5Ctx);
1395      CvtHex(RespHash, Response);
1396};
1397</pre><div id="rfc.figure.u.30"></div>
1398      <p>File "digtest.c":</p><pre class="text">#include &lt;stdio.h&gt;
1399#include "digcalc.h"
1400
1401void main(int argc, char ** argv) {
1402
1403      char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
1404      char * pszCNonce = "0a4f113b";
1405      char * pszUser = "Mufasa";
1406      char * pszRealm = "testrealm@host.com";
1407      char * pszPass = "Circle Of Life";
1408      char * pszAlg = "md5";
1409      char szNonceCount[9] = "00000001";
1410      char * pszMethod = "GET";
1411      char * pszQop = "auth";
1412      char * pszURI = "/dir/index.html";
1413      HASHHEX HA1;
1414      HASHHEX HA2 = "";
1415      HASHHEX Response;
1416
1417      DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass, pszNonce,
1418pszCNonce, HA1);
1419      DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
1420       pszMethod, pszURI, HA2, Response);
1421      printf("Response = %s\n", Response);
1422};
1423</pre><h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;Acknowledgments
1424      </h1>
1425      <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>
1426      <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,
1427         Ned Freed, and David M. Kristol.
1428      </p>
1429      <p id="rfc.section.6.p.3">Jim Gettys and Larry Masinter edited this document for update.</p>
1430      <h1 id="rfc.references"><a href="#rfc.section.7" id="rfc.section.7">7.</a> References
1431      </h1>
1432      <table summary="References"> 
1433         <tr>
1434            <td class="reference"><b id="RFC1945">[1]</b></td>
1435            <td class="top"><a href="mailto:timbl@w3.org" title="MIT, Laboratory for Computer Science">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:frystyk@w3.org" title="W3 Consortium, MIT Laboratory for Computer Science">H.F. Nielsen</a>, “<a href="http://tools.ietf.org/html/rfc1945">Hypertext Transfer Protocol -- HTTP/1.0</a>”, RFC&nbsp;1945, May&nbsp;1996.
1436            </td>
1437         </tr> 
1438         <tr>
1439            <td class="reference"><b id="RFC2616">[2]</b></td>
1440            <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Information and Computer Science">Fielding, R.T.</a>, <a href="mailto:jg@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation, Western Research Laboratory">Mogul, J.C.</a>, <a href="mailto:frystyk@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">Nielsen, H.F.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.J.</a>, and <a href="mailto:timbl@w3.org" title="World Wide Web Consortium, MIT Laboratory for Computer Science">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC&nbsp;2616, June&nbsp;1999.
1441            </td>
1442         </tr> 
1443         <!--WARNING: unused reference 'RFC1321'-->
1444         <tr>
1445            <td class="reference"><b id="RFC1321">[3]</b></td>
1446            <td class="top"><a href="mailto:rivest@theory.lcs.mit.edu" title="Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science">Rivest, R.</a>, “<a href="http://tools.ietf.org/html/rfc1321">The MD5 Message-Digest Algorithm</a>”, RFC&nbsp;1321, April&nbsp;1992.
1447            </td>
1448         </tr> 
1449         <!--WARNING: unused reference 'RFC2045'-->
1450         <tr>
1451            <td class="reference"><b id="RFC2045">[4]</b></td>
1452            <td class="top"><a href="mailto:ned@innosoft.com" title="Innosoft International, Inc.">Freed, N.</a> and <a href="mailto:nsb@nsb.fv.com" title="First Virtual Holdings">N.S. Borenstein</a>, “<a href="http://tools.ietf.org/html/rfc2045">Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</a>”, RFC&nbsp;2045, November&nbsp;1996.
1453            </td>
1454         </tr> 
1455         <tr>
1456            <td class="reference"><b id="RFC2246">[5]</b></td>
1457            <td class="top"><a href="mailto:tdierks@certicom.com" title="Certicom">Dierks, T.</a> and <a href="mailto:callen@certicom.com" title="Certicom">C. Allen</a>, “<a href="http://tools.ietf.org/html/rfc2246">The TLS Protocol Version 1.0</a>”, RFC&nbsp;2246, January&nbsp;1999.
1458            </td>
1459         </tr> 
1460         <tr>
1461            <td class="reference"><b id="RFC2069">[6]</b></td>
1462            <td class="top"><a href="mailto:john@math.nwu.edu" title="Northwestern University,  Department of Mathematics">Franks, J.</a>, <a href="mailto:hallam@w3.org" title="CERN">Hallam-Baker, P.</a>, <a href="mailto:jeff@spyglass.com" title="Spyglass, Inc.">Hostetler, J.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:luotonen@netscape.com" title="Netscape Communications Corporation">Luotonen, A.</a>, <a href="mailto:eric@spyglass.com" title="Spyglass, Inc.">Sink, E.</a>, and <a href="mailto:stewart@OpenMarket.com" title="Open Market, Inc.">L. Stewart</a>, “<a href="http://tools.ietf.org/html/rfc2069">An Extension to HTTP : Digest Access Authentication</a>”, RFC&nbsp;2069, January&nbsp;1997.
1463            </td>
1464         </tr> 
1465         <tr>
1466            <td class="reference"><b id="RFC2396">[7]</b></td>
1467            <td class="top"><a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:fielding@ics.uci.edu" title="Department of Information and Computer Science">Fielding, R.T.</a>, and <a href="mailto:masinter@parc.xerox.com" title="Xerox PARC">L. Masinter</a>, “<a href="http://tools.ietf.org/html/rfc2396">Uniform Resource Identifiers (URI): Generic Syntax</a>”, RFC&nbsp;2396, August&nbsp;1998.
1468            </td>
1469         </tr> 
1470         <tr>
1471            <td class="reference"><b id="ref8">[8]</b></td>
1472            <td class="top">Kaliski, B. and M. Robshaw, “<a href="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">Message Authentication with MD5</a>”, 1995, &lt;<a href="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm</a>&gt;.<br>CryptoBytes, Spring 1995
1473            </td>
1474         </tr> 
1475         <tr>
1476            <td class="reference"><b id="RFC2195">[9]</b></td>
1477            <td class="top"><a href="mailto:klensin@mci.net" title="MCI">Klensin, J.C.</a>, <a href="mailto:randy@mci.net" title="MCI">Catoe, R.</a>, and <a href="mailto:paul@mci.net" title="MCI">P. Krumviede</a>, “<a href="http://tools.ietf.org/html/rfc2195">IMAP/POP AUTHorize Extension for Simple Challenge/Response</a>”, RFC&nbsp;2195, September&nbsp;1997.
1478            </td>
1479         </tr> 
1480         <tr>
1481            <td class="reference"><b id="ref10">[10]</b></td>
1482            <td class="top">Morgan, B., Alvestrand, H., Hodges, J., and M. Wahl, “Authentication Methods for LDAP”.<br>Work in progress.
1483            </td>
1484         </tr> 
1485      </table>
1486      <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
1487      <address class="vcard"><span class="vcardline"><span class="fn">John Franks</span><span class="n hidden"><span class="family-name">Franks</span><span class="given-name">John</span></span></span><span class="org vcardline">Northwestern University, Department of Mathematics</span><span class="adr"><span class="street-address vcardline">Northwestern University</span><span class="vcardline"><span class="locality">Evanston</span>, <span class="region">IL</span>&nbsp;<span class="postal-code">60208-2730</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:john@math.nwu.edu"><span class="email">john@math.nwu.edu</span></a></span></address>
1488      <address class="vcard"><span class="vcardline"><span class="fn">Phillip M. Hallam-Baker</span><span class="n hidden"><span class="family-name">Hallam-Baker</span><span class="given-name">Phillip M.</span></span></span><span class="org vcardline">Verisign Inc.</span><span class="adr"><span class="street-address vcardline">301 Edgewater Place</span><span class="street-address vcardline">Suite 210</span><span class="vcardline"><span class="locality">Wakefield</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01880</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:pbaker@verisign.com"><span class="email">pbaker@verisign.com</span></a></span></address>
1489      <address class="vcard"><span class="vcardline"><span class="fn">Jeffery L. Hostetler</span><span class="n hidden"><span class="family-name">Hostetler</span><span class="given-name">Jeffery L.</span></span></span><span class="org vcardline">AbiSource, Inc.</span><span class="adr"><span class="street-address vcardline">6 Dunlap Court</span><span class="vcardline"><span class="locality">Savoy</span>, <span class="region">IL</span>&nbsp;<span class="postal-code">61874</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:jeff@AbiSource.com"><span class="email">jeff@AbiSource.com</span></a></span></address>
1490      <address class="vcard"><span class="vcardline"><span class="fn">Scott D. Lawrence</span><span class="n hidden"><span class="family-name">Lawrence</span><span class="given-name">Scott D.</span></span></span><span class="org vcardline">Agranat Systems, Inc.</span><span class="adr"><span class="street-address vcardline">5 Clocktower Place</span><span class="street-address vcardline">Suite 400</span><span class="vcardline"><span class="locality">Maynard</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">01754</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:lawrence@agranat.com"><span class="email">lawrence@agranat.com</span></a></span></address>
1491      <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span>&nbsp;<span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address>
1492      <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>
1493      <address class="vcard"><span class="vcardline"><span class="fn">Lawrence C. Stewart</span><span class="n hidden"><span class="family-name">Stewart</span><span class="given-name">Lawrence C.</span></span></span><span class="org vcardline">Open Market, Inc.</span><span class="adr"><span class="street-address vcardline">215 First Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span>&nbsp;<span class="postal-code">02142</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">EMail: <a href="mailto:stewart@OpenMarket.com"><span class="email">stewart@OpenMarket.com</span></a></span></address>
1494      <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1>
1495      <p>Copyright © The Internet Society (1999). All Rights Reserved.</p>
1496      <p>This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise
1497         explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without
1498         restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative
1499         works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references
1500         to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards
1501         in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to
1502         translate it into languages other than English.
1503      </p>
1504      <p>The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees.</p>
1505      <p>This document and the information contained herein is provided on an “AS IS” basis and THE INTERNET SOCIETY AND THE INTERNET
1506         ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
1507         OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
1508         PURPOSE.
1509      </p>
1510      <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1>
1511      <p>The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed
1512         to pertain to the implementation or use of the technology described in this document or the extent to which any license under
1513         such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights.
1514         Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be
1515         found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available,
1516         or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors
1517         or users of this specification can be obtained from the IETF Secretariat.
1518      </p>
1519      <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary
1520         rights which may cover technology that may be required to practice this standard. Please address the information to the IETF
1521         Executive Director.
1522      </p>
1523      <h1>Acknowledgment</h1>
1524      <p>Funding for the RFC Editor function is currently provided by the Internet Society.</p>
1525      <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
1526      <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> 
1527      </p>
1528      <div class="print2col">
1529         <ul class="ind">
1530            <li class="indline0"><a id="rfc.index.A" href="#rfc.index.A"><b>A</b></a><ul class="ind">
1531                  <li class="indline1"><tt>algorithm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.3"><b>3.2.1</b></a></li>
1532                  <li class="indline1"><tt>auth-info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.7"><b>3.2.3</b></a></li>
1533                  <li class="indline1"><tt>auth-param</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.2"><b>1.2</b></a></li>
1534                  <li class="indline1"><tt>auth-scheme</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.1"><b>1.2</b></a></li>
1535                  <li class="indline1"><tt>Authentication-Info</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.6"><b>3.2.3</b></a></li>
1536                  <li class="indline1">Authentication-Info header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.5"><b>3.2.3</b></a></li>
1537                  <li class="indline1">Authorization header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.a.4"><b>3.2.2</b></a></li>
1538               </ul>
1539            </li>
1540            <li class="indline0"><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul class="ind">
1541                  <li class="indline1"><tt>base64-user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.2"><b>2</b></a></li>
1542                  <li class="indline1"><tt>basic-credentials</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.b.1"><b>2</b></a></li>
1543               </ul>
1544            </li>
1545            <li class="indline0"><a id="rfc.index.C" href="#rfc.index.C"><b>C</b></a><ul class="ind">
1546                  <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>
1547                  <li class="indline1"><tt>cnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.2</b></a></li>
1548                  <li class="indline1"><tt>cnonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.2</b></a></li>
1549                  <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>
1550               </ul>
1551            </li>
1552            <li class="indline0"><a id="rfc.index.D" href="#rfc.index.D"><b>D</b></a><ul class="ind">
1553                  <li class="indline1"><tt>digest-challenge</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.1"><b>3.2.1</b></a></li>
1554                  <li class="indline1"><tt>digest-response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.3"><b>3.2.2</b></a></li>
1555                  <li class="indline1"><tt>digest-uri</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.4"><b>3.2.2</b></a></li>
1556                  <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>
1557                  <li class="indline1"><tt>domain</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.d.2"><b>3.2.1</b></a></li>
1558               </ul>
1559            </li>
1560            <li class="indline0"><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul class="ind">
1561                  <li class="indline1">Headers&nbsp;&nbsp;
1562                     <ul class="ind">
1563                        <li class="indline1">Authentication-Info&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.3"><b>3.2.3</b></a></li>
1564                        <li class="indline1">Authorization&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.2"><b>3.2.2</b></a></li>
1565                        <li class="indline1">WWW-Authenticate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.h.1"><b>3.2.1</b></a></li>
1566                     </ul>
1567                  </li>
1568               </ul>
1569            </li>
1570            <li class="indline0"><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul class="ind">
1571                  <li class="indline1"><tt>LHEX</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.l.1"><b>3.2.2</b></a></li>
1572               </ul>
1573            </li>
1574            <li class="indline0"><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul class="ind">
1575                  <li class="indline1"><tt>message-qop</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.2</b></a></li>
1576               </ul>
1577            </li>
1578            <li class="indline0"><a id="rfc.index.N" href="#rfc.index.N"><b>N</b></a><ul class="ind">
1579                  <li class="indline1"><tt>nc-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
1580                  <li class="indline1"><tt>nextnonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.5"><b>3.2.3</b></a></li>
1581                  <li class="indline1"><tt>nonce</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a></li>
1582                  <li class="indline1"><tt>nonce-count</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.2</b></a></li>
1583                  <li class="indline1"><tt>nonce-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a></li>
1584               </ul>
1585            </li>
1586            <li class="indline0"><a id="rfc.index.O" href="#rfc.index.O"><b>O</b></a><ul class="ind">
1587                  <li class="indline1"><tt>opaque</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
1588               </ul>
1589            </li>
1590            <li class="indline0"><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul class="ind">
1591                  <li class="indline1"><tt>password</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>2</b></a></li>
1592               </ul>
1593            </li>
1594            <li class="indline0"><a id="rfc.index.Q" href="#rfc.index.Q"><b>Q</b></a><ul class="ind">
1595                  <li class="indline1"><tt>qop-options</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.1"><b>3.2.1</b></a></li>
1596                  <li class="indline1"><tt>qop-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.q.2"><b>3.2.1</b></a></li>
1597               </ul>
1598            </li>
1599            <li class="indline0"><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul class="ind">
1600                  <li class="indline1"><tt>realm</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.1"><b>1.2</b></a></li>
1601                  <li class="indline1"><tt>realm-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.2"><b>1.2</b></a></li>
1602                  <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>
1603                  <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>
1604                  <li class="indline1"><tt>request-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.4"><b>3.2.2</b></a></li>
1605                  <li class="indline1"><tt>response</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.3"><b>3.2.2</b></a></li>
1606                  <li class="indline1"><tt>response-auth</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.5"><b>3.2.3</b></a></li>
1607                  <li class="indline1"><tt>response-digest</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.r.6"><b>3.2.3</b></a></li>
1608                  <li class="indline1"><em>RFC1321</em>&nbsp;&nbsp;<a class="iref" href="#RFC1321"><b>7</b></a></li>
1609                  <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>
1610                  <li class="indline1"><em>RFC2045</em>&nbsp;&nbsp;<a class="iref" href="#RFC2045"><b>7</b></a></li>
1611                  <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>
1612                  <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>
1613                  <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>
1614                  <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>
1615                  <li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.2">1.1</a>, <a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.4">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.5">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.6">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.7">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.8">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.9">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.10">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.11">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.12">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.13">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.14">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.15">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.16">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.17">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.18">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.19">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.20">3.6</a>, <a class="iref" href="#rfc.xref.RFC2616.21">3.6</a>, <a class="iref" href="#RFC2616"><b>7</b></a><ul class="ind">
1616                        <li class="indline1"><em>Section 10.33</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.19">3.6</a></li>
1617                        <li class="indline1"><em>Section 10.34</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.20">3.6</a></li>
1618                     </ul>
1619                     <ul class="ind">
1620                        <li class="indline1"><em>Section 2.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.2">1.1</a></li>
1621                        <li class="indline1"><em>Section 5.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.9">3.2.2.5</a></li>
1622                        <li class="indline1"><em>Section 5.1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.3">1.2</a>, <a class="iref" href="#rfc.xref.RFC2616.11">3.2.2.5</a>, <a class="iref" href="#rfc.xref.RFC2616.13">3.2.2.5</a></li>
1623                        <li class="indline1"><em>Section 13.5.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.7">1.2</a></li>
1624                        <li class="indline1"><em>Section 13.7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.15">3.2.2.5</a></li>
1625                        <li class="indline1"><em>Section 14.8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.5">1.2</a></li>
1626                        <li class="indline1"><em>Section 14.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.17">3.2.2.5</a></li>
1627                     </ul>
1628                  </li>
1629               </ul>
1630            </li>
1631            <li class="indline0"><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul class="ind">
1632                  <li class="indline1"><tt>stale</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1"><b>3.2.1</b></a></li>
1633               </ul>
1634            </li>
1635            <li class="indline0"><a id="rfc.index.U" href="#rfc.index.U"><b>U</b></a><ul class="ind">
1636                  <li class="indline1"><tt>URI</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.3"><b>3.2.1</b></a></li>
1637                  <li class="indline1"><tt>user-pass</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.1"><b>2</b></a></li>
1638                  <li class="indline1"><tt>userid</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.2"><b>2</b></a></li>
1639                  <li class="indline1"><tt>username</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.4"><b>3.2.2</b></a></li>
1640                  <li class="indline1"><tt>username-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.u.5"><b>3.2.2</b></a></li>
1641               </ul>
1642            </li>
1643            <li class="indline0"><a id="rfc.index.W" href="#rfc.index.W"><b>W</b></a><ul class="ind">
1644                  <li class="indline1">WWW-Authenticate header&nbsp;&nbsp;<a class="iref" href="#rfc.iref.w.1"><b>3.2.1</b></a></li>
1645               </ul>
1646            </li>
1647         </ul>
1648      </div>
1649   </body>
1650</html>
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