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Code Editor : draft-ietf-sasl-rfc2831bis-xx.txt

INTERNET-DRAFT                                                  P. Leach
Obsoletes: 2831                                                Microsoft
Intended category: Standards track                             C. Newman
                                                        Sun Microsystems
                                                             A. Melnikov
                                                                   Isode
                                                               June 2003

Using Digest Authentication as a SASL Mechanism
                   draft-ietf-sasl-rfc2831bis-02.txt

Status of this Memo

This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

This specification defines how HTTP Digest Authentication [Digest]
   can be used as a SASL [RFC 2222] mechanism for any protocol that has
   a SASL profile. It is intended both as an improvement over CRAM-MD5
   [RFC 2195] and as a convenient way to support a single authentication
   mechanism for web, mail, LDAP, and other protocols.

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Table of Contents

1 INTRODUCTION.....................................................3
    1.1 CONVENTIONS AND NOTATION......................................3
    1.2 REQUIREMENTS..................................................4
   2 AUTHENTICATION...................................................5
    2.1 INITIAL AUTHENTICATION........................................5
     2.1.1 Step One...................................................5
     2.1.2 Step Two...................................................9
     2.1.3 Step Three................................................16
    2.2 SUBSEQUENT AUTHENTICATION....................................17
     2.2.1 Step one..................................................17
     2.2.2 Step Two..................................................17
    2.3 INTEGRITY PROTECTION.........................................18
    2.4 CONFIDENTIALITY PROTECTION...................................18
   3 SECURITY CONSIDERATIONS.........................................21
    3.1 AUTHENTICATION OF CLIENTS USING DIGEST AUTHENTICATION........21
    3.2 COMPARISON OF DIGEST WITH PLAINTEXT PASSWORDS................21
    3.3 REPLAY ATTACKS...............................................21
    3.4 ONLINE DICTIONARY ATTACKS....................................22
    3.5 OFFLINE DICTIONARY ATTACKS...................................22
    3.6 MAN IN THE MIDDLE............................................22
    3.7 CHOSEN PLAINTEXT ATTACKS.....................................22
    3.8 CBC MODE ATTACKS.............................................
    3.9 SPOOFING BY COUNTERFEIT SERVERS..............................23
    3.10 STORING PASSWORDS...........................................23
    3.11 MULTIPLE REALMS.............................................24
    3.12 SUMMARY.....................................................24
   4 EXAMPLE.........................................................24
   5 REFERENCES......................................................26
    5.1 NORMATIVE REFERENCES.........................................26
    5.2 INFORMATIVE REFERENCES.......................................27
   6 AUTHORS' ADDRESSES..............................................28
   7 ABNF............................................................29
    7.1 AUGMENTED BNF................................................29
    7.2 BASIC RULES..................................................31
   8 SAMPLE CODE.....................................................33
   9 INTEROPERABILITY CONSIDERATIONS.................................34
    9.1 Implementing DES cipher in CBC mode..........................34
   10  ACKNOWLEDGEMENTS..............................................34
   11 FULL COPYRIGHT STATEMENT.......................................35
   Appendix A: Changes from 2831.....................................36
   Appendix B: Open Issues...........................................37

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1  Introduction

This specification describes the use of HTTP Digest Access
   Authentication as a SASL mechanism. The authentication type
   associated with the Digest SASL mechanism is "DIGEST-MD5".

This specification is intended to be upward compatible with the
   "md5-sess" algorithm of HTTP/1.1 Digest Access Authentication
   specified in [Digest]. The only difference in the "md5-sess"
   algorithm is that some directives not needed in a SASL mechanism have
   had their values defaulted.

There is one new feature for use as a SASL mechanism: integrity
   protection on application protocol messages after an authentication
   exchange.

Also, compared to CRAM-MD5, DIGEST-MD5 prevents chosen plaintext
   attacks, and permits the use of third party authentication servers,
   mutual authentication, and optimized reauthentication if a client has
   recently authenticated to a server.

1.1  Conventions and Notation

This specification uses the same ABNF notation and lexical
   conventions as HTTP/1.1 specification; see section 7.

Let { a, b, ... } be the concatenation of the octet strings a, b, ...

Let ** denote the power operation.

Let H(s) be the 16 octet MD5 hash [RFC 1321] of the octet string s.

Let KD(k, s) be H({k, ":", s}), i.e., the 16 octet hash of the string
   k, a colon and the string s.

Let HEX(n) be the representation of the 16 octet MD5 hash n as a
   string of 32 hex digits (with alphabetic characters always in lower
   case, since MD5 is case sensitive).

Let HMAC(k, s) be the 16 octet HMAC-MD5 [RFC 2104] of the octet
   string s using the octet string k as a key.

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Let unq(X) be the value of the quoted-string X without the
   surrounding quotes and with all escape characters "\\" removed. For
   example for the quoted-string "Babylon" the value of unq("Babylon")
   is Babylon; for the quoted string "ABC\"123\\" the value of
   unq("ABC\"123\\") is ABC"123\.

The value of a quoted string constant as an octet string does not
   include any terminating null character.

1.2  Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC 2119].

An implementation is not compliant if it fails to satisfy one or more
   of the MUST level requirements for the protocols it implements. An
   implementation that satisfies all the MUST level and all the SHOULD
   level requirements for its protocols is said to be "unconditionally
   compliant"; one that satisfies all the MUST level requirements but
   not all the SHOULD level requirements for its protocols is said to be
   "conditionally compliant."

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2  Authentication

The following sections describe how to use Digest as a SASL
   authentication mechanism.

2.1  Initial Authentication

If the client has not recently authenticated to the server, then it
   must perform "initial authentication", as defined in this section. If
   it has recently authenticated, then a more efficient form is
   available, defined in the next section.

2.1.1  Step One

The server starts by sending a challenge. The data encoded in the
   challenge contains a string formatted according to the rules for a
   "digest-challenge" defined as follows:

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realm             = "realm" "=" <"> realm-value <">
        realm-value       = qdstr-val
        nonce             = "nonce" "=" <"> nonce-value <">
        nonce-value       = *qdtext
        qop-options       = "qop" "=" <"> qop-list <">
        qop-list          = 1#qop-value
        qop-value         = "auth" | "auth-int" | "auth-conf" |
                             token
        stale             = "stale" "=" "true"
        server_maxbuf     = "maxbuf" "=" maxbuf-value
        maxbuf-value      = 1*DIGIT
        charset           = "charset" "=" "utf-8"
        algorithm         = "algorithm" "=" "md5-sess"
        cipher-opts       = "cipher" "=" <"> 1#cipher-value <">
        cipher-value      = "3des" | "des" | "rc4-40" | "rc4" |
                            "rc4-56" | "aes" | token
        auth-param        = token "=" ( token | quoted-string )

The meanings of the values of the directives used above are as
   follows:

realm
      Mechanistically, a string which can enable users to know which
      username and password to use, in case they might have different
      ones for different servers. Conceptually, it is the name of a
      collection of accounts that might include the user's account. 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.example.com".  This directive is
      optional; if not present, the client SHOULD solicit it from the
      user or be able to compute a default; a plausible default might be
      the realm supplied by the user when they logged in to the client
      system.  Multiple realm directives are allowed, in which case the
      user or client must choose one as the realm for which to supply to
      username and password.

If at least one realm is present and the charset directive is also
      specified (which means that realm(s) are encoded as UTF-8), the
      client should prepare each instance of realm using the "SASLPrep"
      profile [SASLPrep] of the "stringprep" algorithm [StringPrep]. If
      preparation of a realm instance fails or results in an empty
      string, the client should abort the authentication exchange.

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nonce
      A server-specified data string which MUST be different each time a
      digest-challenge is sent as part of initial authentication.  It is
      recommended that this string be base64 or hexadecimal data. Note
      that since the string is passed as a quoted string, the
      double-quote character is not allowed unless escaped (see section
      7.2). The contents of the nonce are implementation dependent. The
      security of the implementation depends on a good choice. It is
      RECOMMENDED that it contain at least 64 bits of entropy. The nonce
      is opaque to the client. This directive is required and MUST
      appear exactly once; if not present, or if multiple instances are
      present, the client should abort the authentication exchange.

qop-options
      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; the value "auth-conf"
      indicates authentication with integrity protection and encryption.
      This directive is optional; if not present it defaults to "auth".
      The client MUST ignore unrecognized options; if the client
      recognizes no option, it should abort the authentication exchange.

stale
      The "stale" directive is not used in initial authentication. See
      the next section for its use in subsequent authentications. This
      directive may appear at most once; if multiple instances are
      present, the client should abort the authentication exchange.

server_maxbuf ("maximal ciphertext buffer size")
      A number indicating the size of the largest buffer the server is
      able to receive when using "auth-int" or "auth-conf". The value
      MUST be bigger than 16 and smaller or equal to 16777215 (i.e.
      2**24-1). If this directive is missing, the default value is
      65536. This directive may appear at most once; if multiple
      instances are present, the client MUST abort the authentication
      exchange.

Let call "maximal cleartext buffer size" (or "maximal sender
      size") the maximal size of a cleartext buffer that, after being
      transformed by integrity (section 2.3) or confidentiality (section
      2.4) protection function, will produce a SASL block of the maxbuf
      size.  As it should be clear from the name, the sender MUST never
      pass a block of data bigger than the "maximal sender size" through
      the selected protection function.  This will guaranty that the
      receiver will never get a block bigger than the maxbuf.

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charset
      This directive, if present, specifies that the server supports
      UTF-8 [UTF-8] encoding for the username, realm and password. If
      present, the username, realm and password are in Unicode, prepared
      using the "SASLPrep" profile [SASLPrep] of the "stringprep"
      algorithm [StringPrep] and than encoded as UTF-8 [UTF-8].  If not
      present, the username, realm and password MUST be encoded in ISO
      8859-1 [ISO-8859] (of which US-ASCII [USASCII] is a subset). The
      directive is needed for backwards compatibility with HTTP Digest,
      which only supports ISO 8859-1.  This directive may appear at most
      once; if multiple instances are present, the client should abort
      the authentication exchange.

Note, that this directive doesn't affect authorization id
      ("authzid").

algorithm
      This directive is required for backwards compatibility with HTTP
      Digest, which supports other algorithms. This directive is
      required and MUST appear exactly once; if not present, or if
      multiple instances are present, the client should abort the
      authentication exchange.

cipher-opts
      A list of ciphers that the server supports. This directive must be
      present exactly once if "auth-conf" is offered in the
      "qop-options" directive, in which case the "3des" cipher is
      mandatory-to-implement. The client MUST ignore unrecognized
      options; if the client recognizes no option, it should abort the
      authentication exchange. See section 2.4 for more detailed
      description of the ciphers.

des
         the Data Encryption Standard (DES) cipher [FIPS] in cipher
         block chaining (CBC) mode with a 56 bit key.

3des
         the "triple DES" cipher in CBC mode with EDE
         (Encrypt,Decrypt,Encrypt) with the same key for each E stage
         (aka "two keys mode") for a total key length of 112 bits.

rc4, rc4-40, rc4-56
         the RC4 cipher with a 128 bit, 40 bit, and 56 bit key,
         respectively.

aes
         the Advanced Encryption Standard (AES) cipher [AES] in cipher
         block chaining (CBC) mode with a 128 bit key. This mode

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requires an Initialization Vector (IV) that has the same size
         as the block size.

auth-param
      This construct allows for future extensions; it may appear more
      than once. The client MUST ignore any unrecognized directives.

For use as a SASL mechanism, note that the following changes are made
   to "digest-challenge" from HTTP: the following Digest options (called
   "directives" in HTTP terminology) are unused (i.e., MUST NOT be sent,
   and MUST be ignored if received):

opaque
    domain

The size of a digest-challenge MUST be less than 2048 bytes.

2.1.2  Step Two

The client makes note of the "digest-challenge" and then responds
   with a string formatted and computed according to the rules for a
   "digest-response" defined as follows:

username         = "username" "=" <"> username-value <">
       username-value   = qdstr-val
       cnonce           = "cnonce" "=" <"> cnonce-value <">
       cnonce-value     = *qdtext
       nonce-count      = "nc" "=" nc-value
       nc-value         = 8LHEX
       client_maxbuf    = "maxbuf" "=" maxbuf-value
       qop              = "qop" "=" qop-value
       digest-uri       = "digest-uri" "=" <"> digest-uri-value <">
       digest-uri-value  = serv-type "/" host [ "/" serv-name ]
       serv-type        = 1*ALPHA
       serv-name        = host
       response         = "response" "=" response-value
       response-value   = 32LHEX
       LHEX             = "0" | "1" | "2" | "3" |
                          "4" | "5" | "6" | "7" |
                          "8" | "9" | "a" | "b" |
                          "c" | "d" | "e" | "f"
       cipher           = "cipher" "=" cipher-value
       authzid          = "authzid" "=" <"> authzid-value <">

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authzid-value    = qdstr-val

The 'host' non-terminal is defined in [RFC 2732] as

host          = hostname | IPv4address | IPv6reference
       ipv6reference = "[" IPv6address "]"

where IPv6address and IPv4address are defined in [RFC 2373]
   and 'hostname' is defined in [RFC 2396].

username
      The user's name in the specified realm, encoded according to the
      value of the "charset" directive. This directive is required and
      MUST be present exactly once; otherwise, authentication fails.

Upon the receipt of this value and if the charset directive is
      also specified (which means that the username is encoded as
      UTF-8), the server MUST prepare the username using the "SASLPrep"
      profile [SASLPrep] of the "stringprep" algorithm [StringPrep]. If
      preparation of the username fails or results in an empty string,
      the server MUST fail the authentication exchange.

realm
      The realm containing the user's account, encoded according to the
      value of the "charset" directive. This directive is required if
      the server provided any realms in the
      "digest-challenge", in which case it may appear exactly once and
      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).

If realm was provided by the client and if the charset directive
      was also specified (which means that the realm is encoded as
      UTF-8), the server MUST prepare the realm using the "SASLPrep"
      profile [SASLPrep] of the "stringprep" algorithm [StringPrep]. If
      preparation of the realm fails or results in an empty string, the
      server MUST fail the authentication exchange.

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 is required and MUST be present
      exactly once; otherwise, authentication fails.

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 "http" 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 (IPv4 or IPv6) 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 [RFC
      2052], 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, as well as some
   redirection attacks. 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.

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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.

client_maxbuf
      A number indicating the size of the largest ciphertext buffer the
      client is able to receive when using "auth-int" or "auth-conf". If
      this directive is missing, the default value is 65536. This
      directive may appear at most once; if multiple instances are
      present, the server MUST abort the authentication exchange. If the
      value is less or equal to 16 or bigger than 16777215 (i.e.
      2**24-1), the server MUST abort the authentication exchange.

Upon processing/sending of the client_maxbuf value both the server
      and the client calculate their "maximal ciphertext buffer size" as
      the minimum of the server_maxbuf (Step One) and the client_maxbuf
      (Step Two).  The "maximal sender size" can be calculated by
      subtracting 16 from the calculated "maximal ciphertext buffer
      size".

When sending a block of data the client/server MUST NOT pass more
      than the "maximal sender size" bytes of data to the selected
      protection function (2.3 or 2.4).

charset
      This directive, if present, specifies that the client has used
      UTF-8 [UTF-8] encoding for the username, realm and password. If
      present, the username, realm and password are in Unicode, prepared
      using the "SASLPrep" profile [SASLPrep] of the "stringprep"
      algorithm [StringPrep] and than encoded as UTF-8 [UTF-8].  If not
      present, the username and password must be encoded in ISO 8859-1
      [ISO-8859] (of which
      US-ASCII [USASCII] is a subset). The client should send this
      directive only if the server has indicated it supports UTF-8
      [UTF-8]. The directive is needed for backwards compatibility with
      HTTP Digest, which only supports ISO 8859-1.

Note, that this directive doesn't affect authorization id
      ("authzid").

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

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present, authentication fails.

authzid
      The "authorization ID" 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 calculated on the server will not match the one
      calculated on the client and authentication will fail.

The authzid MUST NOT be an empty string.

The authorization identifier MUST NOT be converted to ISO 8859-1
      even if the authentication identifier ("username") is converted
      for compatibility as directed by "charset" directive.

The server SHOULD verify the correctness of an authzid as
      specified by the corresponding SASL protocol profile.

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

If authzid is not specified, then A1 is

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

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":", 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 "username-value" are, after
   preparing using the "SASLPrep" profile [SASLPrep] of the "stringprep"
   algorithm [StringPrep], in the ISO 8859-1 character set, then it must
   be converted to ISO 8859-1 before being hashed. The same
   transformation has to be done for "realm-value" and "passwd". 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 }

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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
   "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

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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.

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 o including the next
   subsequent authentication, between the client and the server 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"})

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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)

where Ki is Kic for messages sent by the client and Kis for those
   sent by the server. The sequence number (SeqNum) is an unsigned
   number initialized to zero after initial or subsequent
   authentication, and incremented by one for each message
   sent/successfully verified. (Note, that there are two independent
   counters for sending and receiving.) The sequence number wraps around
   to 0 after 2**32-1.

Upon receipt, MAC(Ki, SeqNum, msg) is computed and compared with the
   received value; the message is discarded if they differ. The
   receiver's sequence counter is incremented if they match.

2.4   Confidentiality Protection

If the server sent a "cipher-opts" directive and the client responded
   with a "cipher" directive, then subsequent messages between the
   client and the server MUST be confidentiality 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 confidentiality protecting messages from client to server
   is:

Kcc = MD5({H(A1)[0..n-1],
   "Digest H(A1) to client-to-server sealing key magic constant"})

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

Kcs = MD5({H(A1)[0..n-1],
   "Digest H(A1) to server-to-client sealing key magic constant"})

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where MD5 is as specified in [RFC 1321]. For cipher "rc4-40" n is 5;
   for "rc4-56" n is 7; for the rest n is 16. The key for the "rc4-*"
   and "aes" ciphers is all 16 bytes of Kcc or Kcs; the key for "des" is
   the first 7 bytes; the key for "3des" is the first 14 bytes.

The IV used to send/receive the initial buffer of security encoded
   data for "des" and "3des" is the last 8 bytes of Kcc or Kcs. For all
   subsequent buffers the last 8 bytes of the ciphertext of the buffer
   NNN is used as the IV for the buffer (NNN + 1).

The IV for the "aes" cipher in CBC mode for messages going from the
   client to the server (IVc) consists of 16 bytes calculated as
   follows:

IVc = MD5({Kcc, "aes-128"})

The IV for the "aes" cipher in CBC mode for messages going from the
   server to the client (IVs) consists of 16 bytes calculated as
   follows:

IVs = MD5({Kcs, "aes-128"})

The IV is XOR'd with the first plaintext block before it is encrypted
   with "aes".  Then for successive blocks, the previous ciphertext
   block is XOR'd with the current plaintext, before it is encrypted.

rc4 cipher state MUST NOT be reset before sending/receiving a next
   buffer of security encoded data.

The MAC block is a variable length padding prefix followed by 16
   bytes formatted as follows: 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. If the blocksize of the chosen cipher is not 1 byte, the
   padding prefix is one or more octets each containing the number of
   padding bytes, such that total length of the encrypted part of the
   message is a multiple of the blocksize. The padding and first 10
   bytes of the MAC block are encrypted with the chosen cipher along
   with the message.

SEAL(Ki, Kc, SeqNum, msg) =
         {CIPHER(Kc, {msg, pad, HMAC(Ki, {SeqNum, msg})[0..9]}), 0x0001,
          SeqNum}

where CIPHER is the chosen cipher, Ki and Kc are Kic and Kcc for
   messages sent by the client and Kis and Kcs for those sent by the
   server. The sequence number (SeqNum) is an unsigned number
   initialized to zero after initial or subsequent authentication, and

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incremented by one for each message sent/successfully verified.
   (Note, that there are two independent counters for sending and
   receiving.) The sequence number wraps around to 0 after 2**32-1.

Upon receipt, the message is decrypted, HMAC(Ki, {SeqNum, msg}) is
   computed and compared with the received value; the padding is
   verified.  The message is discarded if the received and the
   calculated HMACs differ and/or the padding is invalid. See also
   section 3.8 for important information about MAC and padding
   verification. The receiver's sequence counter is then compared with
   the received SeqNum value; the message is discarded if they differ.
   The receiver's sequence counter is incremented if they match.

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3  Security Considerations

General SASL security considerations apply to this mechanism.
   "stringprep" and Unicode security considerations also apply.

Detailed discussion of other DIGEST-MD5 specific security issues is
   below.

3.1   Authentication of Clients using Digest Authentication

Digest Authentication does not provide a strong authentication
   mechanism, when compared to public key based mechanisms, for example.
   However, since it prevents chosen plaintext attacks, it is stronger
   than (e.g.) CRAM-MD5, which has been proposed for use with ACAP [RFC
   2244], POP and IMAP [RFC 2195]. It is intended to replace the much
   weaker and even more dangerous use of plaintext passwords; however,
   since it is still a password based mechanism it avoids some of the
   potential deployabilty issues with public-key, OTP or similar
   mechanisms.

Digest Authentication offers no confidentiality protection beyond
   protecting the actual password. All of the rest of the challenge and
   response are available to an eavesdropper, including the user's name
   and authentication realm.

3.2   Comparison of Digest with Plaintext Passwords

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 mail service whose use
   is restricted to paying subscribers. With plaintext password
   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.

3.3   Replay Attacks

Replay attacks are defeated if the client or the server chooses a
   fresh nonce for each authentication, as this specification requires.

As a security precaution, the server, when verifying a response from
   the client, must use the original server nonce ("nonce") it sent, not
   the one returned by the client in the response, as it might have been
   modified by an attacker.

To prevent some redirection attacks it is recommended that the server
   verifies that the "serv-type" part of the "digest-uri" matches the

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service name and that the hostname/IP address belongs to the server.

3.4  Online dictionary attacks

If the attacker can eavesdrop, then it can test any overheard
   nonce/response pairs against a (potentially very large) 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.

3.5  Offline dictionary attacks

If the attacker can choose the challenge, then it can precompute the
   possible responses to that challenge for 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 just once.

Offline dictionary attacks are defeated if the client chooses a fresh
   nonce for each authentication, as this specification requires.

3.6  Man in the Middle

Digest authentication is vulnerable to "man in the middle" (MITM)
   attacks. Clearly, a MITM 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 substitute a weaker
   qop scheme for the one(s) sent by the server; the server will not be
   able to detect this attack. For this reason, the client should always
   use the strongest scheme that it understands from the choices
   offered, and should never choose a scheme that does not meet its
   minimum requirements.

A man-in-the-middle attack may also make the client and the server
   that agreed to use confidentiality protection to use different (and
   possibly weaker) cipher's. This is because the chosen cipher is not
   used in the shared secret calculation.

3.7  Chosen plaintext attacks

A chosen plaintext attack is where a MITM or a malicious server can
   arbitrarily choose the challenge that the client will use to compute
   the response. The ability to choose the challenge is known to make

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cryptanalysis much easier [MD5].

However, Digest does not permit the attack to choose the challenge as
   long as the client chooses a fresh nonce for each authentication, as
   this specification requires.

3.8  CBC Mode attacks

The following attack can be launched when the connection uses
   Confidentiality protection with ciphers in CBC mode. If bad padding
   is treated differently from bad MACs when decrypting a DIGEST-MD5
   buffer of security encoded data, the attacker may be able to launch
   Vaudenay's attack on padding.

An error logfile will suffice to launch the attack if it reveals the
   type of error -- even if file permissions prevent the attacker from
   actually reading the file (the file length increase cause by the
   attack is likely to reveal which of the two errors occured).

A different approach to distinguish these two error cases and launch
   the attack is to examine the timing of error responses: if the MAC
   verification is skipped when bad padding has been found, the error
   will appear quicker in the case of incorrect block cipher padding
   than in the case of an incorrect MAC.

A countermeasure is to compute a MAC of the plaintext anyway, even if
   the usual padding removal step fails because of incorrect padding, to
   obtain (nearly) uniform timing.

3.9  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 obtain challenge/response pairs where it was able to partly
   choose the challenge. There is no known way that this can be
   exploited.

3.10  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({ username-value,
   ":", realm-value, ":", passwd }), which is adequate to compute H(A1)
   as described above without directly exposing the user's password.

The security implications of this are that if this password file is

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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 plaintext
   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 that any single user is likely to use. In
   particular a realm string should include the name of the host doing
   the authentication.

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3.11  Multiple realms

Use of multiple realms may mean both that compromise of a the
   security database for a single realm does not compromise all
   security, and that there are more things to protect in order to keep
   the whole system secure.

3.11  Summary

By modern cryptographic standards Digest Authentication is weak,
   compared to (say) public key based mechanisms. But for a large range
   of purposes it is valuable as a replacement for plaintext passwords.
   Its strength may vary depending on the implementation.

4  Example

This example shows the use of the Digest SASL mechanism with the
   IMAP4 AUTHENTICATE command [RFC 3501].

In this example, "C:" and "S:" represent a line sent by the client or
   server respectively including a CRLF at the end.  Linebreaks and
   indentation within a "C:" or "S:" are editorial and not part of the
   protocol. The password in this example was "secret".  Note that the
   base64 encoding of the challenges and responses is part of the IMAP4
   AUTHENTICATE command, not part of the Digest specification itself.

S: * OK elwood.innosoft.com PMDF IMAP4rev1 V6.0-9
    C: c CAPABILITY
    S: * CAPABILITY IMAP4 IMAP4rev1 ACL LITERAL+ NAMESPACE QUOTA
                UIDPLUS AUTH=CRAM-MD5 AUTH=DIGEST-MD5 AUTH=PLAIN
    S: c OK Completed
    C: a AUTHENTICATE DIGEST-MD5
    S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
         RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
         cnNldD11dGYtOA==
    C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
       QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
       MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9Im
       ltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9uc2U9ZDM4OGRhZDkw
       ZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcscW9wPWF1dGg=
    S: + cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
    C:
    S: a OK User logged in
    ---

The base64-decoded version of the SASL exchange is:

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S: realm="elwood.innosoft.com",nonce="OA6MG9tEQGm2hh",qop="auth",
       algorithm=md5-sess,charset=utf-8
    C: charset=utf-8,username="chris",realm="elwood.innosoft.com",
       nonce="OA6MG9tEQGm2hh",nc=00000001,cnonce="OA6MHXh6VqTrRk",
       digest-uri="imap/elwood.innosoft.com",
       response=d388dad90d4bbd760a152321f2143af7,qop=auth
    S: rspauth=ea40f60335c427b5527b84dbabcdfffd

The password in this example was "secret".

This example shows the use of the Digest SASL mechanism with the
   ACAP, using the same notational conventions and password as in the
   previous example. Note that ACAP does not base64 encode and uses
   fewer round trips that IMAP4.

S: * ACAP (IMPLEMENTATION "Test ACAP server") (SASL "CRAM-MD5"
               "DIGEST-MD5" "PLAIN")
    C: a AUTHENTICATE "DIGEST-MD5"
    S: + {94}
    S: realm="elwood.innosoft.com",nonce="OA9BSXrbuRhWay",qop="auth",
       algorithm=md5-sess,charset=utf-8
    C: {206}
    C: charset=utf-8,username="chris",realm="elwood.innosoft.com",
       nonce="OA9BSXrbuRhWay",nc=00000001,cnonce="OA9BSuZWMSpW8m",
       digest-uri="acap/elwood.innosoft.com",
       response=6084c6db3fede7352c551284490fd0fc,qop=auth
    S: a OK (SASL {40}
    S: rspauth=2f0b3d7c3c2e486600ef710726aa2eae) "AUTHENTICATE
    Completed"
    ---

The server uses the values of all the directives, plus knowledge of
   the users password (or the hash of the user's name, server's realm
   and the user's password) to verify the computations above. If they
   check, then the user has authenticated.

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5   References

5.1   Normative references

[Digest]   Franks, J., et al., "HTTP Authentication: Basic and Digest
              Access Authentication", RFC 2617, June 1999.

[ISO-8859] ISO-8859. International Standard--Information Processing--
              8-bit Single-Byte Coded Graphic Character Sets --
              Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
              Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
              Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
              Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
              Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
              Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
              Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
              Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
              Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.

[RFC 822]  Crocker, D., "Standard for The Format of ARPA Internet
              Text Messages," STD 11, RFC 822, August 1982.

[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

[RFC 2052] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
              location of services (DNS SRV)", RFC 2052, October 1996.

[RFC 2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
              Hashing for  Message Authentication", RFC 2104, February
              1997.

[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC 2222] Myers, J., "Simple Authentication and Security Layer
              (SASL)", RFC 2222, October 1997.

[Stringprep] Hoffman, P., Blanchet, M., "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

[Unicode] The Unicode Consortium, "The Unicode Standard, Version
              3.2.0", defined by: The Unicode Standard, Version 3.0
              (Reading, MA, Addison-Wesley, 2000.  ISBN 0-201-61633-5),
              as amended by the Unicode Standard Annex #28: Unicode 3.2
              (http://www.unicode.org/reports/tr28/tr28-3.html).

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[UTF-8] Yergeau, "UTF-8, a transformation format of ISO 10646", RFC
              2279, Janyary 1998.

[USASCII]  US-ASCII. Coded Character Set - 7-Bit American Standard
              Code for Information Interchange. Standard ANSI X3.4-1986,
              ANSI, 1986.

[SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user names
              and passwords", Work in progress, draft-ietf-sasl-
              saslprep-XX.txt.

[RFC 2732]  Hinden, R., Carpenter, B., Masinter, L., "Format for
              Literal IPv6 Addresses in URL's", RFC 2732, December 1999.

[RFC 2373] Hinden, R., Deering, S., "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

[RFC 2396] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

[FIPS] National Institute of Standards and Technology, "DES Modes of
              Operation", http://www.itl.nist.gov/fipspubs/fip81.htm,
              December 1980.

[AES] Daemen, J., Rijmen, V., "The Rijndael Block Cipher",
              http://csrc.nist.gov/encryption/aes/rijndael/Rijndael.pdf,
              3rd September 1999.

5.2   Informative references

[RFC 2195] Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response", RFC
              2195, September 1997.

[MD5]  Kaliski, B.,Robshaw, M., "Message Authentication with MD5",
              CryptoBytes, Sping 1995, RSA Inc,
              (http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm)

[RFC 2078] Linn, J., "Generic Security Service Application Program
              Interface, Version 2", RFC 2078, January 1997.

[RFC 3501] Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

[RFC 2244] Newman, C., Myers, J., "ACAP -- Application Configuration
              Access Protocol", RFC 2244, November 1997.

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[RFC 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

[TLS-CBC] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS:
              Problems and Countermeasures",
              http://www.openssl.org/~bodo/tls-cbc.txt.

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6  Authors' Addresses

Paul Leach
   Microsoft
   1 Microsoft Way
   Redmond, WA 98052, USA

EMail: paulle@microsoft.com

Chris Newman
   Sun Microsystems
   1050 Lakes Drive
   West Covina, CA 91790, USA

EMail: Chris.Newman@Sun.COM

Alexey Melnikov
   Isode
   28 Gloucester Road,
   Teddington, Middlesex, TW11 0NU, UK

Email: mel@isode.com

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7  ABNF

What follows is the definition of the notation as is used in the
   HTTP/1.1 specification [RFC 2616] and the HTTP authentication
   specification [Digest]; it is reproduced here for ease of reference.
   Since it is intended that a single Digest implementation can support
   both HTTP and SASL-based protocols, the same notation is used in both
   to facilitate comparison and prevention of unwanted differences.
   Since it is cut-and-paste from the HTTP specifications, not all
   productions may be used in this specification. It is also not quite
   legal ABNF; again, the errors were copied from the HTTP
   specifications.

7.1   Augmented BNF

All of the mechanisms specified in this document are described in
   both prose and an augmented Backus-Naur Form (BNF) similar to that
   used by RFC 822 [RFC 822]. Implementers will need to be familiar with
   the notation in order to understand this specification.

The augmented BNF includes the following constructs:

name = definition
      The name of a rule is simply the name itself (without any
      enclosing "<" and ">") and is separated from its definition by the
      equal "=" character. White space is only significant in that
      indentation of continuation lines is used to indicate a rule
      definition that spans more than one line. Certain basic rules are
      in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
      brackets are used within definitions whenever their presence will
      facilitate discerning the use of rule names.

"literal"
      Quotation marks surround literal text. Unless stated otherwise,
      the text is case-insensitive.

rule1 | rule2
      Elements separated by a bar ("|") are alternatives, e.g., "yes |
      no" will accept yes or no.

(rule1 rule2)
      Elements enclosed in parentheses are treated as a single element.
      Thus, "(elem (foo | bar) elem)" allows the token sequences
      "elem foo elem" and "elem bar elem".

*rule
      The character "*" preceding an element indicates repetition. The
      full form is "<n>*<m>element" indicating at least <n> and at most

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<m> occurrences of element. Default values are 0 and infinity so
      that "*(element)" allows any number, including zero; "1*element"
      requires at least one; and "1*2element" allows one or two.

[rule]
      Square brackets enclose optional elements; "[foo bar]" is
      equivalent to "*1(foo bar)".

N rule
      Specific repetition: "<n>(element)" is equivalent to
      "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
      Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
      alphabetic characters.

#rule
      A construct "#" is defined, similar to "*", for defining lists of
      elements. The full form is "<n>#<m>element" indicating at least
      <n> and at most <m> elements, each separated by one or more commas
      (",") and OPTIONAL linear white space (LWS). This makes the usual
      form of lists very easy; a rule such as
        ( *LWS element *( *LWS "," *LWS element ))
      can be shown as
        1#element
      Wherever this construct is used, null elements are allowed, but do
      not contribute to the count of elements present. That is,
      "(element), , (element) " is permitted, but counts as only two
      elements.  Therefore, where at least one element is required, at
      least one non-null element MUST be present. Default values are 0
      and infinity so that "#element" allows any number, including zero;
      "1#element" requires at least one; and "1#2element" allows one or
      two.

; comment
      A semi-colon, set off some distance to the right of rule text,
      starts a comment that continues to the end of line. This is a
      simple way of including useful notes in parallel with the
      specifications.

implied *LWS
      The grammar described by this specification is word-based. Except
      where noted otherwise, linear white space (LWS) can be included
      between any two adjacent words (token or quoted-string), and
      between adjacent words and separators, without changing the
      interpretation of a field. At least one delimiter (LWS and/or
      separators) MUST exist between any two tokens (for the definition
      of "token" below), since they would otherwise be interpreted as a
      single token.

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7.2   Basic Rules

The following rules are used throughout this specification to
   describe basic parsing constructs. The US-ASCII coded character set
   is defined by ANSI X3.4-1986 [USASCII].

OCTET          = <any 8-bit character>
       CHAR           = <any US-ASCII character (octets 0 - 127)>
       UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
       LOALPHA        = <any US-ASCII lowercase letter "a".."z">
       ALPHA          = UPALPHA | LOALPHA
       DIGIT          = <any US-ASCII digit "0".."9">
       CTL            = <any US-ASCII control character
                        (octets 0 - 31) and DEL (127)>
       CR             = <US-ASCII CR, carriage return (13)>
       LF             = <US-ASCII LF, linefeed (10)>
       SP             = <US-ASCII SP, space (32)>
       HT             = <US-ASCII HT, horizontal-tab (9)>
       <">            = <US-ASCII double-quote mark (34)>
       TEXTCHAR       = <any OCTET except CTLs, but including HT>
       CRLF           = CR LF

All linear white space, including folding, has the same semantics as
   SP.  A recipient MAY replace any linear white space with a single SP
   before interpreting the field value or forwarding the message
   downstream.

LWS            = [CRLF] 1*( SP | HT )

The TEXT rule is only used for descriptive field contents and values
   that are not intended to be interpreted by the message parser. Words
   of TEXT contains characters either from ISO-8859-1 [ISO-8859]
   character set or UTF-8 [UTF-8].

TEXT           = <any *OCTET except CTLs,
                        but including LWS>

A CRLF is allowed in the definition of TEXT only as part of a header
   field continuation. It is expected that the folding LWS will be
   replaced with a single SP before interpretation of the TEXT value.

Hexadecimal numeric characters are used in several protocol elements.

HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                      | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

Many HTTP/1.1 header field values consist of words separated by LWS
   or special characters. These special characters MUST be in a quoted

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string to be used within a parameter value.

token          = 1*TOKENCHAR
       separators     = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "\" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | "{" | "}" | SP | HT
       TOKENCHAR      = <any CHAR except CTLs or separators>

A string of text is parsed as a single word if it is quoted using
   double-quote marks.

quoted-string  = ( <"> qdstr-val <"> )
       qdstr-val      = *( qdtext | quoted-pair )
       qdtext         = <any TEXTCHAR except <"> and "\">

Note that LWS is NOT implicit between the double-quote marks (<">)
   surrounding a qdstr-val and the qdstr-val; any LWS will be considered
   part of the qdstr-val.  This is also the case for quotation marks
   surrounding any other construct.

The backslash character ("\") MAY be used as a single-character
   quoting mechanism only within qdstr-val and comment constructs.

quoted-pair    = "\" CHAR

The value of this construct is CHAR. Note that an effect of this rule
   is that backslash itself MUST be quoted.

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8  Sample Code

The sample implementation in [Digest] also applies to DIGEST-MD5.

The following code implements the conversion from UTF-8 to 8859-1 if
   necessary.

/* if the string is entirely in the 8859-1 subset of UTF-8, then
     * translate to 8859-1 prior to MD5
     */
    void MD5_UTF8_8859_1(MD5_CTX *ctx, const unsigned char *base,
        int len)
    {
        const unsigned char *scan, *end;
        unsigned char cbuf;

end = base + len;
        for (scan = base; scan < end; ++scan) {
            if (*scan > 0xC3) break; /* abort if outside 8859-1 */
            if (*scan >= 0xC0 && *scan <= 0xC3) {
                if (++scan == end || *scan < 0x80 || *scan > 0xBF)
                    break;
            }
        }
        /* if we found a character outside 8859-1, don't alter string
         */
        if (scan < end) {
            MD5Update(ctx, base, len);
            return;
        }

/* convert to 8859-1 prior to applying hash
         */
        do {
            for (scan = base; scan < end && *scan < 0xC0; ++scan)
                ;
            if (scan != base) MD5Update(ctx, base, scan - base);
            if (scan + 1 >= end) break;
            cbuf = ((scan[0] & 0x3) << 6) | (scan[1] & 0x3f);
            MD5Update(ctx, &cbuf, 1);
            base = scan + 2;
        } while (base < end);
    }

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9   Interoperability considerations

9.1 Implementing DES cipher in CBC mode

Several cryptographic libraries (Ebones, OpenSSL) provide a convenience
   function des_cbc_encrypt for implementing DES cipher in CBC mode.
   There is a documented bug in this function: the function doesn't update
   IV before returning. If an implementation uses this function to implement
   DES cipher in CBC mode, it MUST update IV by copying the last 8 bytes of
   the des_cbc_encrypt's output to the IV buffer.
   Note that the function des_ede2_cbc_encrypt that may be used to implement
   3DES (in "two keys mode") in CBC mode works as expected.

Care must be taken when configuring the DES keys for most DES
   libraries. This specification gives 56 bits for the DES key (or 112
   bits for the 3DES key); libraries generally expect the key to be given
   in a 64 bit (128 bit for 3DES) form.

The following C function can be used to convert a 56 bit DES key into a
   form acceptable for the libraries. The low order bit in each byte
   would contain parity information and will be corrected by the library.

/* slide the first 7 bytes of 'inbuf' into the high seven bits of the
      first 8 bytes of 'keybuf'. 'keybuf' better be 8 bytes long or longer. */
   void slidebits(unsigned char *keybuf, unsigned char *inbuf)
   {
       keybuf[0] = inbuf[0];
       keybuf[1] = (inbuf[0]<<7) | (inbuf[1]>>1);
       keybuf[2] = (inbuf[1]<<6) | (inbuf[2]>>2);
       keybuf[3] = (inbuf[2]<<5) | (inbuf[3]>>3);
       keybuf[4] = (inbuf[3]<<4) | (inbuf[4]>>4);
       keybuf[5] = (inbuf[4]<<3) | (inbuf[5]>>5);
       keybuf[6] = (inbuf[5]<<2) | (inbuf[6]>>6);
       keybuf[7] = (inbuf[6]<<1);
   }

10  Acknowledgements

The following people had substantial contributions to the development
   and/or refinement of this document:

Lawrence Greenfield John Gardiner Myers Simon Josefsson RL Bob Morgan
   Jeff Hodges Claus Assmann Tony Hansen Sam Hartman

as well as other members of the SASL mailing list.

The text used is section 3.8 was taken from [TLS-CBC] by Bodo
   Moeller.

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11  Full Copyright Statement

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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Appendix A: Changes from 2831

1). Fixed various typos in formulas.

2). Dropped DES as mandatory to implement cipher (3DES is mandatory
   to implement).

3). Tighten ABNF. Fixed some bugs.

4). Clarified nc-value verification and which side is aborting
   exchange.

5). Added text saying that for interoperability
   username/password/realm MUST be prepared using the "SASLPrep" profile
   [SASLPrep] of the "stringprep" algorithm [StringPrep].

6). Clarified that unquoted version of the username, etc. used in A1
   calculation.

7). Various cleanup to References section. Split all references to
   Normative and Informative.

8). Added minimal and maximal limits on maxbuf. Clarified how to
   calculate max sender size.

9). Change ABNF for host to allow for IPv6 addresses. ABNF now
   references RFC 2373 and RFC 2396.

10). Added DES cipher interoperability section.

11). Added man-in-the-middle considerations for ciphers.

12). Clarified how sequence counters are modified.

13). Addition warnings about preventing reply/redirection attacks.

14). Specified that "charset" directive affects "realm" and doesn't
   affect
        "authzid".

15). Removed text that described that "authzid" is in Unicode in
   Normalization
        Form KC, encoded as UTF-8.

16). Clarified that rc4 state is not reset between two sent/received
   buffers
        of encoded data.

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17). Clarified that for DES/3DES the IV for the next buffer of
   encoded data is
        the last 8 bytes of the ciphertext.

18). Clarified how "maximal sender size" is calculated.

19). Prohibit an empty authzid.

20). Added AES cipher defined in "AES Ciphersuite for DIGEST-MD5 SASL
   mechanism"
        document (expired draft-ietf-sasl-digest-aes-00.txt).

21). Minor text clarifications.

Appendix B: Open Issues/ToDo List

1). The latest revision prohibits escaped characters in nonce/cnonce.
   This is different
       from HTTP Digest. Any objections?

2). Do we need/want a new stringprep profile for "realm"?

3). What to do about CBC mode attack that affects TLS document and
   DIGEST-MD5 as well?

One of the proposals is to drop DES/3DES ciphers and define a new one
   (e.g. AES) in such a way that is not susceptible to this kind of
   attack.

AES cipher has to be fixed to prevent this attack.

4). Add reference to CBC mode attack:

This problem is described in LASEC Memo "Password Interception in a
   SSL/TLS Channel" by Brice Canvel, published 2003-02-20:
   http://lasecwww.epfl.ch/memo_ssl.shtml

5). Normative vs. Informative references must be carefully rechecked.

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Code Editor : draft-ietf-sasl-rfc2831bis-xx.txt