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.



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  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



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  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



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  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



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  (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,



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                      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



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  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.








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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 "[email protected]".

  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.



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    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.




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  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





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    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.





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  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.



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  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



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  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", [email protected]

  and the user Mufasa has password "Circle Of Life" then H(A1) would be
  H(Mufasa:[email protected]: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:[email protected]: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




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  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.





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       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)



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  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



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  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="[email protected]",
                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:





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        Authorization: Digest username="Mufasa",
                realm="[email protected]",
                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.



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  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.



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  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.



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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



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  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.









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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.



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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.



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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.





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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>



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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) {



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           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) {



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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 = "[email protected]";
     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);
};











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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.




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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: [email protected]


  Phillip M. Hallam-Baker
  Principal Consultant
  Verisign Inc.
  301 Edgewater Place
  Suite 210
  Wakefield MA 01880, USA

  EMail: [email protected]


  Jeffery L. Hostetler
  Software Craftsman
  AbiSource, Inc.
  6 Dunlap Court
  Savoy, IL 61874

  EMail: [email protected]


  Scott D. Lawrence
  Agranat Systems, Inc.
  5 Clocktower Place, Suite 400
  Maynard, MA 01754, USA

  EMail: [email protected]


  Paul J. Leach
  Microsoft Corporation
  1 Microsoft Way
  Redmond, WA 98052, USA

  EMail: [email protected]







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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: [email protected]






































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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.



















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