GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc2617

Network Working Group J. Franks Request for Comments: 2617 Northwestern University Obsoletes: 2069 P. Hallam-Baker Category: Standards Track Verisign, Inc.

                                                          J. Hostetler
                                                       AbiSource, Inc.
                                                           S. Lawrence
                                                 Agranat Systems, Inc.
                                                              P. Leach
                                                 Microsoft Corporation
                                                           A. Luotonen
                                   Netscape Communications Corporation
                                                            L. Stewart
                                                     Open Market, Inc.
                                                             June 1999
    HTTP Authentication: Basic and Digest Access Authentication

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 "HTTP/1.0", includes the specification for a Basic Access
 Authentication scheme. This scheme is not considered to be a secure
 method of user authentication (unless used in conjunction with some
 external secure system such as SSL [5]), as the user name and
 password are passed over the network as cleartext.
 This document also provides the specification for HTTP's
 authentication framework, the original Basic authentication scheme
 and a scheme based on cryptographic hashes, referred to as "Digest
 Access Authentication".  It is therefore also intended to serve as a
 replacement for RFC 2069 [6].  Some optional elements specified by
 RFC 2069 have been removed from this specification due to problems
 found since its publication; other new elements have been added for
 compatibility, those new elements have been made optional, but are
 strongly recommended.

Franks, et al. Standards Track [Page 1] RFC 2617 HTTP Authentication June 1999

 Like Basic, Digest access authentication verifies that both parties
 to a communication know a shared secret (a password); unlike Basic,
 this verification can be done without sending the password in the
 clear, which is Basic's biggest weakness. As with most other
 authentication protocols, the greatest sources of risks are usually
 found not in the core protocol itself but in policies and procedures
 surrounding its use.

Table of Contents

 1   Access Authentication................................   3
  1.1   Reliance on the HTTP/1.1 Specification............   3
  1.2   Access Authentication Framework...................   3
 2   Basic Authentication Scheme..........................   5
 3   Digest Access Authentication Scheme..................   6
  3.1   Introduction......................................   6
   3.1.1  Purpose.........................................   6
   3.1.2  Overall Operation...............................   6
   3.1.3  Representation of digest values.................   7
   3.1.4  Limitations.....................................   7
  3.2   Specification of Digest Headers...................   7
   3.2.1  The WWW-Authenticate Response Header............   8
   3.2.2  The Authorization Request Header................  11
   3.2.3  The Authentication-Info Header..................  15
  3.3   Digest Operation..................................  17
  3.4   Security Protocol Negotiation.....................  18
  3.5   Example...........................................  18
  3.6   Proxy-Authentication and Proxy-Authorization......  19
 4   Security Considerations..............................  19
  4.1   Authentication of Clients using Basic
        Authentication....................................  19
  4.2   Authentication of Clients using Digest
        Authentication....................................  20
  4.3   Limited Use Nonce Values..........................  21
  4.4   Comparison of Digest with Basic Authentication....  22
  4.5   Replay Attacks....................................  22
  4.6   Weakness Created by Multiple Authentication
        Schemes...........................................  23
  4.7   Online dictionary attacks.........................  23
  4.8   Man in the Middle.................................  24
  4.9   Chosen plaintext attacks..........................  24
  4.10  Precomputed dictionary attacks....................  25
  4.11  Batch brute force attacks.........................  25
  4.12  Spoofing by Counterfeit Servers...................  25
  4.13  Storing passwords.................................  26
  4.14  Summary...........................................  26
 5   Sample implementation................................  27
 6   Acknowledgments......................................  31

Franks, et al. Standards Track [Page 2] RFC 2617 HTTP Authentication June 1999

 7   References...........................................  31
 8   Authors' Addresses...................................  32
 9   Full Copyright Statement.............................  34

1 Access Authentication

1.1 Reliance on the HTTP/1.1 Specification

 This specification is a companion to the HTTP/1.1 specification [2].
 It uses the augmented BNF section 2.1 of that document, and relies on
 both the non-terminals defined in that document and other aspects of
 the HTTP/1.1 specification.

1.2 Access Authentication Framework

 HTTP provides a simple challenge-response authentication mechanism
 that MAY be used by a server to challenge a client request and by a
 client to provide authentication information. It uses an extensible,
 case-insensitive token to identify the authentication scheme,
 followed by a comma-separated list of attribute-value pairs which
 carry the parameters necessary for achieving authentication via that
 scheme.
    auth-scheme    = token
    auth-param     = token "=" ( token | quoted-string )
 The 401 (Unauthorized) response message is used by an origin server
 to challenge the authorization of a user agent. This response MUST
 include a WWW-Authenticate header field containing at least one
 challenge applicable to the requested resource. The 407 (Proxy
 Authentication Required) response message is used by a proxy to
 challenge the authorization of a client and MUST include a Proxy-
 Authenticate header field containing at least one challenge
 applicable to the proxy for the requested resource.
    challenge   = auth-scheme 1*SP 1#auth-param
 Note: User agents will need to take special care in parsing the WWW-
 Authenticate or Proxy-Authenticate header field value if it contains
 more than one challenge, or if more than one WWW-Authenticate header
 field is provided, since the contents of a challenge may itself
 contain a comma-separated list of authentication parameters.
 The authentication parameter realm is defined for all authentication
 schemes:
    realm       = "realm" "=" realm-value
    realm-value = quoted-string

Franks, et al. Standards Track [Page 3] RFC 2617 HTTP Authentication June 1999

 The realm directive (case-insensitive) is required for all
 authentication schemes that issue a challenge. The realm value
 (case-sensitive), in combination with the canonical root URL (the
 absoluteURI for the server whose abs_path is empty; see section 5.1.2
 of [2]) of the server being accessed, defines the protection space.
 These realms allow the protected resources on a server to be
 partitioned into a set of protection spaces, each with its own
 authentication scheme and/or authorization database. The realm value
 is a string, generally assigned by the origin server, which may have
 additional semantics specific to the authentication scheme. Note that
 there may be multiple challenges with the same auth-scheme but
 different realms.
 A user agent that wishes to authenticate itself with an origin
 server--usually, but not necessarily, after receiving a 401
 (Unauthorized)--MAY do so by including an Authorization header field
 with the request. A client that wishes to authenticate itself with a
 proxy--usually, but not necessarily, after receiving a 407 (Proxy
 Authentication Required)--MAY do so by including a Proxy-
 Authorization header field with the request.  Both the Authorization
 field value and the Proxy-Authorization field value consist of
 credentials containing the authentication information of the client
 for the realm of the resource being requested. The user agent MUST
 choose to use one of the challenges with the strongest auth-scheme it
 understands and request credentials from the user based upon that
 challenge.
 credentials = auth-scheme #auth-param
    Note that many browsers will only recognize Basic and will require
    that it be the first auth-scheme presented. Servers should only
    include Basic if it is minimally acceptable.
 The protection space determines the domain over which credentials can
 be automatically applied. If a prior request has been authorized, the
 same credentials MAY be reused for all other requests within that
 protection space for a period of time determined by the
 authentication scheme, parameters, and/or user preference. Unless
 otherwise defined by the authentication scheme, a single protection
 space cannot extend outside the scope of its server.
 If the origin server does not wish to accept the credentials sent
 with a request, it SHOULD return a 401 (Unauthorized) response. The
 response MUST include a WWW-Authenticate header field containing at
 least one (possibly new) challenge applicable to the requested
 resource. If a proxy does not accept the credentials sent with a
 request, it SHOULD return a 407 (Proxy Authentication Required). The
 response MUST include a Proxy-Authenticate header field containing a

Franks, et al. Standards Track [Page 4] RFC 2617 HTTP Authentication June 1999

 (possibly new) challenge applicable to the proxy for the requested
 resource.
 The HTTP protocol does not restrict applications to this simple
 challenge-response mechanism for access authentication. Additional
 mechanisms MAY be used, such as encryption at the transport level or
 via message encapsulation, and with additional header fields
 specifying authentication information. However, these additional
 mechanisms are not defined by this specification.
 Proxies MUST be completely transparent regarding user agent
 authentication by origin servers. That is, they must forward the
 WWW-Authenticate and Authorization headers untouched, and follow the
 rules found in section 14.8 of [2]. Both the Proxy-Authenticate and
 the Proxy-Authorization header fields are hop-by-hop headers (see
 section 13.5.1 of [2]).

2 Basic Authentication Scheme

 The "basic" authentication scheme is based on the model that the
 client must authenticate itself with a user-ID and a password for
 each realm.  The realm value should be considered an opaque string
 which can only be compared for equality with other realms on that
 server. The server will service the request only if it can validate
 the user-ID and password for the protection space of the Request-URI.
 There are no optional authentication parameters.
 For Basic, the framework above is utilized as follows:
    challenge   = "Basic" realm
    credentials = "Basic" basic-credentials
 Upon receipt of an unauthorized request for a URI within the
 protection space, the origin server MAY respond with a challenge like
 the following:
    WWW-Authenticate: Basic realm="WallyWorld"
 where "WallyWorld" is the string assigned by the server to identify
 the protection space of the Request-URI. A proxy may respond with the
 same challenge using the Proxy-Authenticate header field.
 To receive authorization, the client sends the userid and password,
 separated by a single colon (":") character, within a base64 [7]
 encoded string in the credentials.
    basic-credentials = base64-user-pass
    base64-user-pass  = <base64 [4] encoding of user-pass,

Franks, et al. Standards Track [Page 5] RFC 2617 HTTP Authentication June 1999

                     except not limited to 76 char/line>
    user-pass   = userid ":" password
    userid      = *<TEXT excluding ":">
    password    = *TEXT
 Userids might be case sensitive.
 If the user agent wishes to send the userid "Aladdin" and password
 "open sesame", it would use the following header field:
    Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
 A client SHOULD assume that all paths at or deeper than the depth of
 the last symbolic element in the path field of the Request-URI also
 are within the protection space specified by the Basic realm value of
 the current challenge. A client MAY preemptively send the
 corresponding Authorization header with requests for resources in
 that space without receipt of another challenge from the server.
 Similarly, when a client sends a request to a proxy, it may reuse a
 userid and password in the Proxy-Authorization header field without
 receiving another challenge from the proxy server. See section 4 for
 security considerations associated with Basic authentication.

3 Digest Access Authentication Scheme

3.1 Introduction

3.1.1 Purpose

 The protocol referred to as "HTTP/1.0" includes the specification for
 a Basic Access Authentication scheme[1]. That scheme is not
 considered to be a secure method of user authentication, as the user
 name and password are passed over the network in an unencrypted form.
 This section provides the specification for a scheme that does not
 send the password in cleartext,  referred to as "Digest Access
 Authentication".
 The Digest Access Authentication scheme is not intended to be a
 complete answer to the need for security in the World Wide Web. This
 scheme provides no encryption of message content. The intent is
 simply to create an access authentication method that avoids the most
 serious flaws of Basic authentication.

3.1.2 Overall Operation

 Like Basic Access Authentication, the Digest scheme is based on a
 simple challenge-response paradigm. The Digest scheme challenges
 using a nonce value. A valid response contains a checksum (by

Franks, et al. Standards Track [Page 6] RFC 2617 HTTP Authentication June 1999

 default, the MD5 checksum) of the username, the password, the given
 nonce value, the HTTP method, and the requested URI. In this way, the
 password is never sent in the clear. Just as with the Basic scheme,
 the username and password must be prearranged in some fashion not
 addressed by this document.

3.1.3 Representation of digest values

 An optional header allows the server to specify the algorithm used to
 create the checksum or digest. By default the MD5 algorithm is used
 and that is the only algorithm described in this document.
 For the purposes of this document, an MD5 digest of 128 bits is
 represented as 32 ASCII printable characters. The bits in the 128 bit
 digest are converted from most significant to least significant bit,
 four bits at a time to their ASCII presentation as follows. Each four
 bits is represented by its familiar hexadecimal notation from the
 characters 0123456789abcdef. That is, binary 0000 gets represented by
 the character '0', 0001, by '1', and so on up to the representation
 of 1111 as 'f'.

3.1.4 Limitations

 The Digest authentication scheme described in this document suffers
 from many known limitations. It is intended as a replacement for
 Basic authentication and nothing more. It is a password-based system
 and (on the server side) suffers from all the same problems of any
 password system. In particular, no provision is made in this protocol
 for the initial secure arrangement between user and server to
 establish the user's password.
 Users and implementors should be aware that this protocol is not as
 secure as Kerberos, and not as secure as any client-side private-key
 scheme. Nevertheless it is better than nothing, better than what is
 commonly used with telnet and ftp, and better than Basic
 authentication.

3.2 Specification of Digest Headers

 The Digest Access Authentication scheme is conceptually similar to
 the Basic scheme. The formats of the modified WWW-Authenticate header
 line and the Authorization header line are specified below. In
 addition, a new header, Authentication-Info, is specified.

Franks, et al. Standards Track [Page 7] RFC 2617 HTTP Authentication June 1999

3.2.1 The WWW-Authenticate Response Header

 If a server receives a request for an access-protected object, and an
 acceptable Authorization header is not sent, the server responds with
 a "401 Unauthorized" status code, and a WWW-Authenticate header as
 per the framework defined above, which for the digest scheme is
 utilized as follows:
    challenge        =  "Digest" digest-challenge
    digest-challenge  = 1#( realm | [ domain ] | nonce |
                        [ opaque ] |[ stale ] | [ algorithm ] |
                        [ qop-options ] | [auth-param] )
    domain            = "domain" "=" <"> URI ( 1*SP URI ) <">
    URI               = absoluteURI | abs_path
    nonce             = "nonce" "=" nonce-value
    nonce-value       = quoted-string
    opaque            = "opaque" "=" quoted-string
    stale             = "stale" "=" ( "true" | "false" )
    algorithm         = "algorithm" "=" ( "MD5" | "MD5-sess" |
                         token )
    qop-options       = "qop" "=" <"> 1#qop-value <">
    qop-value         = "auth" | "auth-int" | token
 The meanings of the values of the directives used above are as
 follows:
 realm
   A string to be displayed to users so they know which username and
   password to use. This string should contain at least the name of
   the host performing the authentication and might additionally
   indicate the collection of users who might have access. An example
   might be "registered_users@gotham.news.com".
 domain
   A quoted, space-separated list of URIs, as specified in RFC XURI
   [7], that define the protection space.  If a URI is an abs_path, it
   is relative to the canonical root URL (see section 1.2 above) of
   the server being accessed. An absoluteURI in this list may refer to
   a different server than the one being accessed. The client can use
   this list to determine the set of URIs for which the same
   authentication information may be sent: any URI that has a URI in
   this list as a prefix (after both have been made absolute) may be
   assumed to be in the same protection space. If this directive is
   omitted or its value is empty, the client should assume that the
   protection space consists of all URIs on the responding server.

Franks, et al. Standards Track [Page 8] RFC 2617 HTTP Authentication June 1999

   This directive is not meaningful in Proxy-Authenticate headers, for
   which the protection space is always the entire proxy; if present
   it should be ignored.
 nonce
   A server-specified data string which should be uniquely generated
   each time a 401 response is made. It is recommended that this
   string be base64 or hexadecimal data. Specifically, since the
   string is passed in the header lines as a quoted string, the
   double-quote character is not allowed.
   The contents of the nonce are implementation dependent. The quality
   of the implementation depends on a good choice. A nonce might, for
   example, be constructed as the base 64 encoding of
       time-stamp H(time-stamp ":" ETag ":" private-key)
   where time-stamp is a server-generated time or other non-repeating
   value, ETag is the value of the HTTP ETag header associated with
   the requested entity, and private-key is data known only to the
   server.  With a nonce of this form a server would recalculate the
   hash portion after receiving the client authentication header and
   reject the request if it did not match the nonce from that header
   or if the time-stamp value is not recent enough. In this way the
   server can limit the time of the nonce's validity. The inclusion of
   the ETag prevents a replay request for an updated version of the
   resource.  (Note: including the IP address of the client in the
   nonce would appear to offer the server the ability to limit the
   reuse of the nonce to the same client that originally got it.
   However, that would break proxy farms, where requests from a single
   user often go through different proxies in the farm. Also, IP
   address spoofing is not that hard.)
   An implementation might choose not to accept a previously used
   nonce or a previously used digest, in order to protect against a
   replay attack. Or, an implementation might choose to use one-time
   nonces or digests for POST or PUT requests and a time-stamp for GET
   requests.  For more details on the issues involved see section 4.
   of this document.
   The nonce is opaque to the client.
 opaque
   A string of data, specified by the server, which should be returned
   by the client unchanged in the Authorization header of subsequent
   requests with URIs in the same protection space. It is recommended
   that this string be base64 or hexadecimal data.

Franks, et al. Standards Track [Page 9] RFC 2617 HTTP Authentication June 1999

 stale
   A flag, indicating that the previous request from the client was
   rejected because the nonce value was stale. If stale is TRUE
   (case-insensitive), the client may wish to simply retry the request
   with a new encrypted response, without reprompting the user for a
   new username and password. The server should only set stale to TRUE
   if it receives a request for which the nonce is invalid but with a
   valid digest for that nonce (indicating that the client knows the
   correct username/password). If stale is FALSE, or anything other
   than TRUE, or the stale directive is not present, the username
   and/or password are invalid, and new values must be obtained.
 algorithm
   A string indicating a pair of algorithms used to produce the digest
   and a checksum. If this is not present it is assumed to be "MD5".
   If the algorithm is not understood, the challenge should be ignored
   (and a different one used, if there is more than one).
   In this document the string obtained by applying the digest
   algorithm to the data "data" with secret "secret" will be denoted
   by KD(secret, data), and the string obtained by applying the
   checksum algorithm to the data "data" will be denoted H(data). The
   notation unq(X) means the value of the quoted-string X without the
   surrounding quotes.
   For the "MD5" and "MD5-sess" algorithms
       H(data) = MD5(data)
   and
       KD(secret, data) = H(concat(secret, ":", data))
   i.e., the digest is the MD5 of the secret concatenated with a colon
   concatenated with the data. The "MD5-sess" algorithm is intended to
   allow efficient 3rd party authentication servers; for the
   difference in usage, see the description in section 3.2.2.2.
 qop-options
   This directive is optional, but is made so only for backward
   compatibility with RFC 2069 [6]; it SHOULD be used by all
   implementations compliant with this version of the Digest scheme.
   If present, it is a quoted string of one or more tokens indicating
   the "quality of protection" values supported by the server.  The
   value "auth" indicates authentication; the value "auth-int"
   indicates authentication with integrity protection; see the

Franks, et al. Standards Track [Page 10] RFC 2617 HTTP Authentication June 1999

   descriptions below for calculating the response directive value for
   the application of this choice. Unrecognized options MUST be
   ignored.
 auth-param
   This directive allows for future extensions. Any unrecognized
   directive MUST be ignored.

3.2.2 The Authorization Request Header

 The client is expected to retry the request, passing an Authorization
 header line, which is defined according to the framework above,
 utilized as follows.
     credentials      = "Digest" digest-response
     digest-response  = 1#( username | realm | nonce | digest-uri
                     | response | [ algorithm ] | [cnonce] |
                     [opaque] | [message-qop] |
                         [nonce-count]  | [auth-param] )
     username         = "username" "=" username-value
     username-value   = quoted-string
     digest-uri       = "uri" "=" digest-uri-value
     digest-uri-value = request-uri   ; As specified by HTTP/1.1
     message-qop      = "qop" "=" qop-value
     cnonce           = "cnonce" "=" cnonce-value
     cnonce-value     = nonce-value
     nonce-count      = "nc" "=" nc-value
     nc-value         = 8LHEX
     response         = "response" "=" request-digest
     request-digest = <"> 32LHEX <">
     LHEX             =  "0" | "1" | "2" | "3" |
                         "4" | "5" | "6" | "7" |
                         "8" | "9" | "a" | "b" |
                         "c" | "d" | "e" | "f"
 The values of the opaque and algorithm fields must be those supplied
 in the WWW-Authenticate response header for the entity being
 requested.
 response
   A string of 32 hex digits computed as defined below, which proves
   that the user knows a password
 username
   The user's name in the specified realm.

Franks, et al. Standards Track [Page 11] RFC 2617 HTTP Authentication June 1999

 digest-uri
   The URI from Request-URI of the Request-Line; duplicated here
   because proxies are allowed to change the Request-Line in transit.
 qop
   Indicates what "quality of protection" the client has applied to
   the message. If present, its value MUST be one of the alternatives
   the server indicated it supports in the WWW-Authenticate header.
   These values affect the computation of the request-digest. Note
   that this is a single token, not a quoted list of alternatives as
   in WWW- Authenticate.  This directive is optional in order to
   preserve backward compatibility with a minimal implementation of
   RFC 2069 [6], but SHOULD be used if the server indicated that qop
   is supported by providing a qop directive in the WWW-Authenticate
   header field.
 cnonce
   This MUST be specified if a qop directive is sent (see above), and
   MUST NOT be specified if the server did not send a qop directive in
   the WWW-Authenticate header field.  The cnonce-value is an opaque
   quoted string value provided by the client and used by both client
   and server to avoid chosen plaintext attacks, to provide mutual
   authentication, and to provide some message integrity protection.
   See the descriptions below of the calculation of the response-
   digest and request-digest values.
 nonce-count
   This MUST be specified if a qop directive is sent (see above), and
   MUST NOT be specified if the server did not send a qop directive in
   the WWW-Authenticate header field.  The nc-value is the hexadecimal
   count of the number of requests (including the current request)
   that the client has sent with the nonce value in this request.  For
   example, in the first request sent in response to a given nonce
   value, the client sends "nc=00000001".  The purpose of this
   directive is to allow the server to detect request replays by
   maintaining its own copy of this count - if the same nc-value is
   seen twice, then the request is a replay.   See the description
   below of the construction of the request-digest value.
 auth-param
   This directive allows for future extensions. Any unrecognized
   directive MUST be ignored.
 If a directive or its value is improper, or required directives are
 missing, the proper response is 400 Bad Request. If the request-
 digest is invalid, then a login failure should be logged, since
 repeated login failures from a single client may indicate an attacker
 attempting to guess passwords.

Franks, et al. Standards Track [Page 12] RFC 2617 HTTP Authentication June 1999

 The definition of request-digest above indicates the encoding for its
 value. The following definitions show how the value is computed.

3.2.2.1 Request-Digest

 If the "qop" value is "auth" or "auth-int":
    request-digest  = <"> < KD ( H(A1),     unq(nonce-value)
                                        ":" nc-value
                                        ":" unq(cnonce-value)
                                        ":" unq(qop-value)
                                        ":" H(A2)
                                ) <">
 If the "qop" directive is not present (this construction is for
 compatibility with RFC 2069):
    request-digest  =
               <"> < KD ( H(A1), unq(nonce-value) ":" H(A2) ) >
 <">
 See below for the definitions for A1 and A2.

3.2.2.2 A1

 If the "algorithm" directive's value is "MD5" or is unspecified, then
 A1 is:
    A1       = unq(username-value) ":" unq(realm-value) ":" passwd
 where
    passwd   = < user's password >
 If the "algorithm" directive's value is "MD5-sess", then A1 is
 calculated only once - on the first request by the client following
 receipt of a WWW-Authenticate challenge from the server.  It uses the
 server nonce from that challenge, and the first client nonce value to
 construct A1 as follows:
    A1       = H( unq(username-value) ":" unq(realm-value)
                   ":" passwd )
                   ":" unq(nonce-value) ":" unq(cnonce-value)
 This creates a 'session key' for the authentication of subsequent
 requests and responses which is different for each "authentication
 session", thus limiting the amount of material hashed with any one
 key.  (Note: see further discussion of the authentication session in

Franks, et al. Standards Track [Page 13] RFC 2617 HTTP Authentication June 1999

 section 3.3.) Because the server need only use the hash of the user
 credentials in order to create the A1 value, this construction could
 be used in conjunction with a third party authentication service so
 that the web server would not need the actual password value.  The
 specification of such a protocol is beyond the scope of this
 specification.

3.2.2.3 A2

 If the "qop" directive's value is "auth" or is unspecified, then A2
 is:
    A2       = Method ":" digest-uri-value
 If the "qop" value is "auth-int", then A2 is:
    A2       = Method ":" digest-uri-value ":" H(entity-body)

3.2.2.4 Directive values and quoted-string

 Note that the value of many of the directives, such as "username-
 value", are defined as a "quoted-string". However, the "unq" notation
 indicates that surrounding quotation marks are removed in forming the
 string A1. Thus if the Authorization header includes the fields
   username="Mufasa", realm=myhost@testrealm.com
 and the user Mufasa has password "Circle Of Life" then H(A1) would be
 H(Mufasa:myhost@testrealm.com:Circle Of Life) with no quotation marks
 in the digested string.
 No white space is allowed in any of the strings to which the digest
 function H() is applied unless that white space exists in the quoted
 strings or entity body whose contents make up the string to be
 digested. For example, the string A1 illustrated above must be
      Mufasa:myhost@testrealm.com:Circle Of Life
 with no white space on either side of the colons, but with the white
 space between the words used in the password value.  Likewise, the
 other strings digested by H() must not have white space on either
 side of the colons which delimit their fields unless that white space
 was in the quoted strings or entity body being digested.
 Also note that if integrity protection is applied (qop=auth-int), the
 H(entity-body) is the hash of the entity body, not the message body -
 it is computed before any transfer encoding is applied by the sender

Franks, et al. Standards Track [Page 14] RFC 2617 HTTP Authentication June 1999

 and after it has been removed by the recipient. Note that this
 includes multipart boundaries and embedded headers in each part of
 any multipart content-type.

3.2.2.5 Various considerations

 The "Method" value is the HTTP request method as specified in section
 5.1.1 of [2]. The "request-uri" value is the Request-URI from the
 request line as specified in section 5.1.2 of [2]. This may be "*",
 an "absoluteURL" or an "abs_path" as specified in section 5.1.2 of
 [2], but it MUST agree with the Request-URI. In particular, it MUST
 be an "absoluteURL" if the Request-URI is an "absoluteURL". The
 "cnonce-value" is an optional  client-chosen value whose purpose is
 to foil chosen plaintext attacks.
 The authenticating server must assure that the resource designated by
 the "uri" directive is the same as the resource specified in the
 Request-Line; if they are not, the server SHOULD return a 400 Bad
 Request error. (Since this may be a symptom of an attack, server
 implementers may want to consider logging such errors.) The purpose
 of duplicating information from the request URL in this field is to
 deal with the possibility that an intermediate proxy may alter the
 client's Request-Line. This altered (but presumably semantically
 equivalent) request would not result in the same digest as that
 calculated by the client.
 Implementers should be aware of how authenticated transactions
 interact with shared caches. The HTTP/1.1 protocol specifies that
 when a shared cache (see section 13.7 of [2]) has received a request
 containing an Authorization header and a response from relaying that
 request, it MUST NOT return that response as a reply to any other
 request, unless one of two Cache-Control (see section 14.9 of [2])
 directives was present in the response. If the original response
 included the "must-revalidate" Cache-Control directive, the cache MAY
 use the entity of that response in replying to a subsequent request,
 but MUST first revalidate it with the origin server, using the
 request headers from the new request to allow the origin server to
 authenticate the new request. Alternatively, if the original response
 included the "public" Cache-Control directive, the response entity
 MAY be returned in reply to any subsequent request.

3.2.3 The Authentication-Info Header

 The Authentication-Info header is used by the server to communicate
 some information regarding the successful authentication in the
 response.

Franks, et al. Standards Track [Page 15] RFC 2617 HTTP Authentication June 1999

      AuthenticationInfo = "Authentication-Info" ":" auth-info
      auth-info          = 1#(nextnonce | [ message-qop ]
                             | [ response-auth ] | [ cnonce ]
                             | [nonce-count] )
      nextnonce          = "nextnonce" "=" nonce-value
      response-auth      = "rspauth" "=" response-digest
      response-digest    = <"> *LHEX <">
 The value of the nextnonce directive is the nonce the server wishes
 the client to use for a future authentication response.  The server
 may send the Authentication-Info header with a nextnonce field as a
 means of implementing one-time or otherwise changing  nonces. If the
 nextnonce field is present the client SHOULD use it when constructing
 the Authorization header for its next request. Failure of the client
 to do so may result in a request to re-authenticate from the server
 with the "stale=TRUE".
   Server implementations should carefully consider the performance
   implications of the use of this mechanism; pipelined requests will
   not be possible if every response includes a nextnonce directive
   that must be used on the next request received by the server.
   Consideration should be given to the performance vs. security
   tradeoffs of allowing an old nonce value to be used for a limited
   time to permit request pipelining.  Use of the nonce-count can
   retain most of the security advantages of a new server nonce
   without the deleterious affects on pipelining.
 message-qop
   Indicates the "quality of protection" options applied to the
   response by the server.  The value "auth" indicates authentication;
   the value "auth-int" indicates authentication with integrity
   protection. The server SHOULD use the same value for the message-
   qop directive in the response as was sent by the client in the
   corresponding request.
 The optional response digest in the "response-auth" directive
 supports mutual authentication -- the server proves that it knows the
 user's secret, and with qop=auth-int also provides limited integrity
 protection of the response. The "response-digest" value is calculated
 as for the "request-digest" in the Authorization header, except that
 if "qop=auth" or is not specified in the Authorization header for the
 request, A2 is
    A2       = ":" digest-uri-value
 and if "qop=auth-int", then A2 is
    A2       = ":" digest-uri-value ":" H(entity-body)

Franks, et al. Standards Track [Page 16] RFC 2617 HTTP Authentication June 1999

 where "digest-uri-value" is the value of the "uri" directive on the
 Authorization header in the request. The "cnonce-value" and "nc-
 value" MUST be the ones for the client request to which this message
 is the response. The "response-auth", "cnonce", and "nonce-count"
 directives MUST BE present if "qop=auth" or "qop=auth-int" is
 specified.
 The Authentication-Info header is allowed in the trailer of an HTTP
 message transferred via chunked transfer-coding.

3.3 Digest Operation

 Upon receiving the Authorization header, the server may check its
 validity by looking up the password that corresponds to the submitted
 username. Then, the server must perform the same digest operation
 (e.g., MD5) performed by the client, and compare the result to the
 given request-digest value.
 Note that the HTTP server does not actually need to know the user's
 cleartext password. As long as H(A1) is available to the server, the
 validity of an Authorization header may be verified.
 The client response to a WWW-Authenticate challenge for a protection
 space starts an authentication session with that protection space.
 The authentication session lasts until the client receives another
 WWW-Authenticate challenge from any server in the protection space. A
 client should remember the username, password, nonce, nonce count and
 opaque values associated with an authentication session to use to
 construct the Authorization header in future requests within that
 protection space. The Authorization header may be included
 preemptively; doing so improves server efficiency and avoids extra
 round trips for authentication challenges. The server may choose to
 accept the old Authorization header information, even though the
 nonce value included might not be fresh. Alternatively, the server
 may return a 401 response with a new nonce value, causing the client
 to retry the request; by specifying stale=TRUE with this response,
 the server tells the client to retry with the new nonce, but without
 prompting for a new username and password.
 Because the client is required to return the value of the opaque
 directive given to it by the server for the duration of a session,
 the opaque data may be used to transport authentication session state
 information. (Note that any such use can also be accomplished more
 easily and safely by including the state in the nonce.) For example,
 a server could be responsible for authenticating content that
 actually sits on another server. It would achieve this by having the
 first 401 response include a domain directive whose value includes a
 URI on the second server, and an opaque directive whose value

Franks, et al. Standards Track [Page 17] RFC 2617 HTTP Authentication June 1999

 contains the state information. The client will retry the request, at
 which time the server might respond with a 301/302 redirection,
 pointing to the URI on the second server. The client will follow the
 redirection, and pass an Authorization header , including the
 <opaque> data.
 As with the basic scheme, proxies must be completely transparent in
 the Digest access authentication scheme. That is, they must forward
 the WWW-Authenticate, Authentication-Info and Authorization headers
 untouched. If a proxy wants to authenticate a client before a request
 is forwarded to the server, it can be done using the Proxy-
 Authenticate and Proxy-Authorization headers described in section 3.6
 below.

3.4 Security Protocol Negotiation

 It is useful for a server to be able to know which security schemes a
 client is capable of handling.
 It is possible that a server may want to require Digest as its
 authentication method, even if the server does not know that the
 client supports it. A client is encouraged to fail gracefully if the
 server specifies only authentication schemes it cannot handle.

3.5 Example

 The following example assumes that an access-protected document is
 being requested from the server via a GET request. The URI of the
 document is "http://www.nowhere.org/dir/index.html". Both client and
 server know that the username for this document is "Mufasa", and the
 password is "Circle Of Life" (with one space between each of the
 three words).
 The first time the client requests the document, no Authorization
 header is sent, so the server responds with:
       HTTP/1.1 401 Unauthorized
       WWW-Authenticate: Digest
               realm="testrealm@host.com",
               qop="auth,auth-int",
               nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
               opaque="5ccc069c403ebaf9f0171e9517f40e41"
 The client may prompt the user for the username and password, after
 which it will respond with a new request, including the following
 Authorization header:

Franks, et al. Standards Track [Page 18] RFC 2617 HTTP Authentication June 1999

       Authorization: Digest username="Mufasa",
               realm="testrealm@host.com",
               nonce="dcd98b7102dd2f0e8b11d0f600bfb0c093",
               uri="/dir/index.html",
               qop=auth,
               nc=00000001,
               cnonce="0a4f113b",
               response="6629fae49393a05397450978507c4ef1",
               opaque="5ccc069c403ebaf9f0171e9517f40e41"

3.6 Proxy-Authentication and Proxy-Authorization

 The digest authentication scheme may also be used for authenticating
 users to proxies, proxies to proxies, or proxies to origin servers by
 use of the Proxy-Authenticate and Proxy-Authorization headers. These
 headers are instances of the Proxy-Authenticate and Proxy-
 Authorization headers specified in sections 10.33 and 10.34 of the
 HTTP/1.1 specification [2] and their behavior is subject to
 restrictions described there. The transactions for proxy
 authentication are very similar to those already described. Upon
 receiving a request which requires authentication, the proxy/server
 must issue the "407 Proxy Authentication Required" response with a
 "Proxy-Authenticate" header.  The digest-challenge used in the
 Proxy-Authenticate header is the same as that for the WWW-
 Authenticate header as defined above in section 3.2.1.
 The client/proxy must then re-issue the request with a Proxy-
 Authorization header, with directives as specified for the
 Authorization header in section 3.2.2 above.
 On subsequent responses, the server sends Proxy-Authentication-Info
 with directives the same as those for the Authentication-Info header
 field.
 Note that in principle a client could be asked to authenticate itself
 to both a proxy and an end-server, but never in the same response.

4 Security Considerations

4.1 Authentication of Clients using Basic Authentication

 The Basic authentication scheme is not a secure method of user
 authentication, nor does it in any way protect the entity, which is
 transmitted in cleartext across the physical network used as the
 carrier. HTTP does not prevent additional authentication schemes and
 encryption mechanisms from being employed to increase security or the
 addition of enhancements (such as schemes to use one-time passwords)
 to Basic authentication.

Franks, et al. Standards Track [Page 19] RFC 2617 HTTP Authentication June 1999

 The most serious flaw in Basic authentication is that it results in
 the essentially cleartext transmission of the user's password over
 the physical network. It is this problem which Digest Authentication
 attempts to address.
 Because Basic authentication involves the cleartext transmission of
 passwords it SHOULD NOT be used (without enhancements) to protect
 sensitive or valuable information.
 A common use of Basic authentication is for identification purposes
 -- requiring the user to provide a user name and password as a means
 of identification, for example, for purposes of gathering accurate
 usage statistics on a server. When used in this way it is tempting to
 think that there is no danger in its use if illicit access to the
 protected documents is not a major concern. This is only correct if
 the server issues both user name and password to the users and in
 particular does not allow the user to choose his or her own password.
 The danger arises because naive users frequently reuse a single
 password to avoid the task of maintaining multiple passwords.
 If a server permits users to select their own passwords, then the
 threat is not only unauthorized access to documents on the server but
 also unauthorized access to any other resources on other systems that
 the user protects with the same password. Furthermore, in the
 server's password database, many of the passwords may also be users'
 passwords for other sites. The owner or administrator of such a
 system could therefore expose all users of the system to the risk of
 unauthorized access to all those sites if this information is not
 maintained in a secure fashion.
 Basic Authentication is also vulnerable to spoofing by counterfeit
 servers. If a user can be led to believe that he is connecting to a
 host containing information protected by Basic authentication when,
 in fact, he is connecting to a hostile server or gateway, then the
 attacker can request a password, store it for later use, and feign an
 error. This type of attack is not possible with Digest
 Authentication. Server implementers SHOULD guard against the
 possibility of this sort of counterfeiting by gateways or CGI
 scripts. In particular it is very dangerous for a server to simply
 turn over a connection to a gateway.  That gateway can then use the
 persistent connection mechanism to engage in multiple transactions
 with the client while impersonating the original server in a way that
 is not detectable by the client.

4.2 Authentication of Clients using Digest Authentication

 Digest Authentication does not provide a strong authentication
 mechanism, when compared to public key based mechanisms, for example.

Franks, et al. Standards Track [Page 20] RFC 2617 HTTP Authentication June 1999

 However, it is significantly stronger than (e.g.) CRAM-MD5, which has
 been proposed for use with LDAP [10], POP and IMAP (see RFC 2195
 [9]).  It is intended to replace the much weaker and even more
 dangerous Basic mechanism.
 Digest Authentication offers no confidentiality protection beyond
 protecting the actual password. All of the rest of the request and
 response are available to an eavesdropper.
 Digest Authentication offers only limited integrity protection for
 the messages in either direction. If  qop=auth-int mechanism is used,
 those parts of the message used in the calculation of the WWW-
 Authenticate and Authorization header field response directive values
 (see section 3.2 above) are  protected.  Most header fields and their
 values could be modified as a part of a man-in-the-middle attack.
 Many needs for secure HTTP transactions cannot be met by Digest
 Authentication. For those needs TLS or SHTTP are more appropriate
 protocols. In particular Digest authentication cannot be used for any
 transaction requiring confidentiality protection.  Nevertheless many
 functions remain for which Digest authentication is both useful and
 appropriate.  Any service in present use that uses Basic should be
 switched to Digest as soon as practical.

4.3 Limited Use Nonce Values

 The Digest scheme uses a server-specified nonce to seed the
 generation of the request-digest value (as specified in section
 3.2.2.1 above).  As shown in the example nonce in section 3.2.1, the
 server is free to construct the nonce such that it may only be used
 from a particular client, for a particular resource, for a limited
 period of time or number of uses, or any other restrictions.  Doing
 so strengthens the protection provided against, for example, replay
 attacks (see 4.5).  However, it should be noted that the method
 chosen for generating and checking the nonce also has performance and
 resource implications.  For example, a server may choose to allow
 each nonce value to be used only once by maintaining a record of
 whether or not each recently issued nonce has been returned and
 sending a next-nonce directive in the Authentication-Info header
 field of every response. This protects against even an immediate
 replay attack, but has a high cost checking nonce values, and perhaps
 more important will cause authentication failures for any pipelined
 requests (presumably returning a stale nonce indication).  Similarly,
 incorporating a request-specific element such as the Etag value for a
 resource limits the use of the nonce to that version of the resource
 and also defeats pipelining. Thus it may be useful to do so for
 methods with side effects but have unacceptable performance for those
 that do not.

Franks, et al. Standards Track [Page 21] RFC 2617 HTTP Authentication June 1999

4.4 Comparison of Digest with Basic Authentication

 Both Digest and Basic Authentication are very much on the weak end of
 the security strength spectrum. But a comparison between the two
 points out the utility, even necessity, of replacing Basic by Digest.
 The greatest threat to the type of transactions for which these
 protocols are used is network snooping. This kind of transaction
 might involve, for example, online access to a database whose use is
 restricted to paying subscribers. With Basic authentication an
 eavesdropper can obtain the password of the user. This not only
 permits him to access anything in the database, but, often worse,
 will permit access to anything else the user protects with the same
 password.
 By contrast, with Digest Authentication the eavesdropper only gets
 access to the transaction in question and not to the user's password.
 The information gained by the eavesdropper would permit a replay
 attack, but only with a request for the same document, and even that
 may be limited by the server's choice of nonce.

4.5 Replay Attacks

 A replay attack against Digest authentication would usually be
 pointless for a simple GET request since an eavesdropper would
 already have seen the only document he could obtain with a replay.
 This is because the URI of the requested document is digested in the
 client request and the server will only deliver that document. By
 contrast under Basic Authentication once the eavesdropper has the
 user's password, any document protected by that password is open to
 him.
 Thus, for some purposes, it is necessary to protect against replay
 attacks. A good Digest implementation can do this in various ways.
 The server created "nonce" value is implementation dependent, but if
 it contains a digest of the client IP, a time-stamp, the resource
 ETag, and a private server key (as recommended above) then a replay
 attack is not simple. An attacker must convince the server that the
 request is coming from a false IP address and must cause the server
 to deliver the document to an IP address different from the address
 to which it believes it is sending the document. An attack can only
 succeed in the period before the time-stamp expires. Digesting the
 client IP and time-stamp in the nonce permits an implementation which
 does not maintain state between transactions.
 For applications where no possibility of replay attack can be
 tolerated the server can use one-time nonce values which will not be
 honored for a second use. This requires the overhead of the server

Franks, et al. Standards Track [Page 22] RFC 2617 HTTP Authentication June 1999

 remembering which nonce values have been used until the nonce time-
 stamp (and hence the digest built with it) has expired, but it
 effectively protects against replay attacks.
 An implementation must give special attention to the possibility of
 replay attacks with POST and PUT requests. Unless the server employs
 one-time or otherwise limited-use nonces and/or insists on the use of
 the integrity protection of qop=auth-int, an attacker could replay
 valid credentials from a successful request with counterfeit form
 data or other message body. Even with the use of integrity protection
 most metadata in header fields is not protected. Proper nonce
 generation and checking provides some protection against replay of
 previously used valid credentials, but see 4.8.

4.6 Weakness Created by Multiple Authentication Schemes

 An HTTP/1.1 server may return multiple challenges with a 401
 (Authenticate) response, and each challenge may use a different
 auth-scheme. A user agent MUST choose to use the strongest auth-
 scheme it understands and request credentials from the user based
 upon that challenge.
    Note that many browsers will only recognize Basic and will require
    that it be the first auth-scheme presented. Servers should only
    include Basic if it is minimally acceptable.
 When the server offers choices of authentication schemes using the
 WWW-Authenticate header, the strength of the resulting authentication
 is only as good as that of the of the weakest of the authentication
 schemes. See section 4.8 below for discussion of particular attack
 scenarios that exploit multiple authentication schemes.

4.7 Online dictionary attacks

 If the attacker can eavesdrop, then it can test any overheard
 nonce/response pairs against a list of common words. Such a list is
 usually much smaller than the total number of possible passwords. The
 cost of computing the response for each password on the list is paid
 once for each challenge.
 The server can mitigate this attack by not allowing users to select
 passwords that are in a dictionary.

Franks, et al. Standards Track [Page 23] RFC 2617 HTTP Authentication June 1999

4.8 Man in the Middle

 Both Basic and Digest authentication are vulnerable to "man in the
 middle" (MITM) attacks, for example, from a hostile or compromised
 proxy. Clearly, this would present all the problems of eavesdropping.
 But it also offers some additional opportunities to the attacker.
 A possible man-in-the-middle attack would be to add a weak
 authentication scheme to the set of choices, hoping that the client
 will use one that exposes the user's credentials (e.g. password). For
 this reason, the client should always use the strongest scheme that
 it understands from the choices offered.
 An even better MITM attack would be to remove all offered choices,
 replacing them with a challenge that requests only Basic
 authentication, then uses the cleartext credentials from the Basic
 authentication to authenticate to the origin server using the
 stronger scheme it requested. A particularly insidious way to mount
 such a MITM attack would be to offer a "free" proxy caching service
 to gullible users.
 User agents should consider measures such as presenting a visual
 indication at the time of the credentials request of what
 authentication scheme is to be used, or remembering the strongest
 authentication scheme ever requested by a server and produce a
 warning message before using a weaker one. It might also be a good
 idea for the user agent to be configured to demand Digest
 authentication in general, or from specific sites.
 Or, a hostile proxy might spoof the client into making a request the
 attacker wanted rather than one the client wanted. Of course, this is
 still much harder than a comparable attack against Basic
 Authentication.

4.9 Chosen plaintext attacks

 With Digest authentication, a MITM or a malicious server can
 arbitrarily choose the nonce that the client will use to compute the
 response. This is called a "chosen plaintext" attack. The ability to
 choose the nonce is known to make cryptanalysis much easier [8].
 However, no way to analyze the MD5 one-way function used by Digest
 using chosen plaintext is currently known.
 The countermeasure against this attack is for clients to be
 configured to require the use of the optional "cnonce" directive;
 this allows the client to vary the input to the hash in a way not
 chosen by the attacker.

Franks, et al. Standards Track [Page 24] RFC 2617 HTTP Authentication June 1999

4.10 Precomputed dictionary attacks

 With Digest authentication, if the attacker can execute a chosen
 plaintext attack, the attacker can precompute the response for many
 common words to a nonce of its choice, and store a dictionary of
 (response, password) pairs. Such precomputation can often be done in
 parallel on many machines. It can then use the chosen plaintext
 attack to acquire a response corresponding to that challenge, and
 just look up the password in the dictionary. Even if most passwords
 are not in the dictionary, some might be. Since the attacker gets to
 pick the challenge, the cost of computing the response for each
 password on the list can be amortized over finding many passwords. A
 dictionary with 100 million password/response pairs would take about
 3.2 gigabytes of disk storage.
 The countermeasure against this attack is to for clients to be
 configured to require the use of the optional "cnonce" directive.

4.11 Batch brute force attacks

 With Digest authentication, a MITM can execute a chosen plaintext
 attack, and can gather responses from many users to the same nonce.
 It can then find all the passwords within any subset of password
 space that would generate one of the nonce/response pairs in a single
 pass over that space. It also reduces the time to find the first
 password by a factor equal to the number of nonce/response pairs
 gathered. This search of the password space can often be done in
 parallel on many machines, and even a single machine can search large
 subsets of the password space very quickly -- reports exist of
 searching all passwords with six or fewer letters in a few hours.
 The countermeasure against this attack is to for clients to be
 configured to require the use of the optional "cnonce" directive.

4.12 Spoofing by Counterfeit Servers

 Basic Authentication is vulnerable to spoofing by counterfeit
 servers.  If a user can be led to believe that she is connecting to a
 host containing information protected by a password she knows, when
 in fact she is connecting to a hostile server, then the hostile
 server can request a password, store it away for later use, and feign
 an error.  This type of attack is more difficult with Digest
 Authentication -- but the client must know to demand that Digest
 authentication be used, perhaps using some of the techniques
 described above to counter "man-in-the-middle" attacks.  Again, the
 user can be helped in detecting this attack by a visual indication of
 the authentication mechanism in use with appropriate guidance in
 interpreting the implications of each scheme.

Franks, et al. Standards Track [Page 25] RFC 2617 HTTP Authentication June 1999

4.13 Storing passwords

 Digest authentication requires that the authenticating agent (usually
 the server) store some data derived from the user's name and password
 in a "password file" associated with a given realm. Normally this
 might contain pairs consisting of username and H(A1), where H(A1) is
 the digested value of the username, realm, and password as described
 above.
 The security implications of this are that if this password file is
 compromised, then an attacker gains immediate access to documents on
 the server using this realm. Unlike, say a standard UNIX password
 file, this information need not be decrypted in order to access
 documents in the server realm associated with this file. On the other
 hand, decryption, or more likely a brute force attack, would be
 necessary to obtain the user's password. This is the reason that the
 realm is part of the digested data stored in the password file. It
 means that if one Digest authentication password file is compromised,
 it does not automatically compromise others with the same username
 and password (though it does expose them to brute force attack).
 There are two important security consequences of this. First the
 password file must be protected as if it contained unencrypted
 passwords, because for the purpose of accessing documents in its
 realm, it effectively does.
 A second consequence of this is that the realm string should be
 unique among all realms which any single user is likely to use. In
 particular a realm string should include the name of the host doing
 the authentication. The inability of the client to authenticate the
 server is a weakness of Digest Authentication.

4.14 Summary

 By modern cryptographic standards Digest Authentication is weak. But
 for a large range of purposes it is valuable as a replacement for
 Basic Authentication. It remedies some, but not all, weaknesses of
 Basic Authentication. Its strength may vary depending on the
 implementation.  In particular the structure of the nonce (which is
 dependent on the server implementation) may affect the ease of
 mounting a replay attack.  A range of server options is appropriate
 since, for example, some implementations may be willing to accept the
 server overhead of one-time nonces or digests to eliminate the
 possibility of replay. Others may satisfied with a nonce like the one
 recommended above restricted to a single IP address and a single ETag
 or with a limited lifetime.

Franks, et al. Standards Track [Page 26] RFC 2617 HTTP Authentication June 1999

 The bottom line is that *any* compliant implementation will be
 relatively weak by cryptographic standards, but *any* compliant
 implementation will be far superior to Basic Authentication.

5 Sample implementation

 The following code implements the calculations of H(A1), H(A2),
 request-digest and response-digest, and a test program which computes
 the values used in the example of section 3.5. It uses the MD5
 implementation from RFC 1321.
 File "digcalc.h":

#define HASHLEN 16 typedef char HASH[HASHLEN]; #define HASHHEXLEN 32 typedef char HASHHEX[HASHHEXLEN+1]; #define IN #define OUT

/* calculate H(A1) as per HTTP Digest spec */ void DigestCalcHA1(

  IN char * pszAlg,
  IN char * pszUserName,
  IN char * pszRealm,
  IN char * pszPassword,
  IN char * pszNonce,
  IN char * pszCNonce,
  OUT HASHHEX SessionKey
  );

/* calculate request-digest/response-digest as per HTTP Digest spec */ void DigestCalcResponse(

  IN HASHHEX HA1,           /* H(A1) */
  IN char * pszNonce,       /* nonce from server */
  IN char * pszNonceCount,  /* 8 hex digits */
  IN char * pszCNonce,      /* client nonce */
  IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
  IN char * pszMethod,      /* method from the request */
  IN char * pszDigestUri,   /* requested URL */
  IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
  OUT HASHHEX Response      /* request-digest or response-digest */
  );

File "digcalc.c":

#include <global.h> #include <md5.h>

Franks, et al. Standards Track [Page 27] RFC 2617 HTTP Authentication June 1999

#include <string.h> #include "digcalc.h"

void CvtHex(

  IN HASH Bin,
  OUT HASHHEX Hex
  )

{

  unsigned short i;
  unsigned char j;
  for (i = 0; i < HASHLEN; i++) {
      j = (Bin[i] >> 4) & 0xf;
      if (j <= 9)
          Hex[i*2] = (j + '0');
       else
          Hex[i*2] = (j + 'a' - 10);
      j = Bin[i] & 0xf;
      if (j <= 9)
          Hex[i*2+1] = (j + '0');
       else
          Hex[i*2+1] = (j + 'a' - 10);
  };
  Hex[HASHHEXLEN] = '\0';

};

/* calculate H(A1) as per spec */ void DigestCalcHA1(

  IN char * pszAlg,
  IN char * pszUserName,
  IN char * pszRealm,
  IN char * pszPassword,
  IN char * pszNonce,
  IN char * pszCNonce,
  OUT HASHHEX SessionKey
  )

{

    MD5_CTX Md5Ctx;
    HASH HA1;
    MD5Init(&Md5Ctx);
    MD5Update(&Md5Ctx, pszUserName, strlen(pszUserName));
    MD5Update(&Md5Ctx, ":", 1);
    MD5Update(&Md5Ctx, pszRealm, strlen(pszRealm));
    MD5Update(&Md5Ctx, ":", 1);
    MD5Update(&Md5Ctx, pszPassword, strlen(pszPassword));
    MD5Final(HA1, &Md5Ctx);
    if (stricmp(pszAlg, "md5-sess") == 0) {

Franks, et al. Standards Track [Page 28] RFC 2617 HTTP Authentication June 1999

          MD5Init(&Md5Ctx);
          MD5Update(&Md5Ctx, HA1, HASHLEN);
          MD5Update(&Md5Ctx, ":", 1);
          MD5Update(&Md5Ctx, pszNonce, strlen(pszNonce));
          MD5Update(&Md5Ctx, ":", 1);
          MD5Update(&Md5Ctx, pszCNonce, strlen(pszCNonce));
          MD5Final(HA1, &Md5Ctx);
    };
    CvtHex(HA1, SessionKey);

};

/* calculate request-digest/response-digest as per HTTP Digest spec */ void DigestCalcResponse(

  IN HASHHEX HA1,           /* H(A1) */
  IN char * pszNonce,       /* nonce from server */
  IN char * pszNonceCount,  /* 8 hex digits */
  IN char * pszCNonce,      /* client nonce */
  IN char * pszQop,         /* qop-value: "", "auth", "auth-int" */
  IN char * pszMethod,      /* method from the request */
  IN char * pszDigestUri,   /* requested URL */
  IN HASHHEX HEntity,       /* H(entity body) if qop="auth-int" */
  OUT HASHHEX Response      /* request-digest or response-digest */
  )

{

    MD5_CTX Md5Ctx;
    HASH HA2;
    HASH RespHash;
     HASHHEX HA2Hex;
    // calculate H(A2)
    MD5Init(&Md5Ctx);
    MD5Update(&Md5Ctx, pszMethod, strlen(pszMethod));
    MD5Update(&Md5Ctx, ":", 1);
    MD5Update(&Md5Ctx, pszDigestUri, strlen(pszDigestUri));
    if (stricmp(pszQop, "auth-int") == 0) {
          MD5Update(&Md5Ctx, ":", 1);
          MD5Update(&Md5Ctx, HEntity, HASHHEXLEN);
    };
    MD5Final(HA2, &Md5Ctx);
     CvtHex(HA2, HA2Hex);
    // calculate response
    MD5Init(&Md5Ctx);
    MD5Update(&Md5Ctx, HA1, HASHHEXLEN);
    MD5Update(&Md5Ctx, ":", 1);
    MD5Update(&Md5Ctx, pszNonce, strlen(pszNonce));
    MD5Update(&Md5Ctx, ":", 1);
    if (*pszQop) {

Franks, et al. Standards Track [Page 29] RFC 2617 HTTP Authentication June 1999

        MD5Update(&Md5Ctx, pszNonceCount, strlen(pszNonceCount));
        MD5Update(&Md5Ctx, ":", 1);
        MD5Update(&Md5Ctx, pszCNonce, strlen(pszCNonce));
        MD5Update(&Md5Ctx, ":", 1);
        MD5Update(&Md5Ctx, pszQop, strlen(pszQop));
        MD5Update(&Md5Ctx, ":", 1);
    };
    MD5Update(&Md5Ctx, HA2Hex, HASHHEXLEN);
    MD5Final(RespHash, &Md5Ctx);
    CvtHex(RespHash, Response);

};

File "digtest.c":

#include <stdio.h> #include "digcalc.h"

void main(int argc, char ** argv) {

    char * pszNonce = "dcd98b7102dd2f0e8b11d0f600bfb0c093";
    char * pszCNonce = "0a4f113b";
    char * pszUser = "Mufasa";
    char * pszRealm = "testrealm@host.com";
    char * pszPass = "Circle Of Life";
    char * pszAlg = "md5";
    char szNonceCount[9] = "00000001";
    char * pszMethod = "GET";
    char * pszQop = "auth";
    char * pszURI = "/dir/index.html";
    HASHHEX HA1;
    HASHHEX HA2 = "";
    HASHHEX Response;
    DigestCalcHA1(pszAlg, pszUser, pszRealm, pszPass, pszNonce,

pszCNonce, HA1);

    DigestCalcResponse(HA1, pszNonce, szNonceCount, pszCNonce, pszQop,
     pszMethod, pszURI, HA2, Response);
    printf("Response = %s\n", Response);

};

Franks, et al. Standards Track [Page 30] RFC 2617 HTTP Authentication June 1999

6 Acknowledgments

 Eric W. Sink, of AbiSource, Inc., was one of the original authors
 before the specification underwent substantial revision.
 In addition to the authors, valuable discussion instrumental in
 creating this document has come from Peter J. Churchyard, Ned Freed,
 and David M.  Kristol.
 Jim Gettys and Larry Masinter edited this document for update.

7 References

 [1]  Berners-Lee, T.,  Fielding, R. and H. Frystyk, "Hypertext
      Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
 [2]  Fielding, R.,  Gettys, J., Mogul, J., Frysyk, H., Masinter, L.,
      Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
      HTTP/1.1", RFC 2616, June 1999.
 [3]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
      1992.
 [4]  Freed, N. and N. Borenstein. "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message Bodies",
      RFC 2045, November 1996.
 [5]  Dierks, T. and C. Allen "The TLS Protocol, Version 1.0", RFC
      2246, January 1999.
 [6]  Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
      Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP :
      Digest Access Authentication", RFC 2069, January 1997.
 [7]  Berners Lee, T, Fielding, R. and L. Masinter, "Uniform Resource
      Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
 [8]  Kaliski, B.,Robshaw, M., "Message Authentication with MD5",
      CryptoBytes, Sping 1995, RSA Inc,
      (http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm)
 [9]  Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP AUTHorize
      Extension for Simple Challenge/Response", RFC 2195, September
      1997.
 [10] Morgan, B., Alvestrand, H., Hodges, J., Wahl, M.,
      "Authentication Methods for LDAP", Work in Progress.

Franks, et al. Standards Track [Page 31] RFC 2617 HTTP Authentication June 1999

8 Authors' Addresses

 John Franks
 Professor of Mathematics
 Department of Mathematics
 Northwestern University
 Evanston, IL 60208-2730, USA
 EMail: john@math.nwu.edu
 Phillip M. Hallam-Baker
 Principal Consultant
 Verisign Inc.
 301 Edgewater Place
 Suite 210
 Wakefield MA 01880, USA
 EMail: pbaker@verisign.com
 Jeffery L. Hostetler
 Software Craftsman
 AbiSource, Inc.
 6 Dunlap Court
 Savoy, IL 61874
 EMail: jeff@AbiSource.com
 Scott D. Lawrence
 Agranat Systems, Inc.
 5 Clocktower Place, Suite 400
 Maynard, MA 01754, USA
 EMail: lawrence@agranat.com
 Paul J. Leach
 Microsoft Corporation
 1 Microsoft Way
 Redmond, WA 98052, USA
 EMail: paulle@microsoft.com

Franks, et al. Standards Track [Page 32] RFC 2617 HTTP Authentication June 1999

 Ari Luotonen
 Member of Technical Staff
 Netscape Communications Corporation
 501 East Middlefield Road
 Mountain View, CA 94043, USA
 Lawrence C. Stewart
 Open Market, Inc.
 215 First Street
 Cambridge, MA  02142, USA
 EMail: stewart@OpenMarket.com

Franks, et al. Standards Track [Page 33] RFC 2617 HTTP Authentication June 1999

9. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Franks, et al. Standards Track [Page 34]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2617.txt · Last modified: 1999/06/09 23:33 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki