source: draft-ietf-httpbis/orig/rfc2617.xml @ 311

Last change on this file since 311 was 311, checked in by julian.reschke@…, 11 years ago

Update orig RFCs, also add 2145 and 5234

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[311]1<?xml version="1.0" encoding="UTF-8"?>
2<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
[9]3<?rfc toc="yes"?>
4<?rfc symrefs="no"?>
5<?rfc sortrefs="no"?>
6<?rfc compact="yes"?>
7<?rfc subcompact="no"?>
8<?rfc-ext allow-markup-in-artwork="yes"?>
9<?rfc-ext include-references-in-index="yes" ?>
10
11<!DOCTYPE rfc [
12  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
13  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
14  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
15  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
16  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
17  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
18  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
19  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
20  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
21  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
22]>
[311]23<rfc number="2617" category="std" obsoletes="2069" xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:grddl='http://www.w3.org/2003/g/data-view#' grddl:transformation='rfc2629grddl.xslt'>
[9]24  <front>
25    <title abbrev="HTTP Authentication">HTTP Authentication: Basic and Digest Access Authentication</title>
26    <author initials="J." surname="Franks" fullname="John Franks">
27      <organization>Northwestern University, Department of Mathematics</organization>
28      <address>
29        <postal>
30          <street>Northwestern University</street>
31          <city>Evanston</city>
32          <region>IL</region>
33          <code>60208-2730</code>
34          <country>USA</country>
35        </postal>
36        <email>john@math.nwu.edu</email>
37      </address>
38    </author>
39    <author initials="P.M." surname="Hallam-Baker" fullname="Phillip M. Hallam-Baker">
40      <organization>Verisign Inc.</organization>
41      <address>
42        <postal>
43          <street>301 Edgewater Place</street>
44          <street>Suite 210</street>
45          <city>Wakefield</city>
46          <region>MA</region>
47          <code>01880</code>
48          <country>USA</country>
49        </postal>
50        <email>pbaker@verisign.com</email>
51      </address>
52    </author>
53    <author initials="J.L." surname="Hostetler" fullname="Jeffery L. Hostetler">
54    <organization>AbiSource, Inc.</organization>
55      <address>
56        <postal>
57          <street>6 Dunlap Court</street>
58          <city>Savoy</city>
59          <region>IL</region>
60          <code>61874</code>
61          <country>USA</country>
62        </postal>
63        <email>jeff@AbiSource.com</email>
64      </address>
65    </author>
66    <author initials="S.D." surname="Lawrence" fullname="Scott D. Lawrence">
67      <organization>Agranat Systems, Inc.</organization>
68      <address>
69        <postal>
70          <street>5 Clocktower Place</street>
71          <street>Suite 400</street>
72          <city>Maynard</city>
73          <region>MA</region>
74          <code>01754</code>
75          <country>USA</country>
76        </postal>
77        <email>lawrence@agranat.com</email>
78      </address>
79    </author>
80    <author initials="P.J." surname="Leach" fullname="Paul J. Leach">
81      <organization>Microsoft Corporation</organization>
82      <address>
83        <postal>
84          <street>1 Microsoft Way</street>
85          <city>Redmond</city>
86          <region>WA</region>
87          <code>98052</code>
88          <country>USA</country>
89        </postal>
90        <email>paulle@microsoft.com</email>
91      </address>
92    </author>
93    <author initials="A." surname="Luotonen" fullname="Ari Luotonen">
94      <organization>Netscape Communications Corporation</organization>
95      <address>
96        <postal>
97          <street>501 East Middlefield Road</street>
98          <city>Mountain View</city>
99          <region>CA</region>
100          <code>94043</code>
101          <country>USA</country>
102        </postal>
103      </address>
104    </author>
105    <author initials="L." surname="Stewart" fullname="Lawrence C. Stewart">
106      <organization>Open Market, Inc.</organization>
107      <address>
108        <postal>
109          <street>215 First Street</street>
110          <city>Cambridge</city>
111          <region>MA</region>
112          <code>02142</code>
113          <country>USA</country>
114        </postal>
115        <email>stewart@OpenMarket.com</email>
116      </address>
117    </author>
118    <date month="June" year="1999"/>
119       
120    <abstract>
121      <t>
122   "HTTP/1.0", includes the specification for a Basic Access
123   Authentication scheme. This scheme is not considered to be a secure
124   method of user authentication (unless used in conjunction with some
125   external secure system such as SSL <xref target="RFC2246"/>), as the user name and
126   password are passed over the network as cleartext.
127      </t><t>
128   This document also provides the specification for HTTP's
129   authentication framework, the original Basic authentication scheme
130   and a scheme based on cryptographic hashes, referred to as "Digest
131   Access Authentication".  It is therefore also intended to serve as a
132   replacement for RFC 2069 <xref target="RFC2069"/>.  Some optional elements specified by
133   RFC 2069 have been removed from this specification due to problems
134   found since its publication; other new elements have been added for
135   compatibility, those new elements have been made optional, but are
136   strongly recommended.
137      </t><t>
138   Like Basic, Digest access authentication verifies that both parties
139   to a communication know a shared secret (a password); unlike Basic,
140   this verification can be done without sending the password in the
141   clear, which is Basic's biggest weakness. As with most other
142   authentication protocols, the greatest sources of risks are usually
143   found not in the core protocol itself but in policies and procedures
144   surrounding its use.
145    </t>
146    </abstract>
147  </front>
148  <middle>
149 
[311]150<section title="Access Authentication">
[9]151
152<section title="Reliance on the HTTP/1.1 Specification">
153<t>
154   This specification is a companion to the HTTP/1.1 specification <xref target="RFC2616"/>.
[311]155   It uses the augmented BNF section <xref target="RFC2616" x:fmt="number" x:rel="#notation.abnf"/> of that document, and relies on
[9]156   both the non-terminals defined in that document and other aspects of
157   the HTTP/1.1 specification.
158</t>
159</section>
160
161<section title="Access Authentication Framework" anchor="access.authentication.framework">
162<t>
163   HTTP provides a simple challenge-response authentication mechanism
164   that &MAY; be used by a server to challenge a client request and by a
165   client to provide authentication information. It uses an extensible,
166   case-insensitive token to identify the authentication scheme,
167   followed by a comma-separated list of attribute-value pairs which
168   carry the parameters necessary for achieving authentication via that
169   scheme.
170</t>
171<figure><artwork type="abnf2616"><iref item="auth-scheme" primary="true"
172/>      auth-scheme    = token
173<iref item="auth-param" primary="true"
174/>      auth-param     = token "=" ( token | quoted-string )
175</artwork></figure>
176<t>
177   The 401 (Unauthorized) response message is used by an origin server
178   to challenge the authorization of a user agent. This response &MUST;
179   include a WWW-Authenticate header field containing at least one
180   challenge applicable to the requested resource. The 407 (Proxy
181   Authentication Required) response message is used by a proxy to
182   challenge the authorization of a client and &MUST; include a Proxy-Authenticate
183   header field containing at least one challenge
184   applicable to the proxy for the requested resource.
185</t>
186<figure><artwork type="abnf2616"><iref item="challenge" primary="true"
187/>      challenge   = auth-scheme 1*SP 1#auth-param
188</artwork></figure>
189<t>
190   Note: User agents will need to take special care in parsing the WWW-Authenticate
191   or Proxy-Authenticate header field value if it contains
192   more than one challenge, or if more than one WWW-Authenticate header
193   field is provided, since the contents of a challenge may itself
194   contain a comma-separated list of authentication parameters.
195</t>
196<t>
197   The authentication parameter realm is defined for all authentication
198   schemes:
199</t>
200<figure><artwork type="abnf2616"><iref item="realm" primary="true"
201/>      realm       = "realm" "=" realm-value
202<iref item="realm-value" primary="true"
203/>      realm-value = quoted-string
204</artwork></figure>
205<t>
206   The realm directive (case-insensitive) is required for all
207   authentication schemes that issue a challenge. The realm value
208   (case-sensitive), in combination with the canonical root URL (the
[311]209   absoluteURI for the server whose abs_path is empty; see section <xref target="RFC2616" x:fmt="number" x:rel="#request-uri"/>
[9]210   of <xref target="RFC2616"/>) of the server being accessed, defines the protection space.
211   These realms allow the protected resources on a server to be
212   partitioned into a set of protection spaces, each with its own
213   authentication scheme and/or authorization database. The realm value
214   is a string, generally assigned by the origin server, which may have
215   additional semantics specific to the authentication scheme. Note that
216   there may be multiple challenges with the same auth-scheme but
217   different realms.
218</t>
219<t>
220   A user agent that wishes to authenticate itself with an origin
221   server--usually, but not necessarily, after receiving a 401
222   (Unauthorized)--MAY do so by including an Authorization header field
223   with the request. A client that wishes to authenticate itself with a
224   proxy--usually, but not necessarily, after receiving a 407 (Proxy
225   Authentication Required)--MAY do so by including a Proxy-Authorization
226   header field with the request.  Both the Authorization
227   field value and the Proxy-Authorization field value consist of
228   credentials containing the authentication information of the client
229   for the realm of the resource being requested. The user agent &MUST;
230   choose to use one of the challenges with the strongest auth-scheme it
231   understands and request credentials from the user based upon that
232   challenge.
233</t>
234<figure><artwork type="abnf2616"><iref item="credentials" primary="true"
235/>   credentials = auth-scheme #auth-param
236</artwork></figure>
237<t>
238  <list><t>
239      Note that many browsers will only recognize Basic and will require
240      that it be the first auth-scheme presented. Servers should only
241      include Basic if it is minimally acceptable.
242  </t></list>
243</t>
244<t>
245   The protection space determines the domain over which credentials can
246   be automatically applied. If a prior request has been authorized, the
247   same credentials &MAY; be reused for all other requests within that
248   protection space for a period of time determined by the
249   authentication scheme, parameters, and/or user preference. Unless
250   otherwise defined by the authentication scheme, a single protection
251   space cannot extend outside the scope of its server.
252</t>
253<t>
254   If the origin server does not wish to accept the credentials sent
255   with a request, it &SHOULD; return a 401 (Unauthorized) response. The
256   response &MUST; include a WWW-Authenticate header field containing at
257   least one (possibly new) challenge applicable to the requested
258   resource. If a proxy does not accept the credentials sent with a
259   request, it &SHOULD; return a 407 (Proxy Authentication Required). The
260   response &MUST; include a Proxy-Authenticate header field containing a
261   (possibly new) challenge applicable to the proxy for the requested
262   resource.
263</t>
264<t>
265   The HTTP protocol does not restrict applications to this simple
266   challenge-response mechanism for access authentication. Additional
267   mechanisms &MAY; be used, such as encryption at the transport level or
268   via message encapsulation, and with additional header fields
269   specifying authentication information. However, these additional
270   mechanisms are not defined by this specification.
271</t>
272<t>
273   Proxies &MUST; be completely transparent regarding user agent
274   authentication by origin servers. That is, they must forward the
275   WWW-Authenticate and Authorization headers untouched, and follow the
[311]276   rules found in section <xref target="RFC2616" x:fmt="number" x:rel="#header.authorization"/> of <xref target="RFC2616"/>. Both the Proxy-Authenticate and
[9]277   the Proxy-Authorization header fields are hop-by-hop headers (see
[311]278   section <xref target="RFC2616" x:fmt="number" x:rel="#end-to-end.and.hop-by-hop.headers"/> of <xref target="RFC2616"/>).
[9]279</t>
280</section>
281</section>
282   
283<section title="Basic Authentication Scheme">
284<t>
285   The "basic" authentication scheme is based on the model that the
286   client must authenticate itself with a user-ID and a password for
287   each realm.  The realm value should be considered an opaque string
288   which can only be compared for equality with other realms on that
289   server. The server will service the request only if it can validate
290   the user-ID and password for the protection space of the Request-URI.
291   There are no optional authentication parameters.
292</t>
293<t>
294   For Basic, the framework above is utilized as follows:
295</t>
296<figure><artwork type="abnf2616"><iref item="challenge"
297/>      challenge   = "Basic" realm
298<iref item="credentials"
299/>      credentials = "Basic" basic-credentials
300</artwork></figure>
301<t>
302   Upon receipt of an unauthorized request for a URI within the
303   protection space, the origin server &MAY; respond with a challenge like
304   the following:
305</t>
306<figure><artwork type="example">
307      WWW-Authenticate: Basic realm="WallyWorld"
308</artwork></figure>
309<t>
310   where "WallyWorld" is the string assigned by the server to identify
311   the protection space of the Request-URI. A proxy may respond with the
312   same challenge using the Proxy-Authenticate header field.
313</t>
314<t>
315   To receive authorization, the client sends the userid and password,
316   separated by a single colon (":") character, within a base64 <xref target="RFC2396"/>
317   encoded string in the credentials.
318</t>
319<figure><artwork type="abnf2616"><iref item="basic-credentials" primary="true"
320/>      basic-credentials = base64-user-pass
321<iref item="base64-user-pass" primary="true"
322/>      base64-user-pass  = &lt;base64 [4] encoding of user-pass,
323                       except not limited to 76 char/line>
324<iref item="user-pass" primary="true"
325/>      user-pass   = userid ":" password
326<iref item="userid" primary="true"
327/>      userid      = *&lt;TEXT excluding ":">
328<iref item="password" primary="true"
329/>      password    = *TEXT
330</artwork></figure>
331<t>
332   Userids might be case sensitive.
333</t>
334<t>
335   If the user agent wishes to send the userid "Aladdin" and password
336   "open sesame", it would use the following header field:
337</t>
338<figure><artwork type="example">
339      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
340</artwork></figure>
341<t>
342   A client &SHOULD; assume that all paths at or deeper than the depth of
343   the last symbolic element in the path field of the Request-URI also
344   are within the protection space specified by the Basic realm value of
345   the current challenge. A client &MAY; preemptively send the
346   corresponding Authorization header with requests for resources in
347   that space without receipt of another challenge from the server.
348   Similarly, when a client sends a request to a proxy, it may reuse a
349   userid and password in the Proxy-Authorization header field without
350   receiving another challenge from the proxy server. See <xref target="security.considerations"/> for
351   security considerations associated with Basic authentication.
352</t>
353</section>
354   
355<section title="Digest Access Authentication Scheme">
356
357<section title="Introduction">
358
359<section title="Purpose">
360<t>
361   The protocol referred to as "HTTP/1.0" includes the specification for
362   a Basic Access Authentication scheme<xref target="RFC1945"/>. That scheme is not
363   considered to be a secure method of user authentication, as the user
364   name and password are passed over the network in an unencrypted form.
365   This section provides the specification for a scheme that does not
366   send the password in cleartext,  referred to as "Digest Access
367   Authentication".
368</t>
369<t>
370   The Digest Access Authentication scheme is not intended to be a
371   complete answer to the need for security in the World Wide Web. This
372   scheme provides no encryption of message content. The intent is
373   simply to create an access authentication method that avoids the most
374   serious flaws of Basic authentication.
375</t>
376</section>
377
378<section title="Overall Operation">
379<t>
380   Like Basic Access Authentication, the Digest scheme is based on a
381   simple challenge-response paradigm. The Digest scheme challenges
382   using a nonce value. A valid response contains a checksum (by
383   default, the MD5 checksum) of the username, the password, the given
384   nonce value, the HTTP method, and the requested URI. In this way, the
385   password is never sent in the clear. Just as with the Basic scheme,
386   the username and password must be prearranged in some fashion not
387   addressed by this document.
388</t>
389</section>
390
391<section title="Representation of digest values">
392<t>
393   An optional header allows the server to specify the algorithm used to
394   create the checksum or digest. By default the MD5 algorithm is used
395   and that is the only algorithm described in this document.
396</t>
397<t>
398   For the purposes of this document, an MD5 digest of 128 bits is
399   represented as 32 ASCII printable characters. The bits in the 128 bit
400   digest are converted from most significant to least significant bit,
401   four bits at a time to their ASCII presentation as follows. Each four
402   bits is represented by its familiar hexadecimal notation from the
403   characters 0123456789abcdef. That is, binary 0000 gets represented by
404   the character '0', 0001, by '1', and so on up to the representation
405   of 1111 as 'f'.
406</t>
407</section>
408
409<section title="Limitations">
410<t>
411   The Digest authentication scheme described in this document suffers
412   from many known limitations. It is intended as a replacement for
413   Basic authentication and nothing more. It is a password-based system
414   and (on the server side) suffers from all the same problems of any
415   password system. In particular, no provision is made in this protocol
416   for the initial secure arrangement between user and server to
417   establish the user's password.
418</t>
419<t>
420   Users and implementors should be aware that this protocol is not as
421   secure as Kerberos, and not as secure as any client-side private-key
422   scheme. Nevertheless it is better than nothing, better than what is
423   commonly used with telnet and ftp, and better than Basic
424   authentication.
425</t>
426</section>
427</section>
428
429<section title="Specification of Digest Headers" anchor="specification.of.digest.headers">
430<t>
431   The Digest Access Authentication scheme is conceptually similar to
432   the Basic scheme. The formats of the modified WWW-Authenticate header
433   line and the Authorization header line are specified below. In
434   addition, a new header, Authentication-Info, is specified.
435</t>
436
437<section title="The WWW-Authenticate Response Header" anchor="the.www-authenticate.response.header">
438<iref item="Headers" subitem="WWW-Authenticate" primary="true"/>
439<iref item="WWW-Authenticate header" primary="true"/>
440<t>
441   If a server receives a request for an access-protected object, and an
442   acceptable Authorization header is not sent, the server responds with
443   a "401 Unauthorized" status code, and a WWW-Authenticate header as
444   per the framework defined above, which for the digest scheme is
445   utilized as follows:
446</t>
447<figure><artwork type="abnf2616">
448<iref item="challenge"
449/>      challenge        =  "Digest" digest-challenge
450
451<iref item="digest-challenge" primary="true"
452/>      digest-challenge  = 1#( realm | [ domain ] | nonce |
453                          [ opaque ] |[ stale ] | [ algorithm ] |
454                          [ qop-options ] | [auth-param] )
455
456
457<iref item="domain" primary="true"
458/>      domain            = "domain" "=" &lt;"> URI ( 1*SP URI ) &lt;">
459<iref item="URI" primary="true"
460/>      URI               = absoluteURI | abs_path
461<iref item="nonce" primary="true"
462/>      nonce             = "nonce" "=" nonce-value
463<iref item="nonce-value" primary="true"
464/>      nonce-value       = quoted-string
465<iref item="opaque" primary="true"
466/>      opaque            = "opaque" "=" quoted-string
467<iref item="stale" primary="true"
468/>      stale             = "stale" "=" ( "true" | "false" )
469<iref item="algorithm" primary="true"
470/>      algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
471                           token )
472<iref item="qop-options" primary="true"
473/>      qop-options       = "qop" "=" &lt;"> 1#qop-value &lt;">
474<iref item="qop-value" primary="true"
475/>      qop-value         = "auth" | "auth-int" | token
476</artwork></figure>
477<t>
478   The meanings of the values of the directives used above are as
479   follows:
480</t>
481<t>
482   realm
483   <list><t>
484     A string to be displayed to users so they know which username and
485     password to use. This string should contain at least the name of
486     the host performing the authentication and might additionally
487     indicate the collection of users who might have access. An example
488     might be "registered_users@gotham.news.com".
489   </t></list>
490</t>
491<t>
492   domain
493   <list><t>
494     A quoted, space-separated list of URIs, as specified in RFC XURI
495     <xref target="RFC2396"/>, that define the protection space.  If a URI is an abs_path, it
496     is relative to the canonical root URL (see <xref target="access.authentication.framework"/> above) of
497     the server being accessed. An absoluteURI in this list may refer to
498     a different server than the one being accessed. The client can use
499     this list to determine the set of URIs for which the same
500     authentication information may be sent: any URI that has a URI in
501     this list as a prefix (after both have been made absolute) may be
502     assumed to be in the same protection space. If this directive is
503     omitted or its value is empty, the client should assume that the
504     protection space consists of all URIs on the responding server.
505     This directive is not meaningful in Proxy-Authenticate headers, for
506     which the protection space is always the entire proxy; if present
507     it should be ignored.
508   </t></list>
509</t>
510<t>
511   nonce
512   <list><t>
513     A server-specified data string which should be uniquely generated
514     each time a 401 response is made. It is recommended that this
515     string be base64 or hexadecimal data. Specifically, since the
516     string is passed in the header lines as a quoted string, the
517     double-quote character is not allowed.
518  </t>
519  <t>
520     The contents of the nonce are implementation dependent. The quality
521     of the implementation depends on a good choice. A nonce might, for
522     example, be constructed as the base 64 encoding of
523  </t>
524  <t><figure><artwork type="code">
525         time-stamp H(time-stamp ":" ETag ":" private-key)
526  </artwork></figure></t>
527  <t>
528     where time-stamp is a server-generated time or other non-repeating
529     value, ETag is the value of the HTTP ETag header associated with
530     the requested entity, and private-key is data known only to the
531     server.  With a nonce of this form a server would recalculate the
532     hash portion after receiving the client authentication header and
533     reject the request if it did not match the nonce from that header
534     or if the time-stamp value is not recent enough. In this way the
535     server can limit the time of the nonce's validity. The inclusion of
536     the ETag prevents a replay request for an updated version of the
537     resource.  (Note: including the IP address of the client in the
538     nonce would appear to offer the server the ability to limit the
539     reuse of the nonce to the same client that originally got it.
540     However, that would break proxy farms, where requests from a single
541     user often go through different proxies in the farm. Also, IP
542     address spoofing is not that hard.)
543  </t>
544  <t>
545     An implementation might choose not to accept a previously used
546     nonce or a previously used digest, in order to protect against a
547     replay attack. Or, an implementation might choose to use one-time
548     nonces or digests for POST or PUT requests and a time-stamp for GET
549     requests.  For more details on the issues involved see <xref target="security.considerations"/>
550     of this document.
551  </t>
552  <t>
553     The nonce is opaque to the client.
554   </t></list></t>
555<t>
556   opaque
557   <list><t>
558     A string of data, specified by the server, which should be returned
559     by the client unchanged in the Authorization header of subsequent
560     requests with URIs in the same protection space. It is recommended
561     that this string be base64 or hexadecimal data.
562   </t></list>
563</t>
564<t>
565   stale
566   <list><t>
567     A flag, indicating that the previous request from the client was
568     rejected because the nonce value was stale. If stale is TRUE
569     (case-insensitive), the client may wish to simply retry the request
570     with a new encrypted response, without reprompting the user for a
571     new username and password. The server should only set stale to TRUE
572     if it receives a request for which the nonce is invalid but with a
573     valid digest for that nonce (indicating that the client knows the
574     correct username/password). If stale is FALSE, or anything other
575     than TRUE, or the stale directive is not present, the username
576     and/or password are invalid, and new values must be obtained.
577   </t></list>
578</t>
579<t>
580   algorithm
581   <list><t>
582     A string indicating a pair of algorithms used to produce the digest
583     and a checksum. If this is not present it is assumed to be "MD5".
584     If the algorithm is not understood, the challenge should be ignored
585     (and a different one used, if there is more than one).
586    </t>
587    <t>
588     In this document the string obtained by applying the digest
589     algorithm to the data "data" with secret "secret" will be denoted
590     by KD(secret, data), and the string obtained by applying the
591     checksum algorithm to the data "data" will be denoted H(data). The
592     notation unq(X) means the value of the quoted-string X without the
593     surrounding quotes.
594    </t>
595    <t>
596     For the "MD5" and "MD5-sess" algorithms
597    </t>
598    <t><figure><artwork type="code">
599         H(data) = MD5(data)
600    </artwork></figure></t>
601    <t>
602     and
603    </t>
604    <t><figure><artwork type="code">
605         KD(secret, data) = H(concat(secret, ":", data))
606    </artwork></figure></t>
607    <t>
608     i.e., the digest is the MD5 of the secret concatenated with a colon
609     concatenated with the data. The "MD5-sess" algorithm is intended to
610     allow efficient 3rd party authentication servers; for the
611     difference in usage, see the description in <xref target="A1"/>.
612   </t></list>
613</t>
614<t>
615   qop-options
616   <list><t>
617     This directive is optional, but is made so only for backward
618     compatibility with RFC 2069 <xref target="RFC2069"/>; it &SHOULD; be used by all
619     implementations compliant with this version of the Digest scheme.
620     If present, it is a quoted string of one or more tokens indicating
621     the "quality of protection" values supported by the server.  The
622     value "auth" indicates authentication; the value "auth-int"
623     indicates authentication with integrity protection; see the
624     descriptions below for calculating the response directive value for
625     the application of this choice. Unrecognized options &MUST; be
626     ignored.
627   </t></list>
628</t>
629<t>
630   auth-param
631   <list><t>
632     This directive allows for future extensions. Any unrecognized
633     directive &MUST; be ignored.
634   </t></list>
635</t>
636</section>
637
638<section title="The Authorization Request Header" anchor="the.authorization.request.header">
639<iref item="Headers" subitem="Authorization" primary="true"/>
640<iref item="Authorization header" primary="true"/>
641<t>
642   The client is expected to retry the request, passing an Authorization
643   header line, which is defined according to the framework above,
644   utilized as follows.
645</t>
646<figure><artwork type="abnf2616">
647<iref item="credentials"
648/>       credentials      = "Digest" digest-response
649<iref item="digest-response" primary="true"
650/>       digest-response  = 1#( username | realm | nonce | digest-uri
651                       | response | [ algorithm ] | [cnonce] |
652                       [opaque] | [message-qop] |
653                           [nonce-count]  | [auth-param] )
654
655<iref item="username" primary="true"
656/>       username         = "username" "=" username-value
657<iref item="username-value" primary="true"
658/>       username-value   = quoted-string
659<iref item="digest-uri" primary="true"
660/>       digest-uri       = "uri" "=" digest-uri-value
661<iref item="digest-uri-value" primary="true"
662/>       digest-uri-value = request-uri   ; As specified by HTTP/1.1
663<iref item="message-qop" primary="true"
664/>       message-qop      = "qop" "=" qop-value
665<iref item="cnonce" primary="true"
666/>       cnonce           = "cnonce" "=" cnonce-value
667<iref item="cnonce-value" primary="true"
668/>       cnonce-value     = nonce-value
669<iref item="nonce-count" primary="true"
670/>       nonce-count      = "nc" "=" nc-value
671<iref item="nc-value" primary="true"
672/>       nc-value         = 8LHEX
673<iref item="response" primary="true"
674/>       response         = "response" "=" request-digest
675<iref item="request-digest" primary="true"
676/>       request-digest = &lt;"> 32LHEX &lt;">
677<iref item="LHEX" primary="true"
678/>       LHEX             =  "0" | "1" | "2" | "3" |
679                           "4" | "5" | "6" | "7" |
680                           "8" | "9" | "a" | "b" |
681                           "c" | "d" | "e" | "f"
682</artwork></figure>
683<t>
684   The values of the opaque and algorithm fields must be those supplied
685   in the WWW-Authenticate response header for the entity being
686   requested.
687</t>
688<t>
689   response
690   <list><t>
691     A string of 32 hex digits computed as defined below, which proves
692     that the user knows a password
693   </t></list>
694</t>
695<t>
696   username
697   <list><t>
698     The user's name in the specified realm.
699   </t></list>
700</t>
701<t>
702   digest-uri
703   <list><t>
704     The URI from Request-URI of the Request-Line; duplicated here
705     because proxies are allowed to change the Request-Line in transit.
706   </t></list>
707</t>
708<t>
709   qop
710   <list><t>
711     Indicates what "quality of protection" the client has applied to
712     the message. If present, its value &MUST; be one of the alternatives
713     the server indicated it supports in the WWW-Authenticate header.
714     These values affect the computation of the request-digest. Note
715     that this is a single token, not a quoted list of alternatives as
716     in WWW-Authenticate.  This directive is optional in order to
717     preserve backward compatibility with a minimal implementation of
718     RFC 2069 <xref target="RFC2069"/>, but &SHOULD; be used if the server indicated that qop
719     is supported by providing a qop directive in the WWW-Authenticate
720     header field.
721   </t></list>
722</t>
723<t>
724   cnonce
725   <list><t>
726     This &MUST; be specified if a qop directive is sent (see above), and
727     &MUST-NOT; be specified if the server did not send a qop directive in
728     the WWW-Authenticate header field.  The cnonce-value is an opaque
729     quoted string value provided by the client and used by both client
730     and server to avoid chosen plaintext attacks, to provide mutual
731     authentication, and to provide some message integrity protection.
732     See the descriptions below of the calculation of the response-digest
733     and request-digest values.
734   </t></list>
735</t>
736<t>
737   nonce-count
738   <list><t>
739     This &MUST; be specified if a qop directive is sent (see above), and
740     &MUST-NOT; be specified if the server did not send a qop directive in
741     the WWW-Authenticate header field.  The nc-value is the hexadecimal
742     count of the number of requests (including the current request)
743     that the client has sent with the nonce value in this request.  For
744     example, in the first request sent in response to a given nonce
745     value, the client sends "nc=00000001".  The purpose of this
746     directive is to allow the server to detect request replays by
747     maintaining its own copy of this count - if the same nc-value is
748     seen twice, then the request is a replay.   See the description
749     below of the construction of the request-digest value.
750   </t></list>
751</t>
752<t>
753   auth-param
754   <list><t>
755     This directive allows for future extensions. Any unrecognized
756     directive &MUST; be ignored.
757   </t></list>
758</t>
759<t>
760   If a directive or its value is improper, or required directives are
761   missing, the proper response is 400 Bad Request. If the request-digest
762   is invalid, then a login failure should be logged, since
763   repeated login failures from a single client may indicate an attacker
764   attempting to guess passwords.
765</t>
766<t>
767   The definition of request-digest above indicates the encoding for its
768   value. The following definitions show how the value is computed.
769</t>
770
771<section title="Request-Digest" anchor="request-digest">
772<t>
773   If the "qop" value is "auth" or "auth-int":
774</t>
775<figure><artwork type="abnf2616">
776      request-digest  = &lt;"> &lt; KD ( H(A1),     unq(nonce-value)
777                                          ":" nc-value
778                                          ":" unq(cnonce-value)
779                                          ":" unq(qop-value)
780                                          ":" H(A2)
781                                  ) &lt;">
782</artwork></figure>
783<t>
784   If the "qop" directive is not present (this construction is for
785   compatibility with RFC 2069):
786</t>
787<figure><artwork type="abnf2616">
788      request-digest  =
789                 &lt;"> &lt; KD ( H(A1), unq(nonce-value) ":" H(A2) ) >
790   &lt;">
791</artwork></figure>
792<t>
793   See below for the definitions for A1 and A2.
794</t>
795</section>
796
797<section title="A1" anchor="A1">
798<t>
799   If the "algorithm" directive's value is "MD5" or is unspecified, then
800   A1 is:
801</t>
802<figure><artwork type="abnf2616">
803      A1       = unq(username-value) ":" unq(realm-value) ":" passwd
804</artwork></figure>
805<t>
806   where
807</t>
808<figure><artwork type="abnf2616">
809      passwd   = &lt; user's password >
810</artwork></figure>
811<t>
812   If the "algorithm" directive's value is "MD5-sess", then A1 is
813   calculated only once - on the first request by the client following
814   receipt of a WWW-Authenticate challenge from the server.  It uses the
815   server nonce from that challenge, and the first client nonce value to
816   construct A1 as follows:
817</t>
818<figure><artwork type="abnf2616">
819      A1       = H( unq(username-value) ":" unq(realm-value)
820                     ":" passwd )
821                     ":" unq(nonce-value) ":" unq(cnonce-value)
822</artwork></figure>
823<t>
824   This creates a 'session key' for the authentication of subsequent
825   requests and responses which is different for each "authentication
826   session", thus limiting the amount of material hashed with any one
827   key.  (Note: see further discussion of the authentication session in
828   <xref target="digest.operation"/>) Because the server need only use the hash of the user
829   credentials in order to create the A1 value, this construction could
830   be used in conjunction with a third party authentication service so
831   that the web server would not need the actual password value.  The
832   specification of such a protocol is beyond the scope of this
833   specification.
834</t>
835</section>
836
837<section title="A2">
838<t>
839   If the "qop" directive's value is "auth" or is unspecified, then A2
840   is:
841</t>
842<figure><artwork type="abnf2616">
843      A2       = Method ":" digest-uri-value
844</artwork></figure>
845<t>
846   If the "qop" value is "auth-int", then A2 is:
847</t>
848<figure><artwork type="abnf2616">
849      A2       = Method ":" digest-uri-value ":" H(entity-body)
850</artwork></figure>
851</section>
852
853
854<section title="Directive values and quoted-string">
855<t>
856   Note that the value of many of the directives, such as "username-value",
857   are defined as a "quoted-string". However, the "unq" notation
858   indicates that surrounding quotation marks are removed in forming the
859   string A1. Thus if the Authorization header includes the fields
860</t>
861<figure><artwork type="example">
862     username="Mufasa", realm=myhost@testrealm.com
863</artwork></figure>
864<t>
865   and the user Mufasa has password "Circle Of Life" then H(A1) would be
866   H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks
867   in the digested string.
868</t>
869<t>
870   No white space is allowed in any of the strings to which the digest
871   function H() is applied unless that white space exists in the quoted
872   strings or entity body whose contents make up the string to be
873   digested. For example, the string A1 illustrated above must be
874</t>
875<figure><artwork type="example">
876        Mufasa:myhost@testrealm.com:Circle Of Life
877</artwork></figure>
878<t>
879   with no white space on either side of the colons, but with the white
880   space between the words used in the password value.  Likewise, the
881   other strings digested by H() must not have white space on either
882   side of the colons which delimit their fields unless that white space
883   was in the quoted strings or entity body being digested.
884</t>
885<t>
886   Also note that if integrity protection is applied (qop=auth-int), the
887   H(entity-body) is the hash of the entity body, not the message body -
888   it is computed before any transfer encoding is applied by the sender
889   and after it has been removed by the recipient. Note that this
890   includes multipart boundaries and embedded headers in each part of
891   any multipart content-type.
892</t>
893</section>
894
895<section title="Various considerations">
896<t>
897   The "Method" value is the HTTP request method as specified in section
[311]898   <xref target="RFC2616" x:fmt="number" x:rel="#method"/> of <xref target="RFC2616"/>. The "request-uri" value is the Request-URI from the
899   request line as specified in section <xref target="RFC2616" x:fmt="number" x:rel="#request-uri"/> of <xref target="RFC2616"/>. This may be "*",
900   an "absoluteURL" or an "abs_path" as specified in section <xref target="RFC2616" x:fmt="number" x:rel="#request-uri"/> of
[9]901   <xref target="RFC2616"/>, but it &MUST; agree with the Request-URI. In particular, it &MUST;
902   be an "absoluteURL" if the Request-URI is an "absoluteURL". The
903   "cnonce-value" is an optional  client-chosen value whose purpose is
904   to foil chosen plaintext attacks.
905</t>
906<t>
907   The authenticating server must assure that the resource designated by
908   the "uri" directive is the same as the resource specified in the
909   Request-Line; if they are not, the server &SHOULD; return a 400 Bad
910   Request error. (Since this may be a symptom of an attack, server
911   implementers may want to consider logging such errors.) The purpose
912   of duplicating information from the request URL in this field is to
913   deal with the possibility that an intermediate proxy may alter the
914   client's Request-Line. This altered (but presumably semantically
915   equivalent) request would not result in the same digest as that
916   calculated by the client.
917</t>
918<t>
919   Implementers should be aware of how authenticated transactions
920   interact with shared caches. The HTTP/1.1 protocol specifies that
[311]921   when a shared cache (see section <xref target="RFC2616" x:fmt="number" x:rel="#shared.and.non-shared.caches"/> of <xref target="RFC2616"/>) has received a request
[9]922   containing an Authorization header and a response from relaying that
923   request, it &MUST-NOT; return that response as a reply to any other
[311]924   request, unless one of two Cache-Control (see section <xref target="RFC2616" x:fmt="number" x:rel="#header.cache-control"/> of <xref target="RFC2616"/>)
[9]925   directives was present in the response. If the original response
926   included the "must-revalidate" Cache-Control directive, the cache &MAY;
927   use the entity of that response in replying to a subsequent request,
928   but &MUST; first revalidate it with the origin server, using the
929   request headers from the new request to allow the origin server to
930   authenticate the new request. Alternatively, if the original response
931   included the "public" Cache-Control directive, the response entity
932   &MAY; be returned in reply to any subsequent request.
933</t>
934</section>
935</section>
936
937<section title="The Authentication-Info Header">
938<iref item="Headers" subitem="Authentication-Info" primary="true"/>
939<iref item="Authentication-Info header" primary="true"/>
940<t>
941   The Authentication-Info header is used by the server to communicate
942   some information regarding the successful authentication in the
943   response.
944</t>
945<figure><artwork type="abnf2616"><iref item="Authentication-Info" primary="true"
946/>        AuthenticationInfo = "Authentication-Info" ":" auth-info
947<iref item="auth-info" primary="true"
948/>        auth-info          = 1#(nextnonce | [ message-qop ]
949                               | [ response-auth ] | [ cnonce ]
950                               | [nonce-count] )
951<iref item="nextnonce" primary="true"
952/>        nextnonce          = "nextnonce" "=" nonce-value
953<iref item="response-auth" primary="true"
954/>        response-auth      = "rspauth" "=" response-digest
955<iref item="response-digest" primary="true"
956/>        response-digest    = &lt;"> *LHEX &lt;">
957</artwork></figure>
958<t>
959   The value of the nextnonce directive is the nonce the server wishes
960   the client to use for a future authentication response.  The server
961   may send the Authentication-Info header with a nextnonce field as a
962   means of implementing one-time or otherwise changing  nonces. If the
963   nextnonce field is present the client &SHOULD; use it when constructing
964   the Authorization header for its next request. Failure of the client
965   to do so may result in a request to re-authenticate from the server
966   with the "stale=TRUE".
967</t>
968<t>
969  <list><t>
970     Server implementations should carefully consider the performance
971     implications of the use of this mechanism; pipelined requests will
972     not be possible if every response includes a nextnonce directive
973     that must be used on the next request received by the server.
974     Consideration should be given to the performance vs. security
975     tradeoffs of allowing an old nonce value to be used for a limited
976     time to permit request pipelining.  Use of the nonce-count can
977     retain most of the security advantages of a new server nonce
978     without the deleterious affects on pipelining.
979  </t></list>
980</t>
981<t>
982   message-qop
983</t>
984<t><list><t>
985     Indicates the "quality of protection" options applied to the
986     response by the server.  The value "auth" indicates authentication;
987     the value "auth-int" indicates authentication with integrity
988     protection. The server &SHOULD; use the same value for the message-qop
989     directive in the response as was sent by the client in the
990     corresponding request.
991</t></list></t>
992<t>
993   The optional response digest in the "response-auth" directive
994   supports mutual authentication -- the server proves that it knows the
995   user's secret, and with qop=auth-int also provides limited integrity
996   protection of the response. The "response-digest" value is calculated
997   as for the "request-digest" in the Authorization header, except that
998   if "qop=auth" or is not specified in the Authorization header for the
999   request, A2 is
1000</t>
1001<figure><artwork type="abnf2616">
1002      A2       = ":" digest-uri-value
1003</artwork></figure>
1004<t>
1005   and if "qop=auth-int", then A2 is
1006</t>
1007<figure><artwork type="abnf2616">
1008      A2       = ":" digest-uri-value ":" H(entity-body)
1009</artwork></figure>
1010<t>
1011   where "digest-uri-value" is the value of the "uri" directive on the
1012   Authorization header in the request. The "cnonce-value" and "nc-value"
1013   &MUST; be the ones for the client request to which this message
1014   is the response. The "response-auth", "cnonce", and "nonce-count"
1015   directives &MUST; BE present if "qop=auth" or "qop=auth-int" is
1016   specified.
1017</t>
1018<t>
1019   The Authentication-Info header is allowed in the trailer of an HTTP
1020   message transferred via chunked transfer-coding.
1021</t>
1022</section>
1023</section>
1024
1025<section title="Digest Operation" anchor="digest.operation">
1026<t>
1027   Upon receiving the Authorization header, the server may check its
1028   validity by looking up the password that corresponds to the submitted
1029   username. Then, the server must perform the same digest operation
1030   (e.g., MD5) performed by the client, and compare the result to the
1031   given request-digest value.
1032</t>
1033<t>
1034   Note that the HTTP server does not actually need to know the user's
1035   cleartext password. As long as H(A1) is available to the server, the
1036   validity of an Authorization header may be verified.
1037</t>
1038<t>
1039   The client response to a WWW-Authenticate challenge for a protection
1040   space starts an authentication session with that protection space.
1041   The authentication session lasts until the client receives another
1042   WWW-Authenticate challenge from any server in the protection space. A
1043   client should remember the username, password, nonce, nonce count and
1044   opaque values associated with an authentication session to use to
1045   construct the Authorization header in future requests within that
1046   protection space. The Authorization header may be included
1047   preemptively; doing so improves server efficiency and avoids extra
1048   round trips for authentication challenges. The server may choose to
1049   accept the old Authorization header information, even though the
1050   nonce value included might not be fresh. Alternatively, the server
1051   may return a 401 response with a new nonce value, causing the client
1052   to retry the request; by specifying stale=TRUE with this response,
1053   the server tells the client to retry with the new nonce, but without
1054   prompting for a new username and password.
1055</t>
1056<t>
1057   Because the client is required to return the value of the opaque
1058   directive given to it by the server for the duration of a session,
1059   the opaque data may be used to transport authentication session state
1060   information. (Note that any such use can also be accomplished more
1061   easily and safely by including the state in the nonce.) For example,
1062   a server could be responsible for authenticating content that
1063   actually sits on another server. It would achieve this by having the
1064   first 401 response include a domain directive whose value includes a
1065   URI on the second server, and an opaque directive whose value
1066   contains the state information. The client will retry the request, at
1067   which time the server might respond with a 301/302 redirection,
1068   pointing to the URI on the second server. The client will follow the
1069   redirection, and pass an Authorization header , including the
1070   &lt;opaque> data.
1071</t>
1072<t>
1073   As with the basic scheme, proxies must be completely transparent in
1074   the Digest access authentication scheme. That is, they must forward
1075   the WWW-Authenticate, Authentication-Info and Authorization headers
1076   untouched. If a proxy wants to authenticate a client before a request
1077   is forwarded to the server, it can be done using the Proxy-Authenticate
1078   and Proxy-Authorization headers described in <xref target="proxy-authentication.and.proxy-authorization"/>
1079   below.
1080</t>
1081</section>
1082
1083<section title="Security Protocol Negotiation">
1084<t>
1085   It is useful for a server to be able to know which security schemes a
1086   client is capable of handling.
1087</t>
1088<t>
1089   It is possible that a server may want to require Digest as its
1090   authentication method, even if the server does not know that the
1091   client supports it. A client is encouraged to fail gracefully if the
1092   server specifies only authentication schemes it cannot handle.
1093</t>
1094</section>
1095
1096<section title="Example" anchor="specification.of.digest.headers.example">
1097<t>
1098   The following example assumes that an access-protected document is
1099   being requested from the server via a GET request. The URI of the
1100   document is "http://www.nowhere.org/dir/index.html". Both client and
1101   server know that the username for this document is "Mufasa", and the
1102   password is "Circle Of Life" (with one space between each of the
1103   three words).
1104</t>
1105<t>
1106   The first time the client requests the document, no Authorization
1107   header is sent, so the server responds with:
1108</t>
1109<figure><artwork type='message/http; msgytpe="response"'>
1110         HTTP/1.1 401 Unauthorized
1111         WWW-Authenticate: Digest
1112                 realm="testrealm@host.com",
1113                 qop="auth,auth-int",
1114                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1115                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1116</artwork></figure>
1117<t>
1118   The client may prompt the user for the username and password, after
1119   which it will respond with a new request, including the following
1120   Authorization header:
1121</t>
1122<figure><artwork type="example">
1123         Authorization: Digest username="Mufasa",
1124                 realm="testrealm@host.com",
1125                 nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
1126                 uri="/dir/index.html",
1127                 qop=auth,
1128                 nc=00000001,
1129                 cnonce="0a4f113b",
1130                 response="6629fae49393a05397450978507c4ef1",
1131                 opaque="5ccc069c403ebaf9f0171e9517f40e41"
1132</artwork></figure>
1133</section>
1134
1135<section title="Proxy-Authentication and Proxy-Authorization" anchor="proxy-authentication.and.proxy-authorization">
1136<t>
1137   The digest authentication scheme may also be used for authenticating
1138   users to proxies, proxies to proxies, or proxies to origin servers by
1139   use of the Proxy-Authenticate and Proxy-Authorization headers. These
1140   headers are instances of the Proxy-Authenticate and Proxy-Authorization
1141   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
1142   HTTP/1.1 specification <xref target="RFC2616"/> and their behavior is subject to
1143   restrictions described there. The transactions for proxy
1144   authentication are very similar to those already described. Upon
1145   receiving a request which requires authentication, the proxy/server
1146   must issue the "407 Proxy Authentication Required" response with a
1147   "Proxy-Authenticate" header.  The digest-challenge used in the
1148   Proxy-Authenticate header is the same as that for the WWW-Authenticate
1149   header as defined above in <xref target="the.www-authenticate.response.header"/>.
1150</t>
1151<t>
1152   The client/proxy must then re-issue the request with a Proxy-Authorization
1153   header, with directives as specified for the
1154   Authorization header in <xref target="the.authorization.request.header"/> above.
1155</t>
1156<t>
1157   On subsequent responses, the server sends Proxy-Authentication-Info
1158   with directives the same as those for the Authentication-Info header
1159   field.
1160</t>
1161<t>
1162   Note that in principle a client could be asked to authenticate itself
1163   to both a proxy and an end-server, but never in the same response.
1164</t>
1165</section>
1166</section>
1167   
1168<section title="Security Considerations" anchor="security.considerations">
1169
1170<section title="Authentication of Clients using Basic Authentication">
1171<t>
1172   The Basic authentication scheme is not a secure method of user
1173   authentication, nor does it in any way protect the entity, which is
1174   transmitted in cleartext across the physical network used as the
1175   carrier. HTTP does not prevent additional authentication schemes and
1176   encryption mechanisms from being employed to increase security or the
1177   addition of enhancements (such as schemes to use one-time passwords)
1178   to Basic authentication.
1179</t>
1180<t>
1181   The most serious flaw in Basic authentication is that it results in
1182   the essentially cleartext transmission of the user's password over
1183   the physical network. It is this problem which Digest Authentication
1184   attempts to address.
1185</t>
1186<t>
1187   Because Basic authentication involves the cleartext transmission of
1188   passwords it &SHOULD-NOT; be used (without enhancements) to protect
1189   sensitive or valuable information.
1190</t>
1191<t>
1192   A common use of Basic authentication is for identification purposes
1193   -- requiring the user to provide a user name and password as a means
1194   of identification, for example, for purposes of gathering accurate
1195   usage statistics on a server. When used in this way it is tempting to
1196   think that there is no danger in its use if illicit access to the
1197   protected documents is not a major concern. This is only correct if
1198   the server issues both user name and password to the users and in
1199   particular does not allow the user to choose his or her own password.
1200   The danger arises because naive users frequently reuse a single
1201   password to avoid the task of maintaining multiple passwords.
1202</t>
1203<t>
1204   If a server permits users to select their own passwords, then the
1205   threat is not only unauthorized access to documents on the server but
1206   also unauthorized access to any other resources on other systems that
1207   the user protects with the same password. Furthermore, in the
1208   server's password database, many of the passwords may also be users'
1209   passwords for other sites. The owner or administrator of such a
1210   system could therefore expose all users of the system to the risk of
1211   unauthorized access to all those sites if this information is not
1212   maintained in a secure fashion.
1213</t>
1214<t>
1215   Basic Authentication is also vulnerable to spoofing by counterfeit
1216   servers. If a user can be led to believe that he is connecting to a
1217   host containing information protected by Basic authentication when,
1218   in fact, he is connecting to a hostile server or gateway, then the
1219   attacker can request a password, store it for later use, and feign an
1220   error. This type of attack is not possible with Digest
1221   Authentication. Server implementers &SHOULD; guard against the
1222   possibility of this sort of counterfeiting by gateways or CGI
1223   scripts. In particular it is very dangerous for a server to simply
1224   turn over a connection to a gateway.  That gateway can then use the
1225   persistent connection mechanism to engage in multiple transactions
1226   with the client while impersonating the original server in a way that
1227   is not detectable by the client.
1228</t>
1229</section>
1230
1231<section title="Authentication of Clients using Digest Authentication">
1232<t>
1233   Digest Authentication does not provide a strong authentication
1234   mechanism, when compared to public key based mechanisms, for example.
1235   However, it is significantly stronger than (e.g.) CRAM-MD5, which has
1236   been proposed for use with LDAP <xref target="ref10"/>, POP and IMAP (see RFC 2195
1237   <xref target="RFC2195"/>).  It is intended to replace the much weaker and even more
1238   dangerous Basic mechanism.
1239</t>
1240<t>
1241   Digest Authentication offers no confidentiality protection beyond
1242   protecting the actual password. All of the rest of the request and
1243   response are available to an eavesdropper.
1244</t>
1245<t>
1246   Digest Authentication offers only limited integrity protection for
1247   the messages in either direction. If  qop=auth-int mechanism is used,
1248   those parts of the message used in the calculation of the WWW-Authenticate
1249   and Authorization header field response directive values
1250   (see <xref target="specification.of.digest.headers"/> above) are  protected.  Most header fields and their
1251   values could be modified as a part of a man-in-the-middle attack.
1252</t>
1253<t>
1254   Many needs for secure HTTP transactions cannot be met by Digest
1255   Authentication. For those needs TLS or SHTTP are more appropriate
1256   protocols. In particular Digest authentication cannot be used for any
1257   transaction requiring confidentiality protection.  Nevertheless many
1258   functions remain for which Digest authentication is both useful and
1259   appropriate.  Any service in present use that uses Basic should be
1260   switched to Digest as soon as practical.
1261</t>
1262</section>
1263
1264<section title="Limited Use Nonce Values">
1265<t>
1266   The Digest scheme uses a server-specified nonce to seed the
1267   generation of the request-digest value (as specified in <xref target="request-digest"/>
1268   above).  As shown in the example nonce in <xref target="the.www-authenticate.response.header"/>, the
1269   server is free to construct the nonce such that it may only be used
1270   from a particular client, for a particular resource, for a limited
1271   period of time or number of uses, or any other restrictions.  Doing
1272   so strengthens the protection provided against, for example, replay
[311]1273   attacks (see <xref target="replay.attacks" format="counter"/>).  However, it should be noted that the method
[9]1274   chosen for generating and checking the nonce also has performance and
1275   resource implications.  For example, a server may choose to allow
1276   each nonce value to be used only once by maintaining a record of
1277   whether or not each recently issued nonce has been returned and
1278   sending a next-nonce directive in the Authentication-Info header
1279   field of every response. This protects against even an immediate
1280   replay attack, but has a high cost checking nonce values, and perhaps
1281   more important will cause authentication failures for any pipelined
1282   requests (presumably returning a stale nonce indication).  Similarly,
1283   incorporating a request-specific element such as the Etag value for a
1284   resource limits the use of the nonce to that version of the resource
1285   and also defeats pipelining. Thus it may be useful to do so for
1286   methods with side effects but have unacceptable performance for those
1287   that do not.
1288</t>
1289</section>
1290
1291<section title="Comparison of Digest with Basic Authentication">
1292<t>
1293   Both Digest and Basic Authentication are very much on the weak end of
1294   the security strength spectrum. But a comparison between the two
1295   points out the utility, even necessity, of replacing Basic by Digest.
1296</t>
1297<t>
1298   The greatest threat to the type of transactions for which these
1299   protocols are used is network snooping. This kind of transaction
1300   might involve, for example, online access to a database whose use is
1301   restricted to paying subscribers. With Basic authentication an
1302   eavesdropper can obtain the password of the user. This not only
1303   permits him to access anything in the database, but, often worse,
1304   will permit access to anything else the user protects with the same
1305   password.
1306</t>
1307<t>
1308   By contrast, with Digest Authentication the eavesdropper only gets
1309   access to the transaction in question and not to the user's password.
1310   The information gained by the eavesdropper would permit a replay
1311   attack, but only with a request for the same document, and even that
1312   may be limited by the server's choice of nonce.
1313</t>
1314</section>
1315
[311]1316<section title="Replay Attacks" anchor="replay.attacks">
[9]1317<t>
1318   A replay attack against Digest authentication would usually be
1319   pointless for a simple GET request since an eavesdropper would
1320   already have seen the only document he could obtain with a replay.
1321   This is because the URI of the requested document is digested in the
1322   client request and the server will only deliver that document. By
1323   contrast under Basic Authentication once the eavesdropper has the
1324   user's password, any document protected by that password is open to
1325   him.
1326</t>
1327<t>
1328   Thus, for some purposes, it is necessary to protect against replay
1329   attacks. A good Digest implementation can do this in various ways.
1330   The server created "nonce" value is implementation dependent, but if
1331   it contains a digest of the client IP, a time-stamp, the resource
1332   ETag, and a private server key (as recommended above) then a replay
1333   attack is not simple. An attacker must convince the server that the
1334   request is coming from a false IP address and must cause the server
1335   to deliver the document to an IP address different from the address
1336   to which it believes it is sending the document. An attack can only
1337   succeed in the period before the time-stamp expires. Digesting the
1338   client IP and time-stamp in the nonce permits an implementation which
1339   does not maintain state between transactions.
1340</t>
1341<t>
1342   For applications where no possibility of replay attack can be
1343   tolerated the server can use one-time nonce values which will not be
1344   honored for a second use. This requires the overhead of the server
1345   remembering which nonce values have been used until the nonce time-stamp
1346   (and hence the digest built with it) has expired, but it
1347   effectively protects against replay attacks.
1348</t>
1349<t>
1350   An implementation must give special attention to the possibility of
1351   replay attacks with POST and PUT requests. Unless the server employs
1352   one-time or otherwise limited-use nonces and/or insists on the use of
1353   the integrity protection of qop=auth-int, an attacker could replay
1354   valid credentials from a successful request with counterfeit form
1355   data or other message body. Even with the use of integrity protection
1356   most metadata in header fields is not protected. Proper nonce
1357   generation and checking provides some protection against replay of
1358   previously used valid credentials, but see 4.8.
1359</t>
1360</section>
1361
1362<section title="Weakness Created by Multiple Authentication Schemes">
1363<t>
1364   An HTTP/1.1 server may return multiple challenges with a 401
1365   (Authenticate) response, and each challenge may use a different
1366   auth-scheme. A user agent &MUST; choose to use the strongest auth-scheme
1367   it understands and request credentials from the user based
1368   upon that challenge.
1369</t>
1370<t>
1371  <list><t>
1372      Note that many browsers will only recognize Basic and will require
1373      that it be the first auth-scheme presented. Servers should only
1374      include Basic if it is minimally acceptable.
1375  </t></list>
1376</t>
1377<t>
1378   When the server offers choices of authentication schemes using the
1379   WWW-Authenticate header, the strength of the resulting authentication
1380   is only as good as that of the of the weakest of the authentication
1381   schemes. See <xref target="man.in.the.middle"/> below for discussion of particular attack
1382   scenarios that exploit multiple authentication schemes.
1383</t>
1384</section>
1385
1386<section title="Online dictionary attacks">
1387<t>
1388   If the attacker can eavesdrop, then it can test any overheard
1389   nonce/response pairs against a list of common words. Such a list is
1390   usually much smaller than the total number of possible passwords. The
1391   cost of computing the response for each password on the list is paid
1392   once for each challenge.
1393</t>
1394<t>
1395   The server can mitigate this attack by not allowing users to select
1396   passwords that are in a dictionary.
1397</t>
1398</section>
1399
1400<section title="Man in the Middle" anchor="man.in.the.middle">
1401<t>
1402   Both Basic and Digest authentication are vulnerable to "man in the
1403   middle" (MITM) attacks, for example, from a hostile or compromised
1404   proxy. Clearly, this would present all the problems of eavesdropping.
1405   But it also offers some additional opportunities to the attacker.
1406</t>
1407<t>
1408   A possible man-in-the-middle attack would be to add a weak
1409   authentication scheme to the set of choices, hoping that the client
1410   will use one that exposes the user's credentials (e.g. password). For
1411   this reason, the client should always use the strongest scheme that
1412   it understands from the choices offered.
1413</t>
1414<t>
1415   An even better MITM attack would be to remove all offered choices,
1416   replacing them with a challenge that requests only Basic
1417   authentication, then uses the cleartext credentials from the Basic
1418   authentication to authenticate to the origin server using the
1419   stronger scheme it requested. A particularly insidious way to mount
1420   such a MITM attack would be to offer a "free" proxy caching service
1421   to gullible users.
1422</t>
1423<t>
1424   User agents should consider measures such as presenting a visual
1425   indication at the time of the credentials request of what
1426   authentication scheme is to be used, or remembering the strongest
1427   authentication scheme ever requested by a server and produce a
1428   warning message before using a weaker one. It might also be a good
1429   idea for the user agent to be configured to demand Digest
1430   authentication in general, or from specific sites.
1431</t>
1432<t>
1433   Or, a hostile proxy might spoof the client into making a request the
1434   attacker wanted rather than one the client wanted. Of course, this is
1435   still much harder than a comparable attack against Basic
1436   Authentication.
1437</t>
1438</section>
1439
1440<section title="Chosen plaintext attacks">
1441<t>
1442   With Digest authentication, a MITM or a malicious server can
1443   arbitrarily choose the nonce that the client will use to compute the
1444   response. This is called a "chosen plaintext" attack. The ability to
1445   choose the nonce is known to make cryptanalysis much easier <xref target="ref8"/>.
1446</t>
1447<t>
1448   However, no way to analyze the MD5 one-way function used by Digest
1449   using chosen plaintext is currently known.
1450</t>
1451<t>
1452   The countermeasure against this attack is for clients to be
1453   configured to require the use of the optional "cnonce" directive;
1454   this allows the client to vary the input to the hash in a way not
1455   chosen by the attacker.
1456</t>
1457</section>
1458
1459<section title="Precomputed dictionary attacks">
1460<t>
1461   With Digest authentication, if the attacker can execute a chosen
1462   plaintext attack, the attacker can precompute the response for many
1463   common words to a nonce of its choice, and store a dictionary of
1464   (response, password) pairs. Such precomputation can often be done in
1465   parallel on many machines. It can then use the chosen plaintext
1466   attack to acquire a response corresponding to that challenge, and
1467   just look up the password in the dictionary. Even if most passwords
1468   are not in the dictionary, some might be. Since the attacker gets to
1469   pick the challenge, the cost of computing the response for each
1470   password on the list can be amortized over finding many passwords. A
1471   dictionary with 100 million password/response pairs would take about
1472   3.2 gigabytes of disk storage.
1473</t>
1474<t>
1475   The countermeasure against this attack is to for clients to be
1476   configured to require the use of the optional "cnonce" directive.
1477</t>
1478</section>
1479
1480<section title="Batch brute force attacks">
1481<t>
1482   With Digest authentication, a MITM can execute a chosen plaintext
1483   attack, and can gather responses from many users to the same nonce.
1484   It can then find all the passwords within any subset of password
1485   space that would generate one of the nonce/response pairs in a single
1486   pass over that space. It also reduces the time to find the first
1487   password by a factor equal to the number of nonce/response pairs
1488   gathered. This search of the password space can often be done in
1489   parallel on many machines, and even a single machine can search large
1490   subsets of the password space very quickly -- reports exist of
1491   searching all passwords with six or fewer letters in a few hours.
1492</t>
1493<t>
1494   The countermeasure against this attack is to for clients to be
1495   configured to require the use of the optional "cnonce" directive.
1496</t>
1497</section>
1498
1499<section title="Spoofing by Counterfeit Servers">
1500<t>
1501   Basic Authentication is vulnerable to spoofing by counterfeit
1502   servers.  If a user can be led to believe that she is connecting to a
1503   host containing information protected by a password she knows, when
1504   in fact she is connecting to a hostile server, then the hostile
1505   server can request a password, store it away for later use, and feign
1506   an error.  This type of attack is more difficult with Digest
1507   Authentication -- but the client must know to demand that Digest
1508   authentication be used, perhaps using some of the techniques
1509   described above to counter "man-in-the-middle" attacks.  Again, the
1510   user can be helped in detecting this attack by a visual indication of
1511   the authentication mechanism in use with appropriate guidance in
1512   interpreting the implications of each scheme.
1513</t>
1514</section>
1515
1516<section title="Storing passwords">
1517<t>
1518   Digest authentication requires that the authenticating agent (usually
1519   the server) store some data derived from the user's name and password
1520   in a "password file" associated with a given realm. Normally this
1521   might contain pairs consisting of username and H(A1), where H(A1) is
1522   the digested value of the username, realm, and password as described
1523   above.
1524</t>
1525<t>
1526   The security implications of this are that if this password file is
1527   compromised, then an attacker gains immediate access to documents on
1528   the server using this realm. Unlike, say a standard UNIX password
1529   file, this information need not be decrypted in order to access
1530   documents in the server realm associated with this file. On the other
1531   hand, decryption, or more likely a brute force attack, would be
1532   necessary to obtain the user's password. This is the reason that the
1533   realm is part of the digested data stored in the password file. It
1534   means that if one Digest authentication password file is compromised,
1535   it does not automatically compromise others with the same username
1536   and password (though it does expose them to brute force attack).
1537</t>
1538<t>
1539   There are two important security consequences of this. First the
1540   password file must be protected as if it contained unencrypted
1541   passwords, because for the purpose of accessing documents in its
1542   realm, it effectively does.
1543</t>
1544<t>
1545   A second consequence of this is that the realm string should be
1546   unique among all realms which any single user is likely to use. In
1547   particular a realm string should include the name of the host doing
1548   the authentication. The inability of the client to authenticate the
1549   server is a weakness of Digest Authentication.
1550</t>
1551</section>
1552
1553<section title="Summary">
1554<t>
1555   By modern cryptographic standards Digest Authentication is weak. But
1556   for a large range of purposes it is valuable as a replacement for
1557   Basic Authentication. It remedies some, but not all, weaknesses of
1558   Basic Authentication. Its strength may vary depending on the
1559   implementation.  In particular the structure of the nonce (which is
1560   dependent on the server implementation) may affect the ease of
1561   mounting a replay attack.  A range of server options is appropriate
1562   since, for example, some implementations may be willing to accept the
1563   server overhead of one-time nonces or digests to eliminate the
1564   possibility of replay. Others may satisfied with a nonce like the one
1565   recommended above restricted to a single IP address and a single ETag
1566   or with a limited lifetime.
1567</t>
1568<t>
1569   The bottom line is that *any* compliant implementation will be
1570   relatively weak by cryptographic standards, but *any* compliant
1571   implementation will be far superior to Basic Authentication.
1572</t>
1573</section>
1574</section>
1575   
1576<section title="Sample implementation">
1577<t>
1578   The following code implements the calculations of H(A1), H(A2),
1579   request-digest and response-digest, and a test program which computes
1580   the values used in the example of <xref target="specification.of.digest.headers.example"/>. It uses the MD5
1581   implementation from RFC 1321.
1582</t>
1583<figure><preamble>
1584   File "digcalc.h":
1585</preamble><artwork type="code" name="digcalc.h">
1586#define HASHLEN 16
1587typedef char HASH[HASHLEN];
1588#define HASHHEXLEN 32
1589typedef char HASHHEX[HASHHEXLEN+1];
1590#define IN
1591#define OUT
1592
1593/* calculate H(A1) as per HTTP Digest spec */
1594void DigestCalcHA1(
1595    IN char * pszAlg,
1596    IN char * pszUserName,
1597    IN char * pszRealm,
1598    IN char * pszPassword,
1599    IN char * pszNonce,
1600    IN char * pszCNonce,
1601    OUT HASHHEX SessionKey
1602    );
1603
1604/* calculate request-digest/response-digest as per HTTP Digest spec */
1605void DigestCalcResponse(
1606    IN HASHHEX HA1,           /* H(A1) */
1607    IN char * pszNonce,       /* nonce from server */
1608    IN char * pszNonceCount,  /* 8 hex digits */
1609    IN char * pszCNonce,      /* client nonce */
1610    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1611    IN char * pszMethod,      /* method from the request */
1612    IN char * pszDigestUri,   /* requested URL */
1613    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1614    OUT HASHHEX Response      /* request-digest or response-digest */
1615    );
1616</artwork></figure>
1617<figure><preamble>
1618File "digcalc.c":
1619</preamble><artwork type="example" name="digcalc.c">
1620#include &lt;global.h>
1621#include &lt;md5.h>
1622#include &lt;string.h>
1623#include "digcalc.h"
1624
1625void CvtHex(
1626    IN HASH Bin,
1627    OUT HASHHEX Hex
1628    )
1629{
1630    unsigned short i;
1631    unsigned char j;
1632
1633    for (i = 0; i &lt; HASHLEN; i++) {
1634        j = (Bin[i] >> 4) &amp; 0xf;
1635        if (j &lt;= 9)
1636            Hex[i*2] = (j + '0');
1637         else
1638            Hex[i*2] = (j + 'a' - 10);
1639        j = Bin[i] &amp; 0xf;
1640        if (j &lt;= 9)
1641            Hex[i*2+1] = (j + '0');
1642         else
1643            Hex[i*2+1] = (j + 'a' - 10);
1644    };
1645    Hex[HASHHEXLEN] = '\0';
1646};
1647
1648/* calculate H(A1) as per spec */
1649void DigestCalcHA1(
1650    IN char * pszAlg,
1651    IN char * pszUserName,
1652    IN char * pszRealm,
1653    IN char * pszPassword,
1654    IN char * pszNonce,
1655    IN char * pszCNonce,
1656    OUT HASHHEX SessionKey
1657    )
1658{
1659      MD5_CTX Md5Ctx;
1660      HASH HA1;
1661
1662      MD5Init(&amp;Md5Ctx);
1663      MD5Update(&amp;Md5Ctx, pszUserName, strlen(pszUserName));
1664      MD5Update(&amp;Md5Ctx, ":", 1);
1665      MD5Update(&amp;Md5Ctx, pszRealm, strlen(pszRealm));
1666      MD5Update(&amp;Md5Ctx, ":", 1);
1667      MD5Update(&amp;Md5Ctx, pszPassword, strlen(pszPassword));
1668      MD5Final(HA1, &amp;Md5Ctx);
1669      if (stricmp(pszAlg, "md5-sess") == 0) {
1670            MD5Init(&amp;Md5Ctx);
1671            MD5Update(&amp;Md5Ctx, HA1, HASHLEN);
1672            MD5Update(&amp;Md5Ctx, ":", 1);
1673            MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1674            MD5Update(&amp;Md5Ctx, ":", 1);
1675            MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1676            MD5Final(HA1, &amp;Md5Ctx);
1677      };
1678      CvtHex(HA1, SessionKey);
1679};
1680
1681/* calculate request-digest/response-digest as per HTTP Digest spec */
1682void DigestCalcResponse(
1683    IN HASHHEX HA1,           /* H(A1) */
1684    IN char * pszNonce,       /* nonce from server */
1685    IN char * pszNonceCount,  /* 8 hex digits */
1686    IN char * pszCNonce,      /* client nonce */
1687    IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
1688    IN char * pszMethod,      /* method from the request */
1689    IN char * pszDigestUri,   /* requested URL */
1690    IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
1691    OUT HASHHEX Response      /* request-digest or response-digest */
1692    )
1693{
1694      MD5_CTX Md5Ctx;
1695      HASH HA2;
1696      HASH RespHash;
1697       HASHHEX HA2Hex;
1698
1699      // calculate H(A2)
1700      MD5Init(&amp;Md5Ctx);
1701      MD5Update(&amp;Md5Ctx, pszMethod, strlen(pszMethod));
1702      MD5Update(&amp;Md5Ctx, ":", 1);
1703      MD5Update(&amp;Md5Ctx, pszDigestUri, strlen(pszDigestUri));
1704      if (stricmp(pszQop, "auth-int") == 0) {
1705            MD5Update(&amp;Md5Ctx, ":", 1);
1706            MD5Update(&amp;Md5Ctx, HEntity, HASHHEXLEN);
1707      };
1708      MD5Final(HA2, &amp;Md5Ctx);
1709       CvtHex(HA2, HA2Hex);
1710
1711      // calculate response
1712      MD5Init(&amp;Md5Ctx);
1713      MD5Update(&amp;Md5Ctx, HA1, HASHHEXLEN);
1714      MD5Update(&amp;Md5Ctx, ":", 1);
1715      MD5Update(&amp;Md5Ctx, pszNonce, strlen(pszNonce));
1716      MD5Update(&amp;Md5Ctx, ":", 1);
1717      if (*pszQop) {
1718          MD5Update(&amp;Md5Ctx, pszNonceCount, strlen(pszNonceCount));
1719          MD5Update(&amp;Md5Ctx, ":", 1);
1720          MD5Update(&amp;Md5Ctx, pszCNonce, strlen(pszCNonce));
1721          MD5Update(&amp;Md5Ctx, ":", 1);
1722          MD5Update(&amp;Md5Ctx, pszQop, strlen(pszQop));
1723          MD5Update(&amp;Md5Ctx, ":", 1);
1724      };
1725      MD5Update(&amp;Md5Ctx, HA2Hex, HASHHEXLEN);
1726      MD5Final(RespHash, &amp;Md5Ctx);
1727      CvtHex(RespHash, Response);
1728};
1729</artwork></figure>
1730<figure><preamble>
1731File "digtest.c":
1732</preamble><artwork type="example" name="digtest.c">
1733#include &lt;stdio.h>
1734#include "digcalc.h"
1735
1736void main(int argc, char ** argv) {
1737
1738      char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
1739      char * pszCNonce = "0a4f113b";
1740      char * pszUser = "Mufasa";
1741      char * pszRealm = "testrealm@host.com";
1742      char * pszPass = "Circle Of Life";
1743      char * pszAlg = "md5";
1744      char szNonceCount[9] = "00000001";
1745      char * pszMethod = "GET";
1746      char * pszQop = "auth";
1747      char * pszURI = "/dir/index.html";
1748      HASHHEX HA1;
1749      HASHHEX HA2 = "";
1750      HASHHEX Response;
1751
1752      DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass, pszNonce,
1753pszCNonce, HA1);
1754      DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
1755       pszMethod, pszURI, HA2, Response);
1756      printf("Response = %s\n", Response);
1757};
1758</artwork></figure>
1759</section>
1760   
1761<section title="Acknowledgments">
1762<t>
1763   Eric W. Sink, of AbiSource, Inc., was one of the original authors
1764   before the specification underwent substantial revision.
1765</t>
1766<t>
1767   In addition to the authors, valuable discussion instrumental in
1768   creating this document has come from Peter J. Churchyard, Ned Freed,
1769   and David M.  Kristol.
1770</t>
1771<t>
1772   Jim Gettys and Larry Masinter edited this document for update.
1773</t>
1774</section>
1775  </middle>
1776  <back>
1777   
1778<references>
1779
1780<reference anchor='RFC1945'>
1781<front>
1782<title abbrev='HTTP/1.0'>Hypertext Transfer Protocol -- HTTP/1.0</title>
1783<author initials='T.' surname='Berners-Lee' fullname='Tim Berners-Lee'>
1784<organization>MIT, Laboratory for Computer Science</organization>
1785<address>
1786<postal>
1787<street>545 Technology Square</street>
1788<city>Cambridge</city>
1789<region>MA</region>
1790<code>02139</code>
1791<country>US</country></postal>
1792<facsimile>+1 617 258 8682</facsimile>
1793<email>timbl@w3.org</email></address></author>
1794<author initials='R.T.' surname='Fielding' fullname='Roy T. Fielding'>
1795<organization>University of California, Irvine, Department of Information and Computer Science</organization>
1796<address>
1797<postal>
1798<street />
1799<city>Irvine</city>
1800<region>CA</region>
1801<code>92717-3425</code>
1802<country>US</country></postal>
1803<facsimile>+1 714 824 4056</facsimile>
1804<email>fielding@ics.uci.edu</email></address></author>
1805<author initials='H.F.' surname='Nielsen' fullname='Henrik Frystyk Nielsen'>
1806<organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
1807<address>
1808<postal>
1809<street>545 Technology Square</street>
1810<city>Cambridge</city>
1811<region>MA</region>
1812<code>02139</code>
1813<country>US</country></postal>
1814<facsimile>+1 617 258 8682</facsimile>
1815<email>frystyk@w3.org</email></address></author>
1816<date year='1996' month='May' />
1817</front>
1818<seriesInfo name='RFC' value='1945' />
1819</reference>
1820
1821<reference anchor="RFC2616">
1822
1823<front>
1824<title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
1825<author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
1826<organization>University of California, Irvine, Information and Computer Science</organization>
1827<address>
1828<postal>
1829<street/>
1830<city>Irvine</city>
1831<region>CA</region>
1832<code>92697-3425</code>
1833<country>US</country></postal>
1834<phone>+1 949 824 1715</phone>
1835<email>fielding@ics.uci.edu</email></address></author>
1836<author initials="J." surname="Gettys" fullname="James Gettys">
1837<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
1838<address>
1839<postal>
1840<street>545 Technology Square</street>
1841<city>Cambridge</city>
1842<region>MA</region>
1843<code>02139</code>
1844<country>US</country></postal>
1845<phone/>
1846<facsimile>+1 617 258 8682</facsimile>
1847<email>jg@w3.org</email></address></author>
1848<author initials="J.C." surname="Mogul" fullname="Jeffrey C. Mogul">
1849<organization>Compaq Computer Corporation, Western Research Laboratory</organization>
1850<address>
1851<postal>
1852<street>250 University Avenue</street>
1853<city>Palo Alto</city>
1854<region>CA</region>
1855<code>94301</code>
1856<country>US</country></postal>
1857<phone/>
1858<email>mogul@wrl.dec.com</email></address></author>
1859<author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
1860<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
1861<address>
1862<postal>
1863<street>545 Technology Square</street>
1864<city>Cambridge</city>
1865<region>MA</region>
1866<code>02139</code>
1867<country>US</country></postal>
1868<phone/>
1869<facsimile>+1 617 258 8682</facsimile>
1870<email>frystyk@w3.org</email></address></author>
1871<author initials="L." surname="Masinter" fullname="Larry Masinter">
1872<organization>Xerox Corporation</organization>
1873<address>
1874<postal>
1875<street>3333 Coyote Hill Road</street>
1876<city>Palo Alto</city>
1877<region>CA</region>
1878<code>94034</code>
1879<country>US</country></postal>
1880<phone/>
1881<email>masinter@parc.xerox.com</email></address></author>
1882<author initials="P.J." surname="Leach" fullname="Paul J. Leach">
1883<organization>Microsoft Corporation</organization>
1884<address>
1885<postal>
1886<street>1 Microsoft Way</street>
1887<city>Redmond</city>
1888<region>WA</region>
1889<code>98052</code>
1890<country>US</country></postal>
1891<phone/>
1892<email>paulle@microsoft.com</email></address></author>
1893<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
1894<organization>World Wide Web Consortium, MIT Laboratory for Computer Science</organization>
1895<address>
1896<postal>
1897<street>545 Technology Square</street>
1898<city>Cambridge</city>
1899<region>MA</region>
1900<code>02139</code>
1901<country>US</country></postal>
1902<phone>+1 617 258 8682</phone>
1903<facsimile/>
1904<email>timbl@w3.org</email></address></author>
1905<date month="June" year="1999"/>
1906</front>
1907<seriesInfo name="RFC" value="2616"/>
[311]1908
1909  <x:source href="rfc2616.xml"/>
[9]1910</reference>
1911
1912<reference anchor='RFC1321'>
1913<front>
1914<title abbrev='MD5 Message-Digest Algorithm'>The MD5 Message-Digest Algorithm</title>
1915<author initials='R.' surname='Rivest' fullname='Ronald L. Rivest'>
1916<organization>Massachusetts Institute of Technology, (MIT) Laboratory for Computer Science</organization>
1917<address>
1918<postal>
1919<street>545 Technology Square</street>
1920<street>NE43-324</street>
1921<city>Cambridge</city>
1922<region>MA</region>
1923<code>02139-1986</code>
1924<country>US</country></postal>
1925<phone>+1 617 253 5880</phone>
1926<email>rivest@theory.lcs.mit.edu</email></address></author>
1927<date year='1992' month='April' /></front>
1928<seriesInfo name='RFC' value='1321' />
1929</reference>
1930
1931<reference anchor='RFC2045'>
1932<front>
1933<title abbrev='Internet Message Bodies'>Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies</title>
1934<author initials='N.' surname='Freed' fullname='Ned Freed'>
1935<organization>Innosoft International, Inc.</organization>
1936<address>
1937<postal>
1938<street>1050 East Garvey Avenue South</street>
1939<city>West Covina</city>
1940<region>CA</region>
1941<code>91790</code>
1942<country>US</country></postal>
1943<phone>+1 818 919 3600</phone>
1944<facsimile>+1 818 919 3614</facsimile>
1945<email>ned@innosoft.com</email></address></author>
1946<author initials='N.S.' surname='Borenstein' fullname='Nathaniel S. Borenstein'>
1947<organization>First Virtual Holdings</organization>
1948<address>
1949<postal>
1950<street>25 Washington Avenue</street>
1951<city>Morristown</city>
1952<region>NJ</region>
1953<code>07960</code>
1954<country>US</country></postal>
1955<phone>+1 201 540 8967</phone>
1956<facsimile>+1 201 993 3032</facsimile>
1957<email>nsb@nsb.fv.com</email></address></author>
1958<date year='1996' month='November' />
1959</front>
1960<seriesInfo name='RFC' value='2045' />
1961</reference>
1962
1963<reference anchor='RFC2246'>
1964<front>
1965<title>The TLS Protocol Version 1.0</title>
1966<author initials='T.' surname='Dierks' fullname='Tim Dierks'>
1967<organization>Certicom</organization>
1968<address>
1969<email>tdierks@certicom.com</email></address></author>
1970<author initials='C.' surname='Allen' fullname='Christopher Allen'>
1971<organization>Certicom</organization>
1972<address>
1973<email>callen@certicom.com</email></address></author>
1974<date year='1999' month='January' />
1975</front>
1976<seriesInfo name='RFC' value='2246' />
1977<format type='TXT' octets='170401' target='ftp://ftp.isi.edu/in-notes/rfc2246.txt' />
1978</reference>
1979
1980
1981<reference anchor='RFC2069'>
1982<front>
1983<title abbrev='Digest Access Authentication'>An Extension to HTTP : Digest Access Authentication</title>
1984<author initials='J.' surname='Franks' fullname='John Franks'>
1985<organization>Northwestern University,  Department of Mathematics</organization>
1986<address>
1987<postal>
1988<street />
1989<city>Evanston</city>
1990<region>IL</region>
1991<code>60208-2730</code>
1992<country>US</country></postal>
1993<email>john@math.nwu.edu</email></address></author>
1994<author initials='P.' surname='Hallam-Baker' fullname='Phillip M. Hallam-Baker'>
1995<organization>CERN</organization>
1996<address>
1997<postal>
1998<street />
1999<city>Geneva</city>
2000<country>CH</country></postal>
2001<email>hallam@w3.org</email></address></author>
2002<author initials='J.' surname='Hostetler' fullname='Jeffery L. Hostetler'>
2003<organization>Spyglass, Inc.</organization>
2004<address>
2005<postal>
2006<street>3200 Farber Drive</street>
2007<city>Champaign</city>
2008<region>IL</region>
2009<code>61821</code>
2010<country>US</country></postal>
2011<email>jeff@spyglass.com</email></address></author>
2012<author initials='P.' surname='Leach' fullname='Paul J. Leach'>
2013<organization>Microsoft Corporation</organization>
2014<address>
2015<postal>
2016<street>1 Microsoft Way</street>
2017<city>Redmond</city>
2018<region>WA</region>
2019<code>98052</code>
2020<country>US</country></postal>
2021<email>paulle@microsoft.com</email></address></author>
2022<author initials='A.' surname='Luotonen' fullname='Ari Luotonen'>
2023<organization>Netscape Communications Corporation</organization>
2024<address>
2025<postal>
2026<street>501 East Middlefield Road</street>
2027<city>Mountain View</city>
2028<region>CA</region>
2029<code>94043</code>
2030<country>US</country></postal>
2031<email>luotonen@netscape.com</email></address></author>
2032<author initials='E.' surname='Sink' fullname='Eric W. Sink'>
2033<organization>Spyglass, Inc.</organization>
2034<address>
2035<postal>
2036<street>3200 Farber Drive</street>
2037<city>Champaign</city>
2038<region>IL</region>
2039<code>61821</code>
2040<country>US</country></postal>
2041<email>eric@spyglass.com</email></address></author>
2042<author initials='L.' surname='Stewart' fullname='Lawrence C. Stewart'>
2043<organization>Open Market, Inc.</organization>
2044<address>
2045<postal>
2046<street>215 First Street</street>
2047<city>Cambridge</city>
2048<region>MA</region>
2049<code>02142</code>
2050<country>US</country></postal>
2051<email>stewart@OpenMarket.com</email></address></author>
2052<date year='1997' month='January' />
2053</front>
2054<seriesInfo name='RFC' value='2069' />
2055</reference>
2056
2057<reference anchor="RFC2396">
2058  <front>
2059    <title abbrev="URI Generic Syntax">Uniform Resource Identifiers (URI): Generic Syntax</title>
2060    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
2061      <organization>World Wide Web Consortium</organization>
2062      <address>
2063      <email>timbl@w3.org</email></address>
2064    </author>
2065    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
2066      <organization>Department of Information and Computer Science</organization>
2067      <address>
2068        <email>fielding@ics.uci.edu</email>
2069      </address>
2070    </author>
2071    <author initials="L." surname="Masinter" fullname="Larry Masinter">
2072      <organization>Xerox PARC</organization>
2073      <address>
2074        <email>masinter@parc.xerox.com</email>
2075      </address>
2076    </author>
2077    <date month="August" year="1998"/>
2078  </front>
2079  <seriesInfo name="RFC" value="2396"/>
2080</reference>
2081
2082<reference anchor="ref8" target="http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm">
2083  <front>
2084    <title>Message Authentication with MD5</title>
2085    <author initials="B." surname="Kaliski">
2086      <organization/>
2087    </author>
2088    <author initials="M." surname="Robshaw">
2089      <organization/>
2090    </author>
2091    <date year="1995"/>
2092  </front>
2093  <annotation>CryptoBytes, Spring 1995</annotation>
2094</reference>
2095
2096<reference anchor='RFC2195'>
2097<front>
2098<title abbrev='IMAP/POP AUTHorize Extension'>IMAP/POP AUTHorize Extension for Simple Challenge/Response</title>
2099<author initials='J.C.' surname='Klensin' fullname='John C. Klensin'>
2100<organization>MCI</organization>
2101<address>
2102<postal>
2103<street>800 Boylston</street>
2104<street>7th floor</street>
2105<street>Boston</street>
2106<street>MA 02199</street>
2107<country>USA</country></postal>
2108<phone>+1 617 960 1011</phone>
2109<email>klensin@mci.net</email></address></author>
2110<author initials='R.' surname='Catoe' fullname='Randy Catoe'>
2111<organization>MCI</organization>
2112<address>
2113<postal>
2114<street>2100 Reston Parkway</street>
2115<street>Reston</street>
2116<street>VA 22091</street>
2117<country>USA</country></postal>
2118<phone>+1 703 715 7366</phone>
2119<email>randy@mci.net</email></address></author>
2120<author initials='P.' surname='Krumviede' fullname='Paul Krumviede'>
2121<organization>MCI</organization>
2122<address>
2123<postal>
2124<street>2100 Reston Parkway</street>
2125<street>Reston</street>
2126<street>VA 22091</street>
2127<country>USA</country></postal>
2128<phone>+1 703 715 7251</phone>
2129<email>paul@mci.net</email></address></author>
2130<date year='1997' month='September' />
2131<area>Applications</area>
2132<keyword>IMAP</keyword>
2133<keyword>authentication</keyword>
2134<keyword>internet message access protocol</keyword>
2135<keyword>post office protocol</keyword>
2136<keyword>security</keyword>
2137</front>
2138<seriesInfo name='RFC' value='2195' />
2139</reference>
2140
2141<reference anchor="ref10">
2142  <front>
2143    <title>Authentication Methods for LDAP</title>
2144    <author initials="B." surname="Morgan">
2145      <organization/>
2146    </author>
2147    <author initials="H." surname="Alvestrand">
2148      <organization/>
2149    </author>
2150    <author initials="J." surname="Hodges">
2151      <organization/>
2152    </author>
2153    <author initials="M." surname="Wahl">
2154      <organization/>
2155    </author>
2156    <date/>
2157  </front>
2158  <annotation>Work in progress.</annotation>
2159</reference>
2160</references> 
2161 
2162</back>
[311]2163</rfc>
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