rfc2831.txt

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      its value SHOULD be one of those realms. If the directive is
      missing, "realm-value" will set to the empty string when computing
      A1 (see below for details).

   nonce
      The server-specified data string received in the preceding
      digest-challenge. This directive is required and MUST be present
      exactly once; otherwise, authentication fails.






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   cnonce
      A client-specified data string which MUST be different each time a
      digest-response is sent as part of initial authentication. 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, and to provide mutual authentication. The
      security of the implementation depends on a good choice. It is
      RECOMMENDED that it contain at least 64 bits of entropy. This
      directive is required and MUST be present exactly once; otherwise,
      authentication fails.

   nonce-count
      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 response value. This directive may appear at most once; if
      multiple instances are present, the client should abort the
      authentication exchange.

   qop
      Indicates what "quality of protection" the client accepted. If
      present, it may appear exactly once and  its value MUST be one of
      the alternatives in qop-options. If not present, it defaults to
      "auth". These values affect the computation of the response. Note
      that this is a single token, not a quoted list of alternatives.

   serv-type
      Indicates the type of service, such as "www" for web service,
      "ftp" for FTP service, "smtp" for mail delivery service, etc. The
      service name as defined in the SASL profile for the protocol see
      section 4 of [RFC 2222], registered in the IANA registry of
      "service" elements for the GSSAPI host-based service name form
      [RFC 2078].

   host
      The DNS host name or IP address for the service requested.  The
      DNS host name must be the fully-qualified canonical name of the
      host. The DNS host name is the preferred form; see notes on server
      processing of the digest-uri.







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   serv-name
      Indicates the name of the service if it is replicated. The service
      is considered to be replicated if the client's service-location
      process involves resolution using standard DNS lookup operations,
      and if these operations involve DNS records (such as SRV, or MX)
      which resolve one DNS name into a set of other DNS names. In this
      case, the initial name used by the client is the "serv-name", and
      the final name is the "host" component. For example, the incoming
      mail service for "example.com" may be replicated through the use
      of MX records stored in the DNS, one of which points at an SMTP
      server called "mail3.example.com"; it's "serv-name" would be
      "example.com", it's "host" would be "mail3.example.com". If the
      service is not replicated, or the serv-name is identical to the
      host, then the serv-name component MUST be omitted.

   digest-uri
      Indicates the principal name of the service with which the client
      wishes to connect, formed from the serv-type, host, and serv-name.
      For example, the FTP service on "ftp.example.com" would have a
      "digest-uri" value of "ftp/ftp.example.com"; the SMTP server from
      the example above would have a "digest-uri" value of
      "smtp/mail3.example.com/example.com".

   Servers SHOULD check that the supplied value is correct. This will
   detect accidental connection to the incorrect server. It is also so
   that clients will be trained to provide values that will work with
   implementations that use a shared back-end authentication service
   that can provide server authentication.

   The serv-type component should match the service being offered. The
   host component should match one of the host names of the host on
   which the service is running, or it's IP address. Servers SHOULD NOT
   normally support the IP address form, because server authentication
   by IP address is not very useful; they should only do so if the DNS
   is unavailable or unreliable. The serv-name component should match
   one of the service's configured service names.

   This directive may appear at most once; if multiple instances are
   present, the client should abort the authentication exchange.

   Note: In the HTTP use of Digest authentication, the digest-uri is the
   URI (usually a URL) of the resource requested -- hence the name of
   the directive.

   response
      A string of 32 hex digits computed as defined below, which proves
      that the user knows a password. This directive is required and
      MUST be present exactly once; otherwise, authentication fails.



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   maxbuf
      A number indicating the size of the largest buffer the client is
      able to receive. If this directive is missing, the default value
      is 65536. This directive may appear at most once; if multiple
      instances are present, the server should abort the authentication
      exchange.

   charset
      This directive, if present, specifies that the client has used
      UTF-8 encoding for the username and password. If not present, the
      username and password must be encoded in ISO 8859-1 (of which
      US-ASCII is a subset). The client should send this directive only
      if the server has indicated it supports UTF-8. The directive is
      needed for backwards compatibility with HTTP Digest, which only
      supports ISO 8859-1.

   LHEX
      32 hex digits, where the alphabetic characters MUST be lower case,
      because MD5 is not case insensitive.

   cipher
      The cipher chosen by the client. This directive MUST appear
      exactly once if "auth-conf" is negotiated; if required and not
      present, authentication fails.

   authzid
      The "authorization ID" as per RFC 2222, encoded in UTF-8. This
      directive is optional. If present, and the authenticating user has
      sufficient privilege, and the server supports it, then after
      authentication the server will use this identity for making all
      accesses and access checks. If the client specifies it, and the
      server does not support it, then the response-value will be
      incorrect, and authentication will fail.

   The size of a digest-response MUST be less than 4096 bytes.

2.1.2.1   Response-value

   The definition of "response-value" above indicates the encoding for
   its value -- 32 lower case hex characters. The following definitions
   show how the value is computed.

   Although qop-value and components of digest-uri-value may be
   case-insensitive, the case which the client supplies in step two is
   preserved for the purpose of computing and verifying the
   response-value.

      response-value  =



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         HEX( KD ( HEX(H(A1)),
                 { nonce-value, ":" nc-value, ":",
                   cnonce-value, ":", qop-value, ":", HEX(H(A2)) }))

   If authzid is specified, then A1 is


      A1 = { H( { username-value, ":", realm-value, ":", passwd } ),
           ":", nonce-value, ":", cnonce-value, ":", authzid-value }

   If authzid is not specified, then A1 is


      A1 = { H( { username-value, ":", realm-value, ":", passwd } ),
           ":", nonce-value, ":", cnonce-value }

   where

         passwd   = *OCTET

   The "username-value", "realm-value" and "passwd" are encoded
   according to the value of the "charset" directive. If "charset=UTF-8"
   is present, and all the characters of either "username-value" or
   "passwd" are in the ISO 8859-1 character set, then it must be
   converted to ISO 8859-1 before being hashed. This is so that
   authentication databases that store the hashed username, realm and
   password (which is common) can be shared compatibly with HTTP, which
   specifies ISO 8859-1. A sample implementation of this conversion is
   in section 8.

   If the "qop" directive's value is "auth", then A2 is:

      A2       = { "AUTHENTICATE:", digest-uri-value }

   If the "qop" value is "auth-int" or "auth-conf" then A2 is:

      A2       = { "AUTHENTICATE:", digest-uri-value,
               ":00000000000000000000000000000000" }

   Note that "AUTHENTICATE:" must be in upper case, and the second
   string constant is a string with a colon followed by 32 zeros.

   These apparently strange values of A2 are for compatibility with
   HTTP; they were arrived at by setting "Method" to "AUTHENTICATE" and
   the hash of the entity body to zero in the HTTP digest calculation of
   A2.

   Also, in the HTTP usage of Digest, several directives in the



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   "digest-challenge" sent by the server have to be returned by the
   client in the "digest-response". These are:

    opaque
    algorithm

   These directives are not needed when Digest is used as a SASL
   mechanism (i.e., MUST NOT be sent, and MUST be ignored if received).

2.1.3  Step Three

   The server receives and validates the "digest-response". The server
   checks that the nonce-count is "00000001". If it supports subsequent
   authentication (see section 2.2), it saves the value of the nonce and
   the nonce-count. It sends a message formatted as follows:

    response-auth = "rspauth" "=" response-value

   where response-value is calculated as above, using the values sent in
   step two, except that if qop is "auth", then A2 is

       A2 = { ":", digest-uri-value }

   And if qop is "auth-int" or "auth-conf" then A2 is

       A2 = { ":", digest-uri-value, ":00000000000000000000000000000000" }

   Compared to its use in HTTP, the following Digest directives in the
   "digest-response" are unused:

       nextnonce
       qop
       cnonce
       nonce-count

2.2  Subsequent Authentication

   If the client has previously authenticated to the server, and
   remembers the values of username, realm, nonce, nonce-count, cnonce,
   and qop that it used in that authentication, and the SASL profile for
   a protocol permits an initial client response, then it MAY perform
   "subsequent authentication", as defined in this section.









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2.2.1  Step one

   The client uses the values from the previous authentication and sends
   an initial response with a string formatted and computed according to
   the rules for a "digest-response", as defined above, but with a
   nonce-count one greater than used in the last "digest-response".

2.2.2  Step Two

   The server receives the "digest-response". If the server does not
   support subsequent authentication, then it sends a
   "digest-challenge", and authentication proceeds as in initial
   authentication. If the server has no saved nonce and nonce-count from
   a previous authentication, then it sends a "digest-challenge", and
   authentication proceeds as in initial authentication. Otherwise, the
   server validates the "digest-response", checks that the nonce-count
   is one greater than that used in the previous authentication using
   that nonce, and saves the new value of nonce-count.

   If the response is invalid, then the server sends a
   "digest-challenge", and authentication proceeds as in initial
   authentication (and should be configurable to log an authentication
   failure in some sort of security audit log, since the failure may be
   a symptom of an attack). The nonce-count MUST NOT be incremented in
   this case: to do so would allow a denial of service attack by sending
   an out-of-order nonce-count.

   If the response is valid, the server MAY choose to deem that
   authentication has succeeded. However, if it has been too long since
   the previous authentication, or for any other reason, the server MAY
   send a new "digest-challenge" with a new value for nonce. The
   challenge MAY contain a "stale" directive with value "true", which
   says that the client may respond to the challenge using the password
   it used in the previous response; otherwise, the client must solicit
   the password anew from the user. This permits the server to make sure
   that the user has presented their password recently. (The directive
   name refers to the previous nonce being stale, not to the last use of
   the password.) Except for the handling of "stale", after sending the
   "digest-challenge" authentication proceeds as in the case of initial
   authentication.

2.3   Integrity Protection

   If the server offered "qop=auth-int" and the client responded
   "qop=auth-int", then subsequent messages, up to but not including the
   next subsequent authentication, between the client and the server





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   MUST be integrity protected. Using as a base session key the value of
   H(A1) as defined above the client and server calculate a pair of
   message integrity keys as follows.

   The key for integrity protecting messages from client to server is:

   Kic = MD5({H(A1),
   "Digest session key to client-to-server signing key magic constant"})

   The key for integrity protecting messages from server to client is:

   Kis = MD5({H(A1),
   "Digest session key to server-to-client signing key magic constant"})

   where MD5 is as specified in [RFC 1321]. If message integrity is
   negotiated, a MAC block for each message is appended to the message.
   The MAC block is 16 bytes: the first 10 bytes of the HMAC-MD5 [RFC
   2104] of the message, a 2-byte message type number in network byte
   order with value 1, and the 4-byte sequence number in network byte
   order. The message type is to allow for future extensions such as
   rekeying.

   MAC(Ki, SeqNum, msg) = (HMAC(Ki, {SeqNum, msg})[0..9], 0x0001,
   SeqNum)