source: draft-ietf-httpbis/orig/rfc2617.xml @ 50

Last change on this file since 50 was 9, checked in by fielding@…, 12 years ago

RFC 2617: HTTP Authentication: Basic and Digest Access Authentication

Translated from text to XML by Julian Reschke.

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1<?xml version="1.0" encoding="utf-8"?>
2<?rfc toc="yes"?>
3<?rfc symrefs="no"?>
4<?rfc sortrefs="no"?>
5<?rfc compact="yes"?>
6<?rfc subcompact="no"?>
7<?rfc-ext allow-markup-in-artwork="yes"?>
8<?rfc-ext include-references-in-index="yes" ?>
9
10<!DOCTYPE rfc [
11  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
12  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
13  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
14  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
15  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
16  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
17  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
18  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
19  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
20  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
21]>
22<rfc number="2617" category="std" obsoletes="2069" xmlns:x='http://purl.org/net/xml2rfc/ext'>
23  <front>
24    <title abbrev="HTTP Authentication">HTTP Authentication: Basic and Digest Access Authentication</title>
25    <author initials="J." surname="Franks" fullname="John Franks">
26      <organization>Northwestern University, Department of Mathematics</organization>
27      <address>
28        <postal>
29          <street>Northwestern University</street>
30          <city>Evanston</city>
31          <region>IL</region>
32          <code>60208-2730</code>
33          <country>USA</country>
34        </postal>
35        <email>john@math.nwu.edu</email>
36      </address>
37    </author>
38    <author initials="P.M." surname="Hallam-Baker" fullname="Phillip M. Hallam-Baker">
39      <organization>Verisign Inc.</organization>
40      <address>
41        <postal>
42          <street>301 Edgewater Place</street>
43          <street>Suite 210</street>
44          <city>Wakefield</city>
45          <region>MA</region>
46          <code>01880</code>
47          <country>USA</country>
48        </postal>
49        <email>pbaker@verisign.com</email>
50      </address>
51    </author>
52    <author initials="J.L." surname="Hostetler" fullname="Jeffery L. Hostetler">
53    <organization>AbiSource, Inc.</organization>
54      <address>
55        <postal>
56          <street>6 Dunlap Court</street>
57          <city>Savoy</city>
58          <region>IL</region>
59          <code>61874</code>
60          <country>USA</country>
61        </postal>
62        <email>jeff@AbiSource.com</email>
63      </address>
64    </author>
65    <author initials="S.D." surname="Lawrence" fullname="Scott D. Lawrence">
66      <organization>Agranat Systems, Inc.</organization>
67      <address>
68        <postal>
69          <street>5 Clocktower Place</street>
70          <street>Suite 400</street>
71          <city>Maynard</city>
72          <region>MA</region>
73          <code>01754</code>
74          <country>USA</country>
75        </postal>
76        <email>lawrence@agranat.com</email>
77      </address>
78    </author>
79    <author initials="P.J." surname="Leach" fullname="Paul J. Leach">
80      <organization>Microsoft Corporation</organization>
81      <address>
82        <postal>
83          <street>1 Microsoft Way</street>
84          <city>Redmond</city>
85          <region>WA</region>
86          <code>98052</code>
87          <country>USA</country>
88        </postal>
89        <email>paulle@microsoft.com</email>
90      </address>
91    </author>
92    <author initials="A." surname="Luotonen" fullname="Ari Luotonen">
93      <organization>Netscape Communications Corporation</organization>
94      <address>
95        <postal>
96          <street>501 East Middlefield Road</street>
97          <city>Mountain View</city>
98          <region>CA</region>
99          <code>94043</code>
100          <country>USA</country>
101        </postal>
102      </address>
103    </author>
104    <author initials="L." surname="Stewart" fullname="Lawrence C. Stewart">
105      <organization>Open Market, Inc.</organization>
106      <address>
107        <postal>
108          <street>215 First Street</street>
109          <city>Cambridge</city>
110          <region>MA</region>
111          <code>02142</code>
112          <country>USA</country>
113        </postal>
114        <email>stewart@OpenMarket.com</email>
115      </address>
116    </author>
117    <date month="June" year="1999"/>
118       
119    <abstract>
120      <t>
121   "HTTP/1.0", includes the specification for a Basic Access
122   Authentication scheme. This scheme is not considered to be a secure
123   method of user authentication (unless used in conjunction with some
124   external secure system such as SSL <xref target="RFC2246"/>), as the user name and
125   password are passed over the network as cleartext.
126      </t><t>
127   This document also provides the specification for HTTP's
128   authentication framework, the original Basic authentication scheme
129   and a scheme based on cryptographic hashes, referred to as "Digest
130   Access Authentication".  It is therefore also intended to serve as a
131   replacement for RFC 2069 <xref target="RFC2069"/>.  Some optional elements specified by
132   RFC 2069 have been removed from this specification due to problems
133   found since its publication; other new elements have been added for
134   compatibility, those new elements have been made optional, but are
135   strongly recommended.
136      </t><t>
137   Like Basic, Digest access authentication verifies that both parties
138   to a communication know a shared secret (a password); unlike Basic,
139   this verification can be done without sending the password in the
140   clear, which is Basic's biggest weakness. As with most other
141   authentication protocols, the greatest sources of risks are usually
142   found not in the core protocol itself but in policies and procedures
143   surrounding its use.
144    </t>
145    </abstract>
146  </front>
147  <middle>
148 
149<section title="Access Authentication" anchor="access.authentication">
150
151<section title="Reliance on the HTTP/1.1 Specification">
152<t>
153   This specification is a companion to the HTTP/1.1 specification <xref target="RFC2616"/>.
154   It uses the augmented BNF section <xref target="RFC2616" x:fmt="number" x:sec="2.1"/> of that document, and relies on
155   both the non-terminals defined in that document and other aspects of
156   the HTTP/1.1 specification.
157</t>
158</section>
159
160<section title="Access Authentication Framework" anchor="access.authentication.framework">
161<t>
162   HTTP provides a simple challenge-response authentication mechanism
163   that &MAY; be used by a server to challenge a client request and by a
164   client to provide authentication information. It uses an extensible,
165   case-insensitive token to identify the authentication scheme,
166   followed by a comma-separated list of attribute-value pairs which
167   carry the parameters necessary for achieving authentication via that
168   scheme.
169</t>
170<figure><artwork type="abnf2616"><iref item="auth-scheme" primary="true"
171/>      auth-scheme    = token
172<iref item="auth-param" primary="true"
173/>      auth-param     = token "=" ( token | quoted-string )
174</artwork></figure>
175<t>
176   The 401 (Unauthorized) response message is used by an origin server
177   to challenge the authorization of a user agent. This response &MUST;
178   include a WWW-Authenticate header field containing at least one
179   challenge applicable to the requested resource. The 407 (Proxy
180   Authentication Required) response message is used by a proxy to
181   challenge the authorization of a client and &MUST; include a Proxy-Authenticate
182   header field containing at least one challenge
183   applicable to the proxy for the requested resource.
184</t>
185<figure><artwork type="abnf2616"><iref item="challenge" primary="true"
186/>      challenge   = auth-scheme 1*SP 1#auth-param
187</artwork></figure>
188<t>
189   Note: User agents will need to take special care in parsing the WWW-Authenticate
190   or Proxy-Authenticate header field value if it contains
191   more than one challenge, or if more than one WWW-Authenticate header
192   field is provided, since the contents of a challenge may itself
193   contain a comma-separated list of authentication parameters.
194</t>
195<t>
196   The authentication parameter realm is defined for all authentication
197   schemes:
198</t>
199<figure><artwork type="abnf2616"><iref item="realm" primary="true"
200/>      realm       = "realm" "=" realm-value
201<iref item="realm-value" primary="true"
202/>      realm-value = quoted-string
203</artwork></figure>
204<t>
205   The realm directive (case-insensitive) is required for all
206   authentication schemes that issue a challenge. The realm value
207   (case-sensitive), in combination with the canonical root URL (the
208   absoluteURI for the server whose abs_path is empty; see section <xref target="RFC2616" x:fmt="number" x:sec="5.1.2"/>
209   of <xref target="RFC2616"/>) of the server being accessed, defines the protection space.
210   These realms allow the protected resources on a server to be
211   partitioned into a set of protection spaces, each with its own
212   authentication scheme and/or authorization database. The realm value
213   is a string, generally assigned by the origin server, which may have
214   additional semantics specific to the authentication scheme. Note that
215   there may be multiple challenges with the same auth-scheme but
216   different realms.
217</t>
218<t>
219   A user agent that wishes to authenticate itself with an origin
220   server--usually, but not necessarily, after receiving a 401
221   (Unauthorized)--MAY do so by including an Authorization header field
222   with the request. A client that wishes to authenticate itself with a
223   proxy--usually, but not necessarily, after receiving a 407 (Proxy
224   Authentication Required)--MAY do so by including a Proxy-Authorization
225   header field with the request.  Both the Authorization
226   field value and the Proxy-Authorization field value consist of
227   credentials containing the authentication information of the client
228   for the realm of the resource being requested. The user agent &MUST;
229   choose to use one of the challenges with the strongest auth-scheme it
230   understands and request credentials from the user based upon that
231   challenge.
232</t>
233<figure><artwork type="abnf2616"><iref item="credentials" primary="true"
234/>   credentials = auth-scheme #auth-param
235</artwork></figure>
236<t>
237  <list><t>
238      Note that many browsers will only recognize Basic and will require
239      that it be the first auth-scheme presented. Servers should only
240      include Basic if it is minimally acceptable.
241  </t></list>
242</t>
243<t>
244   The protection space determines the domain over which credentials can
245   be automatically applied. If a prior request has been authorized, the
246   same credentials &MAY; be reused for all other requests within that
247   protection space for a period of time determined by the
248   authentication scheme, parameters, and/or user preference. Unless
249   otherwise defined by the authentication scheme, a single protection
250   space cannot extend outside the scope of its server.
251</t>
252<t>
253   If the origin server does not wish to accept the credentials sent
254   with a request, it &SHOULD; return a 401 (Unauthorized) response. The
255   response &MUST; include a WWW-Authenticate header field containing at
256   least one (possibly new) challenge applicable to the requested
257   resource. If a proxy does not accept the credentials sent with a
258   request, it &SHOULD; return a 407 (Proxy Authentication Required). The
259   response &MUST; include a Proxy-Authenticate header field containing a
260   (possibly new) challenge applicable to the proxy for the requested
261   resource.
262</t>
263<t>
264   The HTTP protocol does not restrict applications to this simple
265   challenge-response mechanism for access authentication. Additional
266   mechanisms &MAY; be used, such as encryption at the transport level or
267   via message encapsulation, and with additional header fields
268   specifying authentication information. However, these additional
269   mechanisms are not defined by this specification.
270</t>
271<t>
272   Proxies &MUST; be completely transparent regarding user agent
273   authentication by origin servers. That is, they must forward the
274   WWW-Authenticate and Authorization headers untouched, and follow the
275   rules found in section <xref target="RFC2616" x:fmt="number" x:sec="14.8"/> of <xref target="RFC2616"/>. Both the Proxy-Authenticate and
276   the Proxy-Authorization header fields are hop-by-hop headers (see
277   section <xref target="RFC2616" x:fmt="number" x:sec="13.5.1"/> of <xref target="RFC2616"/>).
278</t>
279</section>
280</section>
281   
282<section title="Basic Authentication Scheme">
283<t>
284   The "basic" authentication scheme is based on the model that the
285   client must authenticate itself with a user-ID and a password for
286   each realm.  The realm value should be considered an opaque string
287   which can only be compared for equality with other realms on that
288   server. The server will service the request only if it can validate
289   the user-ID and password for the protection space of the Request-URI.
290   There are no optional authentication parameters.
291</t>
292<t>
293   For Basic, the framework above is utilized as follows:
294</t>
295<figure><artwork type="abnf2616"><iref item="challenge"
296/>      challenge   = "Basic" realm
297<iref item="credentials"
298/>      credentials = "Basic" basic-credentials
299</artwork></figure>
300<t>
301   Upon receipt of an unauthorized request for a URI within the
302   protection space, the origin server &MAY; respond with a challenge like
303   the following:
304</t>
305<figure><artwork type="example">
306      WWW-Authenticate: Basic realm="WallyWorld"
307</artwork></figure>
308<t>
309   where "WallyWorld" is the string assigned by the server to identify
310   the protection space of the Request-URI. A proxy may respond with the
311   same challenge using the Proxy-Authenticate header field.
312</t>
313<t>
314   To receive authorization, the client sends the userid and password,
315   separated by a single colon (":") character, within a base64 <xref target="RFC2396"/>
316   encoded string in the credentials.
317</t>
318<figure><artwork type="abnf2616"><iref item="basic-credentials" primary="true"
319/>      basic-credentials = base64-user-pass
320<iref item="base64-user-pass" primary="true"
321/>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
322                       except not limited to 76 char/line>
323<iref item="user-pass" primary="true"
324/>      user-pass   = userid ":" password
325<iref item="userid" primary="true"
326/>      userid      = *&lt;TEXT excluding ":">
327<iref item="password" primary="true"
328/>      password    = *TEXT
329</artwork></figure>
330<t>
331   Userids might be case sensitive.
332</t>
333<t>
334   If the user agent wishes to send the userid "Aladdin" and password
335   "open sesame", it would use the following header field:
336</t>
337<figure><artwork type="example">
338      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
339</artwork></figure>
340<t>
341   A client &SHOULD; assume that all paths at or deeper than the depth of
342   the last symbolic element in the path field of the Request-URI also
343   are within the protection space specified by the Basic realm value of
344   the current challenge. A client &MAY; preemptively send the
345   corresponding Authorization header with requests for resources in
346   that space without receipt of another challenge from the server.
347   Similarly, when a client sends a request to a proxy, it may reuse a
348   userid and password in the Proxy-Authorization header field without
349   receiving another challenge from the proxy server. See <xref target="security.considerations"/> for
350   security considerations associated with Basic authentication.
351</t>
352</section>
353   
354<section title="Digest Access Authentication Scheme">
355
356<section title="Introduction">
357
358<section title="Purpose">
359<t>
360   The protocol referred to as "HTTP/1.0" includes the specification for
361   a Basic Access Authentication scheme<xref target="RFC1945"/>. That scheme is not
362   considered to be a secure method of user authentication, as the user
363   name and password are passed over the network in an unencrypted form.
364   This section provides the specification for a scheme that does not
365   send the password in cleartext,  referred to as "Digest Access
366   Authentication".
367</t>
368<t>
369   The Digest Access Authentication scheme is not intended to be a
370   complete answer to the need for security in the World Wide Web. This
371   scheme provides no encryption of message content. The intent is
372   simply to create an access authentication method that avoids the most
373   serious flaws of Basic authentication.
374</t>
375</section>
376
377<section title="Overall Operation">
378<t>
379   Like Basic Access Authentication, the Digest scheme is based on a
380   simple challenge-response paradigm. The Digest scheme challenges
381   using a nonce value. A valid response contains a checksum (by
382   default, the MD5 checksum) of the username, the password, the given
383   nonce value, the HTTP method, and the requested URI. In this way, the
384   password is never sent in the clear. Just as with the Basic scheme,
385   the username and password must be prearranged in some fashion not
386   addressed by this document.
387</t>
388</section>
389
390<section title="Representation of digest values">
391<t>
392   An optional header allows the server to specify the algorithm used to
393   create the checksum or digest. By default the MD5 algorithm is used
394   and that is the only algorithm described in this document.
395</t>
396<t>
397   For the purposes of this document, an MD5 digest of 128 bits is
398   represented as 32 ASCII printable characters. The bits in the 128 bit
399   digest are converted from most significant to least significant bit,
400   four bits at a time to their ASCII presentation as follows. Each four
401   bits is represented by its familiar hexadecimal notation from the
402   characters 0123456789abcdef. That is, binary 0000 gets represented by
403   the character '0', 0001, by '1', and so on up to the representation
404   of 1111 as 'f'.
405</t>
406</section>
407
408<section title="Limitations">
409<t>
410   The Digest authentication scheme described in this document suffers
411   from many known limitations. It is intended as a replacement for
412   Basic authentication and nothing more. It is a password-based system
413   and (on the server side) suffers from all the same problems of any
414   password system. In particular, no provision is made in this protocol
415   for the initial secure arrangement between user and server to
416   establish the user's password.
417</t>
418<t>
419   Users and implementors should be aware that this protocol is not as
420   secure as Kerberos, and not as secure as any client-side private-key
421   scheme. Nevertheless it is better than nothing, better than what is
422   commonly used with telnet and ftp, and better than Basic
423   authentication.
424</t>
425</section>
426</section>
427
428<section title="Specification of Digest Headers" anchor="specification.of.digest.headers">
429<t>
430   The Digest Access Authentication scheme is conceptually similar to
431   the Basic scheme. The formats of the modified WWW-Authenticate header
432   line and the Authorization header line are specified below. In
433   addition, a new header, Authentication-Info, is specified.
434</t>
435
436<section title="The WWW-Authenticate Response Header" anchor="the.www-authenticate.response.header">
437<iref item="Headers" subitem="WWW-Authenticate" primary="true"/>
438<iref item="WWW-Authenticate header" primary="true"/>
439<t>
440   If a server receives a request for an access-protected object, and an
441   acceptable Authorization header is not sent, the server responds with
442   a "401 Unauthorized" status code, and a WWW-Authenticate header as
443   per the framework defined above, which for the digest scheme is
444   utilized as follows:
445</t>
446<figure><artwork type="abnf2616">
447<iref item="challenge"
448/>      challenge        =  "Digest" digest-challenge
449
450<iref item="digest-challenge" primary="true"
451/>      digest-challenge  = 1#( realm | [ domain ] | nonce |
452                          [ opaque ] |[ stale ] | [ algorithm ] |
453                          [ qop-options ] | [auth-param] )
454
455
456<iref item="domain" primary="true"
457/>      domain            = "domain" "=" &lt;"> URI ( 1*SP URI ) &lt;">
458<iref item="URI" primary="true"
459/>      URI               = absoluteURI | abs_path
460<iref item="nonce" primary="true"
461/>      nonce             = "nonce" "=" nonce-value
462<iref item="nonce-value" primary="true"
463/>      nonce-value       = quoted-string
464<iref item="opaque" primary="true"
465/>      opaque            = "opaque" "=" quoted-string
466<iref item="stale" primary="true"
467/>      stale             = "stale" "=" ( "true" | "false" )
468<iref item="algorithm" primary="true"
469/>      algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
470                           token )
471<iref item="qop-options" primary="true"
472/>      qop-options       = "qop" "=" &lt;"> 1#qop-value &lt;">
473<iref item="qop-value" primary="true"
474/>      qop-value         = "auth" | "auth-int" | token
475</artwork></figure>
476<t>
477   The meanings of the values of the directives used above are as
478   follows:
479</t>
480<t>
481   realm
482   <list><t>
483     A string to be displayed to users so they know which username and
484     password to use. This string should contain at least the name of
485     the host performing the authentication and might additionally
486     indicate the collection of users who might have access. An example
487     might be "registered_users@gotham.news.com".
488   </t></list>
489</t>
490<t>
491   domain
492   <list><t>
493     A quoted, space-separated list of URIs, as specified in RFC XURI
494     <xref target="RFC2396"/>, that define the protection space.  If a URI is an abs_path, it
495     is relative to the canonical root URL (see <xref target="access.authentication.framework"/> above) of
496     the server being accessed. An absoluteURI in this list may refer to
497     a different server than the one being accessed. The client can use
498     this list to determine the set of URIs for which the same
499     authentication information may be sent: any URI that has a URI in
500     this list as a prefix (after both have been made absolute) may be
501     assumed to be in the same protection space. If this directive is
502     omitted or its value is empty, the client should assume that the
503     protection space consists of all URIs on the responding server.
504     This directive is not meaningful in Proxy-Authenticate headers, for
505     which the protection space is always the entire proxy; if present
506     it should be ignored.
507   </t></list>
508</t>
509<t>
510   nonce
511   <list><t>
512     A server-specified data string which should be uniquely generated
513     each time a 401 response is made. It is recommended that this
514     string be base64 or hexadecimal data. Specifically, since the
515     string is passed in the header lines as a quoted string, the
516     double-quote character is not allowed.
517  </t>
518  <t>
519     The contents of the nonce are implementation dependent. The quality
520     of the implementation depends on a good choice. A nonce might, for
521     example, be constructed as the base 64 encoding of
522  </t>
523  <t><figure><artwork type="code">
524         time-stamp H(time-stamp ":" ETag ":" private-key)
525  </artwork></figure></t>
526  <t>
527     where time-stamp is a server-generated time or other non-repeating
528     value, ETag is the value of the HTTP ETag header associated with
529     the requested entity, and private-key is data known only to the
530     server.  With a nonce of this form a server would recalculate the
531     hash portion after receiving the client authentication header and
532     reject the request if it did not match the nonce from that header
533     or if the time-stamp value is not recent enough. In this way the
534     server can limit the time of the nonce's validity. The inclusion of
535     the ETag prevents a replay request for an updated version of the
536     resource.  (Note: including the IP address of the client in the
537     nonce would appear to offer the server the ability to limit the
538     reuse of the nonce to the same client that originally got it.
539     However, that would break proxy farms, where requests from a single
540     user often go through different proxies in the farm. Also, IP
541     address spoofing is not that hard.)
542  </t>
543  <t>
544     An implementation might choose not to accept a previously used
545     nonce or a previously used digest, in order to protect against a
546     replay attack. Or, an implementation might choose to use one-time
547     nonces or digests for POST or PUT requests and a time-stamp for GET
548     requests.  For more details on the issues involved see <xref target="security.considerations"/>
549     of this document.
550  </t>
551  <t>
552     The nonce is opaque to the client.
553   </t></list></t>
554<t>
555   opaque
556   <list><t>
557     A string of data, specified by the server, which should be returned
558     by the client unchanged in the Authorization header of subsequent
559     requests with URIs in the same protection space. It is recommended
560     that this string be base64 or hexadecimal data.
561   </t></list>
562</t>
563<t>
564   stale
565   <list><t>
566     A flag, indicating that the previous request from the client was
567     rejected because the nonce value was stale. If stale is TRUE
568     (case-insensitive), the client may wish to simply retry the request
569     with a new encrypted response, without reprompting the user for a
570     new username and password. The server should only set stale to TRUE
571     if it receives a request for which the nonce is invalid but with a
572     valid digest for that nonce (indicating that the client knows the
573     correct username/password). If stale is FALSE, or anything other
574     than TRUE, or the stale directive is not present, the username
575     and/or password are invalid, and new values must be obtained.
576   </t></list>
577</t>
578<t>
579   algorithm
580   <list><t>
581     A string indicating a pair of algorithms used to produce the digest
582     and a checksum. If this is not present it is assumed to be "MD5".
583     If the algorithm is not understood, the challenge should be ignored
584     (and a different one used, if there is more than one).
585    </t>
586    <t>
587     In this document the string obtained by applying the digest
588     algorithm to the data "data" with secret "secret" will be denoted
589     by KD(secret, data), and the string obtained by applying the
590     checksum algorithm to the data "data" will be denoted H(data). The
591     notation unq(X) means the value of the quoted-string X without the
592     surrounding quotes.
593    </t>
594    <t>
595     For the "MD5" and "MD5-sess" algorithms
596    </t>
597    <t><figure><artwork type="code">
598         H(data) = MD5(data)
599    </artwork></figure></t>
600    <t>
601     and
602    </t>
603    <t><figure><artwork type="code">
604         KD(secret, data) = H(concat(secret, ":", data))
605    </artwork></figure></t>
606    <t>
607     i.e., the digest is the MD5 of the secret concatenated with a colon
608     concatenated with the data. The "MD5-sess" algorithm is intended to
609     allow efficient 3rd party authentication servers; for the
610     difference in usage, see the description in <xref target="A1"/>.
611   </t></list>
612</t>
613<t>
614   qop-options
615   <list><t>
616     This directive is optional, but is made so only for backward
617     compatibility with RFC 2069 <xref target="RFC2069"/>; it &SHOULD; be used by all
618     implementations compliant with this version of the Digest scheme.
619     If present, it is a quoted string of one or more tokens indicating
620     the "quality of protection" values supported by the server.  The
621     value "auth" indicates authentication; the value "auth-int"
622     indicates authentication with integrity protection; see the
623     descriptions below for calculating the response directive value for
624     the application of this choice. Unrecognized options &MUST; be
625     ignored.
626   </t></list>
627</t>
628<t>
629   auth-param
630   <list><t>
631     This directive allows for future extensions. Any unrecognized
632     directive &MUST; be ignored.
633   </t></list>
634</t>
635</section>
636
637<section title="The Authorization Request Header" anchor="the.authorization.request.header">
638<iref item="Headers" subitem="Authorization" primary="true"/>
639<iref item="Authorization header" primary="true"/>
640<t>
641   The client is expected to retry the request, passing an Authorization
642   header line, which is defined according to the framework above,
643   utilized as follows.
644</t>
645<figure><artwork type="abnf2616">
646<iref item="credentials"
647/>       credentials      = "Digest" digest-response
648<iref item="digest-response" primary="true"
649/>       digest-response  = 1#( username | realm | nonce | digest-uri
650                       | response | [ algorithm ] | [cnonce] |
651                       [opaque] | [message-qop] |
652                           [nonce-count]  | [auth-param] )
653
654<iref item="username" primary="true"
655/>       username         = "username" "=" username-value
656<iref item="username-value" primary="true"
657/>       username-value   = quoted-string
658<iref item="digest-uri" primary="true"
659/>       digest-uri       = "uri" "=" digest-uri-value
660<iref item="digest-uri-value" primary="true"
661/>       digest-uri-value = request-uri   ; As specified by HTTP/1.1
662<iref item="message-qop" primary="true"
663/>       message-qop      = "qop" "=" qop-value
664<iref item="cnonce" primary="true"
665/>       cnonce           = "cnonce" "=" cnonce-value
666<iref item="cnonce-value" primary="true"
667/>       cnonce-value     = nonce-value
668<iref item="nonce-count" primary="true"
669/>       nonce-count      = "nc" "=" nc-value
670<iref item="nc-value" primary="true"
671/>       nc-value         = 8LHEX
672<iref item="response" primary="true"
673/>       response         = "response" "=" request-digest
674<iref item="request-digest" primary="true"
675/>       request-digest = &lt;"> 32LHEX &lt;">
676<iref item="LHEX" primary="true"
677/>       LHEX             =  "0" | "1" | "2" | "3" |
678                           "4" | "5" | "6" | "7" |
679                           "8" | "9" | "a" | "b" |
680                           "c" | "d" | "e" | "f"
681</artwork></figure>
682<t>
683   The values of the opaque and algorithm fields must be those supplied
684   in the WWW-Authenticate response header for the entity being
685   requested.
686</t>
687<t>
688   response
689   <list><t>
690     A string of 32 hex digits computed as defined below, which proves
691     that the user knows a password
692   </t></list>
693</t>
694<t>
695   username
696   <list><t>
697     The user's name in the specified realm.
698   </t></list>
699</t>
700<t>
701   digest-uri
702   <list><t>
703     The URI from Request-URI of the Request-Line; duplicated here
704     because proxies are allowed to change the Request-Line in transit.
705   </t></list>
706</t>
707<t>
708   qop
709   <list><t>
710     Indicates what "quality of protection" the client has applied to
711     the message. If present, its value &MUST; be one of the alternatives
712     the server indicated it supports in the WWW-Authenticate header.
713     These values affect the computation of the request-digest. Note
714     that this is a single token, not a quoted list of alternatives as
715     in WWW-Authenticate.  This directive is optional in order to
716     preserve backward compatibility with a minimal implementation of
717     RFC 2069 <xref target="RFC2069"/>, but &SHOULD; be used if the server indicated that qop
718     is supported by providing a qop directive in the WWW-Authenticate
719     header field.
720   </t></list>
721</t>
722<t>
723   cnonce
724   <list><t>
725     This &MUST; be specified if a qop directive is sent (see above), and
726     &MUST-NOT; be specified if the server did not send a qop directive in
727     the WWW-Authenticate header field.  The cnonce-value is an opaque
728     quoted string value provided by the client and used by both client
729     and server to avoid chosen plaintext attacks, to provide mutual
730     authentication, and to provide some message integrity protection.
731     See the descriptions below of the calculation of the response-digest
732     and request-digest values.
733   </t></list>
734</t>
735<t>
736   nonce-count
737   <list><t>
738     This &MUST; be specified if a qop directive is sent (see above), and
739     &MUST-NOT; be specified if the server did not send a qop directive in
740     the WWW-Authenticate header field.  The nc-value is the hexadecimal
741     count of the number of requests (including the current request)
742     that the client has sent with the nonce value in this request.  For
743     example, in the first request sent in response to a given nonce
744     value, the client sends "nc=00000001".  The purpose of this
745     directive is to allow the server to detect request replays by
746     maintaining its own copy of this count - if the same nc-value is
747     seen twice, then the request is a replay.   See the description
748     below of the construction of the request-digest value.
749   </t></list>
750</t>
751<t>
752   auth-param
753   <list><t>
754     This directive allows for future extensions. Any unrecognized
755     directive &MUST; be ignored.
756   </t></list>
757</t>
758<t>
759   If a directive or its value is improper, or required directives are
760   missing, the proper response is 400 Bad Request. If the request-digest
761   is invalid, then a login failure should be logged, since
762   repeated login failures from a single client may indicate an attacker
763   attempting to guess passwords.
764</t>
765<t>
766   The definition of request-digest above indicates the encoding for its
767   value. The following definitions show how the value is computed.
768</t>
769
770<section title="Request-Digest" anchor="request-digest">
771<t>
772   If the "qop" value is "auth" or "auth-int":
773</t>
774<figure><artwork type="abnf2616">
775      request-digest  = &lt;"> &lt; KD ( H(A1),     unq(nonce-value)
776                                          ":" nc-value
777                                          ":" unq(cnonce-value)
778                                          ":" unq(qop-value)
779                                          ":" H(A2)
780                                  ) &lt;">
781</artwork></figure>
782<t>
783   If the "qop" directive is not present (this construction is for
784   compatibility with RFC 2069):
785</t>
786<figure><artwork type="abnf2616">
787      request-digest  =
788                 &lt;"> &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) >
789   &lt;">
790</artwork></figure>
791<t>
792   See below for the definitions for A1 and A2.
793</t>
794</section>
795
796<section title="A1" anchor="A1">
797<t>
798   If the "algorithm" directive's value is "MD5" or is unspecified, then
799   A1 is:
800</t>
801<figure><artwork type="abnf2616">
802      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
803</artwork></figure>
804<t>
805   where
806</t>
807<figure><artwork type="abnf2616">
808      passwd   = &lt; user's password >
809</artwork></figure>
810<t>
811   If the "algorithm" directive's value is "MD5-sess", then A1 is
812   calculated only once - on the first request by the client following
813   receipt of a WWW-Authenticate challenge from the server.  It uses the
814   server nonce from that challenge, and the first client nonce value to
815   construct A1 as follows:
816</t>
817<figure><artwork type="abnf2616">
818      A1       = H( unq(username-value) ":" unq(realm-value)
819                     ":" passwd )
820                     ":" unq(nonce-value) ":" unq(cnonce-value)
821</artwork></figure>
822<t>
823   This creates a 'session key' for the authentication of subsequent
824   requests and responses which is different for each "authentication
825   session", thus limiting the amount of material hashed with any one
826   key.  (Note: see further discussion of the authentication session in
827   <xref target="digest.operation"/>) Because the server need only use the hash of the user
828   credentials in order to create the A1 value, this construction could
829   be used in conjunction with a third party authentication service so
830   that the web server would not need the actual password value.  The
831   specification of such a protocol is beyond the scope of this
832   specification.
833</t>
834</section>
835
836<section title="A2">
837<t>
838   If the "qop" directive's value is "auth" or is unspecified, then A2
839   is:
840</t>
841<figure><artwork type="abnf2616">
842      A2       = Method ":" digest-uri-value
843</artwork></figure>
844<t>
845   If the "qop" value is "auth-int", then A2 is:
846</t>
847<figure><artwork type="abnf2616">
848      A2       = Method ":" digest-uri-value ":" H(entity-body)
849</artwork></figure>
850</section>
851
852
853<section title="Directive values and quoted-string">
854<t>
855   Note that the value of many of the directives, such as "username-value",
856   are defined as a "quoted-string". However, the "unq" notation
857   indicates that surrounding quotation marks are removed in forming the
858   string A1. Thus if the Authorization header includes the fields
859</t>
860<figure><artwork type="example">
861     username="Mufasa", realm=myhost@testrealm.com
862</artwork></figure>
863<t>
864   and the user Mufasa has password "Circle Of Life" then H(A1) would be
865   H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks
866   in the digested string.
867</t>
868<t>
869   No white space is allowed in any of the strings to which the digest
870   function H() is applied unless that white space exists in the quoted
871   strings or entity body whose contents make up the string to be
872   digested. For example, the string A1 illustrated above must be
873</t>
874<figure><artwork type="example">
875        Mufasa:myhost@testrealm.com:Circle Of Life
876</artwork></figure>
877<t>
878   with no white space on either side of the colons, but with the white
879   space between the words used in the password value.  Likewise, the
880   other strings digested by H() must not have white space on either
881   side of the colons which delimit their fields unless that white space
882   was in the quoted strings or entity body being digested.
883</t>
884<t>
885   Also note that if integrity protection is applied (qop=auth-int), the
886   H(entity-body) is the hash of the entity body, not the message body -
887   it is computed before any transfer encoding is applied by the sender
888   and after it has been removed by the recipient. Note that this
889   includes multipart boundaries and embedded headers in each part of
890   any multipart content-type.
891</t>
892</section>
893
894<section title="Various considerations">
895<t>
896   The "Method" value is the HTTP request method as specified in section
897   <xref target="RFC2616" x:fmt="number" x:sec="5.1.1"/> of <xref target="RFC2616"/>. The "request-uri" value is the Request-URI from the
898   request line as specified in section <xref target="RFC2616" x:fmt="number" x:sec="5.1.2"/> of <xref target="RFC2616"/>. This may be "*",
899   an "absoluteURL" or an "abs_path" as specified in section <xref target="RFC2616" x:fmt="number" x:sec="5.1.2"/> of
900   <xref target="RFC2616"/>, but it &MUST; agree with the Request-URI. In particular, it &MUST;
901   be an "absoluteURL" if the Request-URI is an "absoluteURL". The
902   "cnonce-value" is an optional  client-chosen value whose purpose is
903   to foil chosen plaintext attacks.
904</t>
905<t>
906   The authenticating server must assure that the resource designated by
907   the "uri" directive is the same as the resource specified in the
908   Request-Line; if they are not, the server &SHOULD; return a 400 Bad
909   Request error. (Since this may be a symptom of an attack, server
910   implementers may want to consider logging such errors.) The purpose
911   of duplicating information from the request URL in this field is to
912   deal with the possibility that an intermediate proxy may alter the
913   client's Request-Line. This altered (but presumably semantically
914   equivalent) request would not result in the same digest as that
915   calculated by the client.
916</t>
917<t>
918   Implementers should be aware of how authenticated transactions
919   interact with shared caches. The HTTP/1.1 protocol specifies that
920   when a shared cache (see section <xref target="RFC2616" x:fmt="number" x:sec="13.7"/> of <xref target="RFC2616"/>) has received a request
921   containing an Authorization header and a response from relaying that
922   request, it &MUST-NOT; return that response as a reply to any other
923   request, unless one of two Cache-Control (see section <xref target="RFC2616" x:fmt="number" x:sec="14.9"/> of <xref target="RFC2616"/>)
924   directives was present in the response. If the original response
925   included the "must-revalidate" Cache-Control directive, the cache &MAY;
926   use the entity of that response in replying to a subsequent request,
927   but &MUST; first revalidate it with the origin server, using the
928   request headers from the new request to allow the origin server to
929   authenticate the new request. Alternatively, if the original response
930   included the "public" Cache-Control directive, the response entity
931   &MAY; be returned in reply to any subsequent request.
932</t>
933</section>
934</section>
935
936<section title="The Authentication-Info Header">
937<iref item="Headers" subitem="Authentication-Info" primary="true"/>
938<iref item="Authentication-Info header" primary="true"/>
939<t>
940   The Authentication-Info header is used by the server to communicate
941   some information regarding the successful authentication in the
942   response.
943</t>
944<figure><artwork type="abnf2616"><iref item="Authentication-Info" primary="true"
945/>        AuthenticationInfo = "Authentication-Info" ":" auth-info
946<iref item="auth-info" primary="true"
947/>        auth-info          = 1#(nextnonce | [ message-qop ]
948                               | [ response-auth ] | [ cnonce ]
949                               | [nonce-count] )
950<iref item="nextnonce" primary="true"
951/>        nextnonce          = "nextnonce" "=" nonce-value
952<iref item="response-auth" primary="true"
953/>        response-auth      = "rspauth" "=" response-digest
954<iref item="response-digest" primary="true"
955/>        response-digest    = &lt;"> *LHEX &lt;">
956</artwork></figure>
957<t>
958   The value of the nextnonce directive is the nonce the server wishes
959   the client to use for a future authentication response.  The server
960   may send the Authentication-Info header with a nextnonce field as a
961   means of implementing one-time or otherwise changing  nonces. If the
962   nextnonce field is present the client &SHOULD; use it when constructing
963   the Authorization header for its next request. Failure of the client
964   to do so may result in a request to re-authenticate from the server
965   with the "stale=TRUE".
966</t>
967<t>
968  <list><t>
969     Server implementations should carefully consider the performance
970     implications of the use of this mechanism; pipelined requests will
971     not be possible if every response includes a nextnonce directive
972     that must be used on the next request received by the server.
973     Consideration should be given to the performance vs. security
974     tradeoffs of allowing an old nonce value to be used for a limited
975     time to permit request pipelining.  Use of the nonce-count can
976     retain most of the security advantages of a new server nonce
977     without the deleterious affects on pipelining.
978  </t></list>
979</t>
980<t>
981   message-qop
982</t>
983<t><list><t>
984     Indicates the "quality of protection" options applied to the
985     response by the server.  The value "auth" indicates authentication;
986     the value "auth-int" indicates authentication with integrity
987     protection. The server &SHOULD; use the same value for the message-qop
988     directive in the response as was sent by the client in the
989     corresponding request.
990</t></list></t>
991<t>
992   The optional response digest in the "response-auth" directive
993   supports mutual authentication -- the server proves that it knows the
994   user's secret, and with qop=auth-int also provides limited integrity
995   protection of the response. The "response-digest" value is calculated
996   as for the "request-digest" in the Authorization header, except that
997   if "qop=auth" or is not specified in the Authorization header for the
998   request, A2 is
999</t>
1000<figure><artwork type="abnf2616">
1001      A2       = ":" digest-uri-value
1002</artwork></figure>
1003<t>
1004   and if "qop=auth-int", then A2 is
1005</t>
1006<figure><artwork type="abnf2616">
1007      A2       = ":" digest-uri-value ":" H(entity-body)
1008</artwork></figure>
1009<t>
1010   where "digest-uri-value" is the value of the "uri" directive on the
1011   Authorization header in the request. The "cnonce-value" and "nc-value"
1012   &MUST; be the ones for the client request to which this message
1013   is the response. The "response-auth", "cnonce", and "nonce-count"
1014   directives &MUST; BE present if "qop=auth" or "qop=auth-int" is
1015   specified.
1016</t>
1017<t>
1018   The Authentication-Info header is allowed in the trailer of an HTTP
1019   message transferred via chunked transfer-coding.
1020</t>
1021</section>
1022</section>
1023
1024<section title="Digest Operation" anchor="digest.operation">
1025<t>
1026   Upon receiving the Authorization header, the server may check its
1027   validity by looking up the password that corresponds to the submitted
1028   username. Then, the server must perform the same digest operation
1029   (e.g., MD5) performed by the client, and compare the result to the
1030   given request-digest value.
1031</t>
1032<t>
1033   Note that the HTTP server does not actually need to know the user's
1034   cleartext password. As long as H(A1) is available to the server, the
1035   validity of an Authorization header may be verified.
1036</t>
1037<t>
1038   The client response to a WWW-Authenticate challenge for a protection
1039   space starts an authentication session with that protection space.
1040   The authentication session lasts until the client receives another
1041   WWW-Authenticate challenge from any server in the protection space. A
1042   client should remember the username, password, nonce, nonce count and
1043   opaque values associated with an authentication session to use to
1044   construct the Authorization header in future requests within that
1045   protection space. The Authorization header may be included
1046   preemptively; doing so improves server efficiency and avoids extra
1047   round trips for authentication challenges. The server may choose to
1048   accept the old Authorization header information, even though the
1049   nonce value included might not be fresh. Alternatively, the server
1050   may return a 401 response with a new nonce value, causing the client
1051   to retry the request; by specifying stale=TRUE with this response,
1052   the server tells the client to retry with the new nonce, but without
1053   prompting for a new username and password.
1054</t>
1055<t>
1056   Because the client is required to return the value of the opaque
1057   directive given to it by the server for the duration of a session,
1058   the opaque data may be used to transport authentication session state
1059   information. (Note that any such use can also be accomplished more
1060   easily and safely by including the state in the nonce.) For example,
1061   a server could be responsible for authenticating content that
1062   actually sits on another server. It would achieve this by having the
1063   first 401 response include a domain directive whose value includes a
1064   URI on the second server, and an opaque directive whose value
1065   contains the state information. The client will retry the request, at
1066   which time the server might respond with a 301/302 redirection,
1067   pointing to the URI on the second server. The client will follow the
1068   redirection, and pass an Authorization header , including the
1069   &lt;opaque> data.
1070</t>
1071<t>
1072   As with the basic scheme, proxies must be completely transparent in
1073   the Digest access authentication scheme. That is, they must forward
1074   the WWW-Authenticate, Authentication-Info and Authorization headers
1075   untouched. If a proxy wants to authenticate a client before a request
1076   is forwarded to the server, it can be done using the Proxy-Authenticate
1077   and Proxy-Authorization headers described in <xref target="proxy-authentication.and.proxy-authorization"/>
1078   below.
1079</t>
1080</section>
1081
1082<section title="Security Protocol Negotiation">
1083<t>
1084   It is useful for a server to be able to know which security schemes a
1085   client is capable of handling.
1086</t>
1087<t>
1088   It is possible that a server may want to require Digest as its
1089   authentication method, even if the server does not know that the
1090   client supports it. A client is encouraged to fail gracefully if the
1091   server specifies only authentication schemes it cannot handle.
1092</t>
1093</section>
1094
1095<section title="Example" anchor="specification.of.digest.headers.example">
1096<t>
1097   The following example assumes that an access-protected document is
1098   being requested from the server via a GET request. The URI of the
1099   document is "http://www.nowhere.org/dir/index.html". Both client and
1100   server know that the username for this document is "Mufasa", and the
1101   password is "Circle Of Life" (with one space between each of the
1102   three words).
1103</t>
1104<t>
1105   The first time the client requests the document, no Authorization
1106   header is sent, so the server responds with:
1107</t>
1108<figure><artwork type='message/http; msgytpe="response"'>
1109         HTTP/1.1 401 Unauthorized
1110         WWW-Authenticate: Digest
1111                 realm="testrealm@host.com",
1112                 qop="auth,auth-int",
1113                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1114                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1115</artwork></figure>
1116<t>
1117   The client may prompt the user for the username and password, after
1118   which it will respond with a new request, including the following
1119   Authorization header:
1120</t>
1121<figure><artwork type="example">
1122         Authorization: Digest username="Mufasa",
1123                 realm="testrealm@host.com",
1124                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1125                 uri="/dir/index.html",
1126                 qop=auth,
1127                 nc=00000001,
1128                 cnonce="0a4f113b",
1129                 response="6629fae49393a05397450978507c4ef1",
1130                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1131</artwork></figure>
1132</section>
1133
1134<section title="Proxy-Authentication and Proxy-Authorization" anchor="proxy-authentication.and.proxy-authorization">
1135<t>
1136   The digest authentication scheme may also be used for authenticating
1137   users to proxies, proxies to proxies, or proxies to origin servers by
1138   use of the Proxy-Authenticate and Proxy-Authorization headers. These
1139   headers are instances of the Proxy-Authenticate and Proxy-Authorization
1140   headers specified in sections <xref target="RFC2616" x:fmt="number" x:sec="10.33"/> and <xref target="RFC2616" x:fmt="number" x:sec="10.34"/> of the
1141   HTTP/1.1 specification <xref target="RFC2616"/> and their behavior is subject to
1142   restrictions described there. The transactions for proxy
1143   authentication are very similar to those already described. Upon
1144   receiving a request which requires authentication, the proxy/server
1145   must issue the "407 Proxy Authentication Required" response with a
1146   "Proxy-Authenticate" header.  The digest-challenge used in the
1147   Proxy-Authenticate header is the same as that for the WWW-Authenticate
1148   header as defined above in <xref target="the.www-authenticate.response.header"/>.
1149</t>
1150<t>
1151   The client/proxy must then re-issue the request with a Proxy-Authorization
1152   header, with directives as specified for the
1153   Authorization header in <xref target="the.authorization.request.header"/> above.
1154</t>
1155<t>
1156   On subsequent responses, the server sends Proxy-Authentication-Info
1157   with directives the same as those for the Authentication-Info header
1158   field.
1159</t>
1160<t>
1161   Note that in principle a client could be asked to authenticate itself
1162   to both a proxy and an end-server, but never in the same response.
1163</t>
1164</section>
1165</section>
1166   
1167<section title="Security Considerations" anchor="security.considerations">
1168
1169<section title="Authentication of Clients using Basic Authentication">
1170<t>
1171   The Basic authentication scheme is not a secure method of user
1172   authentication, nor does it in any way protect the entity, which is
1173   transmitted in cleartext across the physical network used as the
1174   carrier. HTTP does not prevent additional authentication schemes and
1175   encryption mechanisms from being employed to increase security or the
1176   addition of enhancements (such as schemes to use one-time passwords)
1177   to Basic authentication.
1178</t>
1179<t>
1180   The most serious flaw in Basic authentication is that it results in
1181   the essentially cleartext transmission of the user's password over
1182   the physical network. It is this problem which Digest Authentication
1183   attempts to address.
1184</t>
1185<t>
1186   Because Basic authentication involves the cleartext transmission of
1187   passwords it &SHOULD-NOT; be used (without enhancements) to protect
1188   sensitive or valuable information.
1189</t>
1190<t>
1191   A common use of Basic authentication is for identification purposes
1192   -- requiring the user to provide a user name and password as a means
1193   of identification, for example, for purposes of gathering accurate
1194   usage statistics on a server. When used in this way it is tempting to
1195   think that there is no danger in its use if illicit access to the
1196   protected documents is not a major concern. This is only correct if
1197   the server issues both user name and password to the users and in
1198   particular does not allow the user to choose his or her own password.
1199   The danger arises because naive users frequently reuse a single
1200   password to avoid the task of maintaining multiple passwords.
1201</t>
1202<t>
1203   If a server permits users to select their own passwords, then the
1204   threat is not only unauthorized access to documents on the server but
1205   also unauthorized access to any other resources on other systems that
1206   the user protects with the same password. Furthermore, in the
1207   server's password database, many of the passwords may also be users'
1208   passwords for other sites. The owner or administrator of such a
1209   system could therefore expose all users of the system to the risk of
1210   unauthorized access to all those sites if this information is not
1211   maintained in a secure fashion.
1212</t>
1213<t>
1214   Basic Authentication is also vulnerable to spoofing by counterfeit
1215   servers. If a user can be led to believe that he is connecting to a
1216   host containing information protected by Basic authentication when,
1217   in fact, he is connecting to a hostile server or gateway, then the
1218   attacker can request a password, store it for later use, and feign an
1219   error. This type of attack is not possible with Digest
1220   Authentication. Server implementers &SHOULD; guard against the
1221   possibility of this sort of counterfeiting by gateways or CGI
1222   scripts. In particular it is very dangerous for a server to simply
1223   turn over a connection to a gateway.  That gateway can then use the
1224   persistent connection mechanism to engage in multiple transactions
1225   with the client while impersonating the original server in a way that
1226   is not detectable by the client.
1227</t>
1228</section>
1229
1230<section title="Authentication of Clients using Digest Authentication">
1231<t>
1232   Digest Authentication does not provide a strong authentication
1233   mechanism, when compared to public key based mechanisms, for example.
1234   However, it is significantly stronger than (e.g.) CRAM-MD5, which has
1235   been proposed for use with LDAP <xref target="ref10"/>, POP and IMAP (see RFC 2195
1236   <xref target="RFC2195"/>).  It is intended to replace the much weaker and even more
1237   dangerous Basic mechanism.
1238</t>
1239<t>
1240   Digest Authentication offers no confidentiality protection beyond
1241   protecting the actual password. All of the rest of the request and
1242   response are available to an eavesdropper.
1243</t>
1244<t>
1245   Digest Authentication offers only limited integrity protection for
1246   the messages in either direction. If  qop=auth-int mechanism is used,
1247   those parts of the message used in the calculation of the WWW-Authenticate
1248   and Authorization header field response directive values
1249   (see <xref target="specification.of.digest.headers"/> above) are  protected.  Most header fields and their
1250   values could be modified as a part of a man-in-the-middle attack.
1251</t>
1252<t>
1253   Many needs for secure HTTP transactions cannot be met by Digest
1254   Authentication. For those needs TLS or SHTTP are more appropriate
1255   protocols. In particular Digest authentication cannot be used for any
1256   transaction requiring confidentiality protection.  Nevertheless many
1257   functions remain for which Digest authentication is both useful and
1258   appropriate.  Any service in present use that uses Basic should be
1259   switched to Digest as soon as practical.
1260</t>
1261</section>
1262
1263<section title="Limited Use Nonce Values">
1264<t>
1265   The Digest scheme uses a server-specified nonce to seed the
1266   generation of the request-digest value (as specified in <xref target="request-digest"/>
1267   above).  As shown in the example nonce in <xref target="the.www-authenticate.response.header"/>, the
1268   server is free to construct the nonce such that it may only be used
1269   from a particular client, for a particular resource, for a limited
1270   period of time or number of uses, or any other restrictions.  Doing
1271   so strengthens the protection provided against, for example, replay
1272   attacks (see 4.5).  However, it should be noted that the method
1273   chosen for generating and checking the nonce also has performance and
1274   resource implications.  For example, a server may choose to allow
1275   each nonce value to be used only once by maintaining a record of
1276   whether or not each recently issued nonce has been returned and
1277   sending a next-nonce directive in the Authentication-Info header
1278   field of every response. This protects against even an immediate
1279   replay attack, but has a high cost checking nonce values, and perhaps
1280   more important will cause authentication failures for any pipelined
1281   requests (presumably returning a stale nonce indication).  Similarly,
1282   incorporating a request-specific element such as the Etag value for a
1283   resource limits the use of the nonce to that version of the resource
1284   and also defeats pipelining. Thus it may be useful to do so for
1285   methods with side effects but have unacceptable performance for those
1286   that do not.
1287</t>
1288</section>
1289
1290<section title="Comparison of Digest with Basic Authentication">
1291<t>
1292   Both Digest and Basic Authentication are very much on the weak end of
1293   the security strength spectrum. But a comparison between the two
1294   points out the utility, even necessity, of replacing Basic by Digest.
1295</t>
1296<t>
1297   The greatest threat to the type of transactions for which these
1298   protocols are used is network snooping. This kind of transaction
1299   might involve, for example, online access to a database whose use is
1300   restricted to paying subscribers. With Basic authentication an
1301   eavesdropper can obtain the password of the user. This not only
1302   permits him to access anything in the database, but, often worse,
1303   will permit access to anything else the user protects with the same
1304   password.
1305</t>
1306<t>
1307   By contrast, with Digest Authentication the eavesdropper only gets
1308   access to the transaction in question and not to the user's password.
1309   The information gained by the eavesdropper would permit a replay
1310   attack, but only with a request for the same document, and even that
1311   may be limited by the server's choice of nonce.
1312</t>
1313</section>
1314
1315<section title="Replay Attacks">
1316<t>
1317   A replay attack against Digest authentication would usually be
1318   pointless for a simple GET request since an eavesdropper would
1319   already have seen the only document he could obtain with a replay.
1320   This is because the URI of the requested document is digested in the
1321   client request and the server will only deliver that document. By
1322   contrast under Basic Authentication once the eavesdropper has the
1323   user's password, any document protected by that password is open to
1324   him.
1325</t>
1326<t>
1327   Thus, for some purposes, it is necessary to protect against replay
1328   attacks. A good Digest implementation can do this in various ways.
1329   The server created "nonce" value is implementation dependent, but if
1330   it contains a digest of the client IP, a time-stamp, the resource
1331   ETag, and a private server key (as recommended above) then a replay
1332   attack is not simple. An attacker must convince the server that the
1333   request is coming from a false IP address and must cause the server
1334   to deliver the document to an IP address different from the address
1335   to which it believes it is sending the document. An attack can only
1336   succeed in the period before the time-stamp expires. Digesting the
1337   client IP and time-stamp in the nonce permits an implementation which
1338   does not maintain state between transactions.
1339</t>
1340<t>
1341   For applications where no possibility of replay attack can be
1342   tolerated the server can use one-time nonce values which will not be
1343   honored for a second use. This requires the overhead of the server
1344   remembering which nonce values have been used until the nonce time-stamp
1345   (and hence the digest built with it) has expired, but it
1346   effectively protects against replay attacks.
1347</t>
1348<t>
1349   An implementation must give special attention to the possibility of
1350   replay attacks with POST and PUT requests. Unless the server employs
1351   one-time or otherwise limited-use nonces and/or insists on the use of
1352   the integrity protection of qop=auth-int, an attacker could replay
1353   valid credentials from a successful request with counterfeit form
1354   data or other message body. Even with the use of integrity protection
1355   most metadata in header fields is not protected. Proper nonce
1356   generation and checking provides some protection against replay of
1357   previously used valid credentials, but see 4.8.
1358</t>
1359</section>
1360
1361<section title="Weakness Created by Multiple Authentication Schemes">
1362<t>
1363   An HTTP/1.1 server may return multiple challenges with a 401
1364   (Authenticate) response, and each challenge may use a different
1365   auth-scheme. A user agent &MUST; choose to use the strongest auth-scheme
1366   it understands and request credentials from the user based
1367   upon that challenge.
1368</t>
1369<t>
1370  <list><t>
1371      Note that many browsers will only recognize Basic and will require
1372      that it be the first auth-scheme presented. Servers should only
1373      include Basic if it is minimally acceptable.
1374  </t></list>
1375</t>
1376<t>
1377   When the server offers choices of authentication schemes using the
1378   WWW-Authenticate header, the strength of the resulting authentication
1379   is only as good as that of the of the weakest of the authentication
1380   schemes. See <xref target="man.in.the.middle"/> below for discussion of particular attack
1381   scenarios that exploit multiple authentication schemes.
1382</t>
1383</section>
1384
1385<section title="Online dictionary attacks">
1386<t>
1387   If the attacker can eavesdrop, then it can test any overheard
1388   nonce/response pairs against a list of common words. Such a list is
1389   usually much smaller than the total number of possible passwords. The
1390   cost of computing the response for each password on the list is paid
1391   once for each challenge.
1392</t>
1393<t>
1394   The server can mitigate this attack by not allowing users to select
1395   passwords that are in a dictionary.
1396</t>
1397</section>
1398
1399<section title="Man in the Middle" anchor="man.in.the.middle">
1400<t>
1401   Both Basic and Digest authentication are vulnerable to "man in the
1402   middle" (MITM) attacks, for example, from a hostile or compromised
1403   proxy. Clearly, this would present all the problems of eavesdropping.
1404   But it also offers some additional opportunities to the attacker.
1405</t>
1406<t>
1407   A possible man-in-the-middle attack would be to add a weak
1408   authentication scheme to the set of choices, hoping that the client
1409   will use one that exposes the user's credentials (e.g. password). For
1410   this reason, the client should always use the strongest scheme that
1411   it understands from the choices offered.
1412</t>
1413<t>
1414   An even better MITM attack would be to remove all offered choices,
1415   replacing them with a challenge that requests only Basic
1416   authentication, then uses the cleartext credentials from the Basic
1417   authentication to authenticate to the origin server using the
1418   stronger scheme it requested. A particularly insidious way to mount
1419   such a MITM attack would be to offer a "free" proxy caching service
1420   to gullible users.
1421</t>
1422<t>
1423   User agents should consider measures such as presenting a visual
1424   indication at the time of the credentials request of what
1425   authentication scheme is to be used, or remembering the strongest
1426   authentication scheme ever requested by a server and produce a
1427   warning message before using a weaker one. It might also be a good
1428   idea for the user agent to be configured to demand Digest
1429   authentication in general, or from specific sites.
1430</t>
1431<t>
1432   Or, a hostile proxy might spoof the client into making a request the
1433   attacker wanted rather than one the client wanted. Of course, this is
1434   still much harder than a comparable attack against Basic
1435   Authentication.
1436</t>
1437</section>
1438
1439<section title="Chosen plaintext attacks">
1440<t>
1441   With Digest authentication, a MITM or a malicious server can
1442   arbitrarily choose the nonce that the client will use to compute the
1443   response. This is called a "chosen plaintext" attack. The ability to
1444   choose the nonce is known to make cryptanalysis much easier <xref target="ref8"/>.
1445</t>
1446<t>
1447   However, no way to analyze the MD5 one-way function used by Digest
1448   using chosen plaintext is currently known.
1449</t>
1450<t>
1451   The countermeasure against this attack is for clients to be
1452   configured to require the use of the optional "cnonce" directive;
1453   this allows the client to vary the input to the hash in a way not
1454   chosen by the attacker.
1455</t>
1456</section>
1457
1458<section title="Precomputed dictionary attacks">
1459<t>
1460   With Digest authentication, if the attacker can execute a chosen
1461   plaintext attack, the attacker can precompute the response for many
1462   common words to a nonce of its choice, and store a dictionary of
1463   (response, password) pairs. Such precomputation can often be done in
1464   parallel on many machines. It can then use the chosen plaintext
1465   attack to acquire a response corresponding to that challenge, and
1466   just look up the password in the dictionary. Even if most passwords
1467   are not in the dictionary, some might be. Since the attacker gets to
1468   pick the challenge, the cost of computing the response for each
1469   password on the list can be amortized over finding many passwords. A
1470   dictionary with 100 million password/response pairs would take about
1471   3.2 gigabytes of disk storage.
1472</t>
1473<t>
1474   The countermeasure against this attack is to for clients to be
1475   configured to require the use of the optional "cnonce" directive.
1476</t>
1477</section>
1478
1479<section title="Batch brute force attacks">
1480<t>
1481   With Digest authentication, a MITM can execute a chosen plaintext
1482   attack, and can gather responses from many users to the same nonce.
1483   It can then find all the passwords within any subset of password
1484   space that would generate one of the nonce/response pairs in a single
1485   pass over that space. It also reduces the time to find the first
1486   password by a factor equal to the number of nonce/response pairs
1487   gathered. This search of the password space can often be done in
1488   parallel on many machines, and even a single machine can search large
1489   subsets of the password space very quickly -- reports exist of
1490   searching all passwords with six or fewer letters in a few hours.
1491</t>
1492<t>
1493   The countermeasure against this attack is to for clients to be
1494   configured to require the use of the optional "cnonce" directive.
1495</t>
1496</section>
1497
1498<section title="Spoofing by Counterfeit Servers">
1499<t>
1500   Basic Authentication is vulnerable to spoofing by counterfeit
1501   servers.  If a user can be led to believe that she is connecting to a
1502   host containing information protected by a password she knows, when
1503   in fact she is connecting to a hostile server, then the hostile
1504   server can request a password, store it away for later use, and feign
1505   an error.  This type of attack is more difficult with Digest
1506   Authentication -- but the client must know to demand that Digest
1507   authentication be used, perhaps using some of the techniques
1508   described above to counter "man-in-the-middle" attacks.  Again, the
1509   user can be helped in detecting this attack by a visual indication of
1510   the authentication mechanism in use with appropriate guidance in
1511   interpreting the implications of each scheme.
1512</t>
1513</section>
1514
1515<section title="Storing passwords">
1516<t>
1517   Digest authentication requires that the authenticating agent (usually
1518   the server) store some data derived from the user's name and password
1519   in a "password file" associated with a given realm. Normally this
1520   might contain pairs consisting of username and H(A1), where H(A1) is
1521   the digested value of the username, realm, and password as described
1522   above.
1523</t>
1524<t>
1525   The security implications of this are that if this password file is
1526   compromised, then an attacker gains immediate access to documents on
1527   the server using this realm. Unlike, say a standard UNIX password
1528   file, this information need not be decrypted in order to access
1529   documents in the server realm associated with this file. On the other
1530   hand, decryption, or more likely a brute force attack, would be
1531   necessary to obtain the user's password. This is the reason that the
1532   realm is part of the digested data stored in the password file. It
1533   means that if one Digest authentication password file is compromised,
1534   it does not automatically compromise others with the same username
1535   and password (though it does expose them to brute force attack).
1536</t>
1537<t>
1538   There are two important security consequences of this. First the
1539   password file must be protected as if it contained unencrypted
1540   passwords, because for the purpose of accessing documents in its
1541   realm, it effectively does.
1542</t>
1543<t>
1544   A second consequence of this is that the realm string should be
1545   unique among all realms which any single user is likely to use. In
1546   particular a realm string should include the name of the host doing
1547   the authentication. The inability of the client to authenticate the
1548   server is a weakness of Digest Authentication.
1549</t>
1550</section>
1551
1552<section title="Summary">
1553<t>
1554   By modern cryptographic standards Digest Authentication is weak. But
1555   for a large range of purposes it is valuable as a replacement for
1556   Basic Authentication. It remedies some, but not all, weaknesses of
1557   Basic Authentication. Its strength may vary depending on the
1558   implementation.  In particular the structure of the nonce (which is
1559   dependent on the server implementation) may affect the ease of
1560   mounting a replay attack.  A range of server options is appropriate
1561   since, for example, some implementations may be willing to accept the
1562   server overhead of one-time nonces or digests to eliminate the
1563   possibility of replay. Others may satisfied with a nonce like the one
1564   recommended above restricted to a single IP address and a single ETag
1565   or with a limited lifetime.
1566</t>
1567<t>
1568   The bottom line is that *any* compliant implementation will be
1569   relatively weak by cryptographic standards, but *any* compliant
1570   implementation will be far superior to Basic Authentication.
1571</t>
1572</section>
1573</section>
1574   
1575<section title="Sample implementation">
1576<t>
1577   The following code implements the calculations of H(A1), H(A2),
1578   request-digest and response-digest, and a test program which computes
1579   the values used in the example of <xref target="specification.of.digest.headers.example"/>. It uses the MD5
1580   implementation from RFC 1321.
1581</t>
1582<figure><preamble>
1583   File "digcalc.h":
1584</preamble><artwork type="code" name="digcalc.h">
1585#define HASHLEN 16
1586typedef char HASH[HASHLEN];
1587#define HASHHEXLEN 32
1588typedef char HASHHEX[HASHHEXLEN+1];
1589#define IN
1590#define OUT
1591
1592/* calculate H(A1) as per HTTP Digest spec */
1593void DigestCalcHA1(
1594    IN char * pszAlg,
1595    IN char * pszUserName,
1596    IN char * pszRealm,
1597    IN char * pszPassword,
1598    IN char * pszNonce,
1599    IN char * pszCNonce,
1600    OUT HASHHEX SessionKey
1601    );
1602
1603/* calculate request-digest/response-digest as per HTTP Digest spec */
1604void DigestCalcResponse(
1605    IN HASHHEX HA1,           /* H(A1) */
1606    IN char * pszNonce,       /* nonce from server */
1607    IN char * pszNonceCount,  /* 8 hex digits */
1608    IN char * pszCNonce,      /* client nonce */
1609    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1610    IN char * pszMethod,      /* method from the request */
1611    IN char * pszDigestUri,   /* requested URL */
1612    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1613    OUT HASHHEX Response      /* request-digest or response-digest */
1614    );
1615</artwork></figure>
1616<figure><preamble>
1617File "digcalc.c":
1618</preamble><artwork type="example" name="digcalc.c">
1619#include &lt;global.h>
1620#include &lt;md5.h>
1621#include &lt;string.h>
1622#include "digcalc.h"
1623
1624void CvtHex(
1625    IN HASH Bin,
1626    OUT HASHHEX Hex
1627    )
1628{
1629    unsigned short i;
1630    unsigned char j;
1631
1632    for (i = 0; i &lt; HASHLEN; i++) {
1633        j = (Bin[i] >> 4) &amp; 0xf;
1634        if (j &lt;= 9)
1635            Hex[i*2] = (j + '0');
1636         else
1637            Hex[i*2] = (j + 'a' - 10);
1638        j = Bin[i] &amp; 0xf;
1639        if (j &lt;= 9)
1640            Hex[i*2+1] = (j + '0');
1641         else
1642            Hex[i*2+1] = (j + 'a' - 10);
1643    };
1644    Hex[HASHHEXLEN] = '\0';
1645};
1646
1647/* calculate H(A1) as per spec */
1648void DigestCalcHA1(
1649    IN char * pszAlg,
1650    IN char * pszUserName,
1651    IN char * pszRealm,
1652    IN char * pszPassword,
1653    IN char * pszNonce,
1654    IN char * pszCNonce,
1655    OUT HASHHEX SessionKey
1656    )
1657{
1658      MD5_CTX Md5Ctx;
1659      HASH HA1;
1660
1661      MD5Init(&amp;Md5Ctx);
1662      MD5Update(&amp;Md5Ctx, pszUserName, strlen(pszUserName));
1663      MD5Update(&amp;Md5Ctx, ":", 1);
1664      MD5Update(&amp;Md5Ctx, pszRealm, strlen(pszRealm));
1665      MD5Update(&amp;Md5Ctx, ":", 1);
1666      MD5Update(&amp;Md5Ctx, pszPassword, strlen(pszPassword));
1667      MD5Final(HA1, &amp;Md5Ctx);
1668      if (stricmp(pszAlg, "md5-sess") == 0) {
1669            MD5Init(&amp;Md5Ctx);
1670            MD5Update(&amp;Md5Ctx, HA1, HASHLEN);
1671            MD5Update(&amp;Md5Ctx, ":", 1);
1672            MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1673            MD5Update(&amp;Md5Ctx, ":", 1);
1674            MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1675            MD5Final(HA1, &amp;Md5Ctx);
1676      };
1677      CvtHex(HA1, SessionKey);
1678};
1679
1680/* calculate request-digest/response-digest as per HTTP Digest spec */
1681void DigestCalcResponse(
1682    IN HASHHEX HA1,           /* H(A1) */
1683    IN char * pszNonce,       /* nonce from server */
1684    IN char * pszNonceCount,  /* 8 hex digits */
1685    IN char * pszCNonce,      /* client nonce */
1686    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1687    IN char * pszMethod,      /* method from the request */
1688    IN char * pszDigestUri,   /* requested URL */
1689    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1690    OUT HASHHEX Response      /* request-digest or response-digest */
1691    )
1692{
1693      MD5_CTX Md5Ctx;
1694      HASH HA2;
1695      HASH RespHash;
1696       HASHHEX HA2Hex;
1697
1698      // calculate H(A2)
1699      MD5Init(&amp;Md5Ctx);
1700      MD5Update(&amp;Md5Ctx, pszMethod, strlen(pszMethod));
1701      MD5Update(&amp;Md5Ctx, ":", 1);
1702      MD5Update(&amp;Md5Ctx, pszDigestUri, strlen(pszDigestUri));
1703      if (stricmp(pszQop, "auth-int") == 0) {
1704            MD5Update(&amp;Md5Ctx, ":", 1);
1705            MD5Update(&amp;Md5Ctx, HEntity, HASHHEXLEN);
1706      };
1707      MD5Final(HA2, &amp;Md5Ctx);
1708       CvtHex(HA2, HA2Hex);
1709
1710      // calculate response
1711      MD5Init(&amp;Md5Ctx);
1712      MD5Update(&amp;Md5Ctx, HA1, HASHHEXLEN);
1713      MD5Update(&amp;Md5Ctx, ":", 1);
1714      MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1715      MD5Update(&amp;Md5Ctx, ":", 1);
1716      if (*pszQop) {
1717          MD5Update(&amp;Md5Ctx, pszNonceCount, strlen(pszNonceCount));
1718          MD5Update(&amp;Md5Ctx, ":", 1);
1719          MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1720          MD5Update(&amp;Md5Ctx, ":", 1);
1721          MD5Update(&amp;Md5Ctx, pszQop, strlen(pszQop));
1722          MD5Update(&amp;Md5Ctx, ":", 1);
1723      };
1724      MD5Update(&amp;Md5Ctx, HA2Hex, HASHHEXLEN);
1725      MD5Final(RespHash, &amp;Md5Ctx);
1726      CvtHex(RespHash, Response);
1727};
1728</artwork></figure>
1729<figure><preamble>
1730File "digtest.c":
1731</preamble><artwork type="example" name="digtest.c">
1732#include &lt;stdio.h>
1733#include "digcalc.h"
1734
1735void main(int argc, char ** argv) {
1736
1737      char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
1738      char * pszCNonce = "0a4f113b";
1739      char * pszUser = "Mufasa";
1740      char * pszRealm = "testrealm@host.com";
1741      char * pszPass = "Circle Of Life";
1742      char * pszAlg = "md5";
1743      char szNonceCount[9] = "00000001";
1744      char * pszMethod = "GET";
1745      char * pszQop = "auth";
1746      char * pszURI = "/dir/index.html";
1747      HASHHEX HA1;
1748      HASHHEX HA2 = "";
1749      HASHHEX Response;
1750
1751      DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass, pszNonce,
1752pszCNonce, HA1);
1753      DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
1754       pszMethod, pszURI, HA2, Response);
1755      printf("Response = %s\n", Response);
1756};
1757</artwork></figure>
1758</section>
1759   
1760<section title="Acknowledgments">
1761<t>
1762   Eric W. Sink, of AbiSource, Inc., was one of the original authors
1763   before the specification underwent substantial revision.
1764</t>
1765<t>
1766   In addition to the authors, valuable discussion instrumental in
1767   creating this document has come from Peter J. Churchyard, Ned Freed,
1768   and David M.  Kristol.
1769</t>
1770<t>
1771   Jim Gettys and Larry Masinter edited this document for update.
1772</t>
1773</section>
1774  </middle>
1775  <back>
1776   
1777<references>
1778
1779<reference anchor='RFC1945'>
1780<front>
1781<title abbrev='HTTP/1.0'>Hypertext Transfer Protocol -- HTTP/1.0</title>
1782<author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
1783<organization>MIT, Laboratory for Computer Science</organization>
1784<address>
1785<postal>
1786<street>545 Technology Square</street>
1787<city>Cambridge</city>
1788<region>MA</region>
1789<code>02139</code>
1790<country>US</country></postal>
1791<facsimile>+1 617 258 8682</facsimile>
1792<email>timbl@w3.org</email></address></author>
1793<author initials='R.T.' surname='Fielding' fullname='Roy T. Fielding'>
1794<organization>University of California, Irvine, Department of Information and Computer Science</organization>
1795<address>
1796<postal>
1797<street />
1798<city>Irvine</city>
1799<region>CA</region>
1800<code>92717-3425</code>
1801<country>US</country></postal>
1802<facsimile>+1 714 824 4056</facsimile>
1803<email>fielding@ics.uci.edu</email></address></author>
1804<author initials='H.F.' surname='Nielsen' fullname='Henrik Frystyk Nielsen'>
1805<organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
1806<address>
1807<postal>
1808<street>545 Technology Square</street>
1809<city>Cambridge</city>
1810<region>MA</region>
1811<code>02139</code>
1812<country>US</country></postal>
1813<facsimile>+1 617 258 8682</facsimile>
1814<email>frystyk@w3.org</email></address></author>
1815<date year='1996' month='May' />
1816</front>
1817<seriesInfo name='RFC' value='1945' />
1818</reference>
1819
1820<reference anchor="RFC2616">
1821
1822<front>
1823<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1824<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
1825<organization>University of California, Irvine, Information and Computer Science</organization>
1826<address>
1827<postal>
1828<street/>
1829<city>Irvine</city>
1830<region>CA</region>
1831<code>92697-3425</code>
1832<country>US</country></postal>
1833<phone>+1 949 824 1715</phone>
1834<email>fielding@ics.uci.edu</email></address></author>
1835<author initials="J." surname="Gettys" fullname="James Gettys">
1836<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
1837<address>
1838<postal>
1839<street>545 Technology Square</street>
1840<city>Cambridge</city>
1841<region>MA</region>
1842<code>02139</code>
1843<country>US</country></postal>
1844<phone/>
1845<facsimile>+1 617 258 8682</facsimile>
1846<email>jg@w3.org</email></address></author>
1847<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
1848<organization>Compaq Computer Corporation, Western Research Laboratory</organization>
1849<address>
1850<postal>
1851<street>250 University Avenue</street>
1852<city>Palo Alto</city>
1853<region>CA</region>
1854<code>94301</code>
1855<country>US</country></postal>
1856<phone/>
1857<email>mogul@wrl.dec.com</email></address></author>
1858<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1859<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
1860<address>
1861<postal>
1862<street>545 Technology Square</street>
1863<city>Cambridge</city>
1864<region>MA</region>
1865<code>02139</code>
1866<country>US</country></postal>
1867<phone/>
1868<facsimile>+1 617 258 8682</facsimile>
1869<email>frystyk@w3.org</email></address></author>
1870<author initials="L." surname="Masinter" fullname="Larry Masinter">
1871<organization>Xerox Corporation</organization>
1872<address>
1873<postal>
1874<street>3333 Coyote Hill Road</street>
1875<city>Palo Alto</city>
1876<region>CA</region>
1877<code>94034</code>
1878<country>US</country></postal>
1879<phone/>
1880<email>masinter@parc.xerox.com</email></address></author>
1881<author initials="P.J." surname="Leach" fullname="Paul J. Leach">
1882<organization>Microsoft Corporation</organization>
1883<address>
1884<postal>
1885<street>1 Microsoft Way</street>
1886<city>Redmond</city>
1887<region>WA</region>
1888<code>98052</code>
1889<country>US</country></postal>
1890<phone/>
1891<email>paulle@microsoft.com</email></address></author>
1892<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1893<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
1894<address>
1895<postal>
1896<street>545 Technology Square</street>
1897<city>Cambridge</city>
1898<region>MA</region>
1899<code>02139</code>
1900<country>US</country></postal>
1901<phone>+1 617 258 8682</phone>
1902<facsimile/>
1903<email>timbl@w3.org</email></address></author>
1904<date month="June" year="1999"/>
1905</front>
1906<seriesInfo name="RFC" value="2616"/>
1907</reference>
1908
1909<reference anchor='RFC1321'>
1910<front>
1911<title abbrev='MD5 Message-Digest Algorithm'>The MD5 Message-Digest Algorithm</title>
1912<author initials='R.' surname='Rivest' fullname='Ronald L. Rivest'>
1913<organization>Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science</organization>
1914<address>
1915<postal>
1916<street>545 Technology Square</street>
1917<street>NE43-324</street>
1918<city>Cambridge</city>
1919<region>MA</region>
1920<code>02139-1986</code>
1921<country>US</country></postal>
1922<phone>+1 617 253 5880</phone>
1923<email>rivest@theory.lcs.mit.edu</email></address></author>
1924<date year='1992' month='April' /></front>
1925<seriesInfo name='RFC' value='1321' />
1926</reference>
1927
1928<reference anchor='RFC2045'>
1929<front>
1930<title abbrev='Internet Message Bodies'>Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
1931<author initials='N.' surname='Freed' fullname='Ned Freed'>
1932<organization>Innosoft International, Inc.</organization>
1933<address>
1934<postal>
1935<street>1050 East Garvey Avenue South</street>
1936<city>West Covina</city>
1937<region>CA</region>
1938<code>91790</code>
1939<country>US</country></postal>
1940<phone>+1 818 919 3600</phone>
1941<facsimile>+1 818 919 3614</facsimile>
1942<email>ned@innosoft.com</email></address></author>
1943<author initials='N.S.' surname='Borenstein' fullname='Nathaniel S. Borenstein'>
1944<organization>First Virtual Holdings</organization>
1945<address>
1946<postal>
1947<street>25 Washington Avenue</street>
1948<city>Morristown</city>
1949<region>NJ</region>
1950<code>07960</code>
1951<country>US</country></postal>
1952<phone>+1 201 540 8967</phone>
1953<facsimile>+1 201 993 3032</facsimile>
1954<email>nsb@nsb.fv.com</email></address></author>
1955<date year='1996' month='November' />
1956</front>
1957<seriesInfo name='RFC' value='2045' />
1958</reference>
1959
1960<reference anchor='RFC2246'>
1961<front>
1962<title>The TLS Protocol Version 1.0</title>
1963<author initials='T.' surname='Dierks' fullname='Tim Dierks'>
1964<organization>Certicom</organization>
1965<address>
1966<email>tdierks@certicom.com</email></address></author>
1967<author initials='C.' surname='Allen' fullname='Christopher Allen'>
1968<organization>Certicom</organization>
1969<address>
1970<email>callen@certicom.com</email></address></author>
1971<date year='1999' month='January' />
1972</front>
1973<seriesInfo name='RFC' value='2246' />
1974<format type='TXT' octets='170401' target='ftp://ftp.isi.edu/in-notes/rfc2246.txt' />
1975</reference>
1976
1977
1978<reference anchor='RFC2069'>
1979<front>
1980<title abbrev='Digest Access Authentication'>An Extension to HTTP : Digest Access Authentication</title>
1981<author initials='J.' surname='Franks' fullname='John Franks'>
1982<organization>Northwestern University,  Department of Mathematics</organization>
1983<address>
1984<postal>
1985<street />
1986<city>Evanston</city>
1987<region>IL</region>
1988<code>60208-2730</code>
1989<country>US</country></postal>
1990<email>john@math.nwu.edu</email></address></author>
1991<author initials='P.' surname='Hallam-Baker' fullname='Phillip M. Hallam-Baker'>
1992<organization>CERN</organization>
1993<address>
1994<postal>
1995<street />
1996<city>Geneva</city>
1997<country>CH</country></postal>
1998<email>hallam@w3.org</email></address></author>
1999<author initials='J.' surname='Hostetler' fullname='Jeffery L. Hostetler'>
2000<organization>Spyglass, Inc.</organization>
2001<address>
2002<postal>
2003<street>3200 Farber Drive</street>
2004<city>Champaign</city>
2005<region>IL</region>
2006<code>61821</code>
2007<country>US</country></postal>
2008<email>jeff@spyglass.com</email></address></author>
2009<author initials='P.' surname='Leach' fullname='Paul J. Leach'>
2010<organization>Microsoft Corporation</organization>
2011<address>
2012<postal>
2013<street>1 Microsoft Way</street>
2014<city>Redmond</city>
2015<region>WA</region>
2016<code>98052</code>
2017<country>US</country></postal>
2018<email>paulle@microsoft.com</email></address></author>
2019<author initials='A.' surname='Luotonen' fullname='Ari Luotonen'>
2020<organization>Netscape Communications Corporation</organization>
2021<address>
2022<postal>
2023<street>501 East Middlefield Road</street>
2024<city>Mountain View</city>
2025<region>CA</region>
2026<code>94043</code>
2027<country>US</country></postal>
2028<email>luotonen@netscape.com</email></address></author>
2029<author initials='E.' surname='Sink' fullname='Eric W. Sink'>
2030<organization>Spyglass, Inc.</organization>
2031<address>
2032<postal>
2033<street>3200 Farber Drive</street>
2034<city>Champaign</city>
2035<region>IL</region>
2036<code>61821</code>
2037<country>US</country></postal>
2038<email>eric@spyglass.com</email></address></author>
2039<author initials='L.' surname='Stewart' fullname='Lawrence C. Stewart'>
2040<organization>Open Market, Inc.</organization>
2041<address>
2042<postal>
2043<street>215 First Street</street>
2044<city>Cambridge</city>
2045<region>MA</region>
2046<code>02142</code>
2047<country>US</country></postal>
2048<email>stewart@OpenMarket.com</email></address></author>
2049<date year='1997' month='January' />
2050</front>
2051<seriesInfo name='RFC' value='2069' />
2052</reference>
2053
2054<reference anchor="RFC2396">
2055  <front>
2056    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
2057    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2058      <organization>World Wide Web Consortium</organization>
2059      <address>
2060      <email>timbl@w3.org</email></address>
2061    </author>
2062    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2063      <organization>Department of Information and Computer Science</organization>
2064      <address>
2065        <email>fielding@ics.uci.edu</email>
2066      </address>
2067    </author>
2068    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2069      <organization>Xerox PARC</organization>
2070      <address>
2071        <email>masinter@parc.xerox.com</email>
2072      </address>
2073    </author>
2074    <date month="August" year="1998"/>
2075  </front>
2076  <seriesInfo name="RFC" value="2396"/>
2077</reference>
2078
2079<reference anchor="ref8" target="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">
2080  <front>
2081    <title>Message Authentication with MD5</title>
2082    <author initials="B." surname="Kaliski">
2083      <organization/>
2084    </author>
2085    <author initials="M." surname="Robshaw">
2086      <organization/>
2087    </author>
2088    <date year="1995"/>
2089  </front>
2090  <annotation>CryptoBytes, Spring 1995</annotation>
2091</reference>
2092
2093<reference anchor='RFC2195'>
2094<front>
2095<title abbrev='IMAP/POP AUTHorize Extension'>IMAP/POP AUTHorize Extension for Simple Challenge/Response</title>
2096<author initials='J.C.' surname='Klensin' fullname='John C. Klensin'>
2097<organization>MCI</organization>
2098<address>
2099<postal>
2100<street>800 Boylston</street>
2101<street>7th floor</street>
2102<street>Boston</street>
2103<street>MA 02199</street>
2104<country>USA</country></postal>
2105<phone>+1 617 960 1011</phone>
2106<email>klensin@mci.net</email></address></author>
2107<author initials='R.' surname='Catoe' fullname='Randy Catoe'>
2108<organization>MCI</organization>
2109<address>
2110<postal>
2111<street>2100 Reston Parkway</street>
2112<street>Reston</street>
2113<street>VA 22091</street>
2114<country>USA</country></postal>
2115<phone>+1 703 715 7366</phone>
2116<email>randy@mci.net</email></address></author>
2117<author initials='P.' surname='Krumviede' fullname='Paul Krumviede'>
2118<organization>MCI</organization>
2119<address>
2120<postal>
2121<street>2100 Reston Parkway</street>
2122<street>Reston</street>
2123<street>VA 22091</street>
2124<country>USA</country></postal>
2125<phone>+1 703 715 7251</phone>
2126<email>paul@mci.net</email></address></author>
2127<date year='1997' month='September' />
2128<area>Applications</area>
2129<keyword>IMAP</keyword>
2130<keyword>authentication</keyword>
2131<keyword>internet message access protocol</keyword>
2132<keyword>post office protocol</keyword>
2133<keyword>security</keyword>
2134</front>
2135<seriesInfo name='RFC' value='2195' />
2136</reference>
2137
2138<reference anchor="ref10">
2139  <front>
2140    <title>Authentication Methods for LDAP</title>
2141    <author initials="B." surname="Morgan">
2142      <organization/>
2143    </author>
2144    <author initials="H." surname="Alvestrand">
2145      <organization/>
2146    </author>
2147    <author initials="J." surname="Hodges">
2148      <organization/>
2149    </author>
2150    <author initials="M." surname="Wahl">
2151      <organization/>
2152    </author>
2153    <date/>
2154  </front>
2155  <annotation>Work in progress.</annotation>
2156</reference>
2157</references> 
2158 
2159</back>
2160</rfc>
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