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Versions: 00 01 02 03 04 05 06 07 08                                    
Network Working Group                                        K.R. Burdis
Internet-Draft                                         Rhodes University
Expires: July 2, 2001                                          R. Naffah
                                                          Forge Research
                                                            January 2001


                 Secure Remote Password SASL Mechanism
                      draft-burdis-cat-srp-sasl-04

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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.

   This Internet-Draft will expire on July 2, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

   This document describes a family of SASL mechanisms based on the
   Secure Remote Password protocol.  These mechanisms perform mutual
   authentication and can provide a security layer with replay
   detection, integrity protection and/or confidentiality protection.









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Table of Contents

   1.  Mechanism Names  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in this Document  . . . . . . . . . . . . . .  4
   3.  Data Element Formats . . . . . . . . . . . . . . . . . . . . .  5
   3.1 Scalar numbers . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.2 Multi-Precision Integers . . . . . . . . . . . . . . . . . . .  5
   3.3 Octet Sequences  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.4 Extended Octet Sequences . . . . . . . . . . . . . . . . . . .  6
   3.5 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.6 Buffers  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.7 Data Element Size Limits . . . . . . . . . . . . . . . . . . .  7
   4.  Protocol Description . . . . . . . . . . . . . . . . . . . . .  8
   4.1 Client sends its authentication identity . . . . . . . . . . .  9
   4.2 Server sends initial protocol elements . . . . . . . . . . . .  9
   4.3 Client sends its ephemeral public key  . . . . . . . . . . . . 10
   4.4 Server sends its ephemeral public key  . . . . . . . . . . . . 11
   4.5 Client sends its evidence  . . . . . . . . . . . . . . . . . . 11
   4.6 Server sends its evidence  . . . . . . . . . . . . . . . . . . 11
   5.  Security Layer . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.1 Confidentiality Protection . . . . . . . . . . . . . . . . . . 14
   5.2 Replay Detection . . . . . . . . . . . . . . . . . . . . . . . 16
   5.3 Integrity Protection . . . . . . . . . . . . . . . . . . . . . 16
   5.4 Summary of Security Layer Output . . . . . . . . . . . . . . . 16
   6.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
   A.  Modulus and Generator values . . . . . . . . . . . . . . . . . 24
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 26



















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1. Mechanism Names

   The family of SASL mechanisms associated with the protocol described
   in this document are named "SRP-<MDA name>" where <MDA name> is the
   canonical name of a Message Digest Algorithm.

   For example, "SRP-SHA-160" shall denote the SASL mechanism using the
   protocol described in this document with SHA-1 (20-octet output
   length, or 160 bits) being used to compute both client-side and
   server-side digests.  Similarly, "SRP-RIPEMD-160" shall denote the
   SASL mechanism using the protocol described in this document with
   RIPEMD-160 as the underlying Message Digest Algorithm.







































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2. Conventions Used in this Document

   o  A hex digit is an element of the set:

         {0, 1, 2, 3, 4, 5, 6, 7, 8 , 9, A, B, C, D, E, F}

      A hex digit is the representation of a 4-bit string.  Examples:

         7 = 0111

         A = 1010

   o  An octet is an 8-bit string.  In this document an octet may be
      written as a pair of hex digits.  Examples:

         7A = 01111010

         02 = 00000010

   o  All data is encoded and sent in network byte order (big-endian).

   o  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
      RFC2119 [1].


























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3. Data Element Formats

   This section describes the encoding of the data elements used by the
   SASL mechanisms described in this document.

3.1 Scalar numbers

   Scalar numbers are unsigned quantities.  Using b[k] to refer to the
   k-th octet being processed, the value of a two-octet scalar is:

      ((b[0] << 8) + b[1]),

   where << is the bit left-shift operator.  The value of a four-octet
   scalar is:

      ((b[0] << 24) + (b[1] << 16) + (b[2] << 8) + b[3]).

3.2 Multi-Precision Integers

   Multi-Precision Integers, or MPIs, are positive integers used to
   hold large integers used in cryptographic computations.

   MPIs are encoded using a scheme inspired by that used by OpenPGP -
   RFC2440 (section 3.2) [2] - for encoding such entities:

      The encoded form of an MPI SHALL consist of two pieces: a
      two-octet scalar that represents the length of the entity, in
      octets, followed by a sequence of octets that contain the actual
      integer.

      These octets form a big-endian number;  A big-endian number can
      be encoded by prefixing it with the appropriate length.

      Examples: (all numbers are in hexadecimal)

         The sequence of octets [00 01 01] encodes an MPI with the
         value 1, while the sequence [00 02 01 FF] encodes an MPI with
         the value of 511

      Additional rule:

      *  The length field of an encoded MPI describes the octet count
         starting from the MPI's first non-zero octet, containing the
         most significant non-zero bit.  Thus, the encoding [00 02 01]
         is not formed correctly; It should be [00 01 01].

   We shall use the syntax mpi(A) to denote the encoded form of the
   multi-precision integer A.  Furthermore, we shall use the syntax
   bytes(A) to denote the big-endian sequence of octets forming the


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   multi-precision integer with the most significant octet being the
   first non-zero octet containing the most significant bit of A.

3.3 Octet Sequences

   These mechanisms generate, use and exchange sequences of octets;
   e.g. output values of message digest algorithm functions.  When such
   entities travel on the wire, they shall be preceded by a one-octet
   scalar quantity representing the count of following octets.

   We shall use the syntax os(s) to denote the encoded form of the
   octet sequence.  Furthermore, we shall use the syntax bytes(s) to
   denote the sequence of octets s, in big-endian order.

3.4 Extended Octet Sequences

   Extended sequences of octets are exchanged when using the security
   layer. When these sequences travel on the wire, they shall be
   preceded by a four-octet scalar quantity representing the count of
   following octets.

   We shall use the syntax eos(s) to denote the encoded form of the
   extended octet sequence.  Furthermore, we shall use the syntax
   bytes(s) to denote the sequence of octets s, in big-endian order.

3.5 Text

   The only character set for text is the UTF-8 [3] encoding of Unicode
   characters [4].

   We shall use the syntax utf8(L) to denote the string L in UTF-8
   encoding, preceded by a two-octet scalar quantity representing the
   count of following octets.  Furthermore, we shall use the syntax
   bytes(L) to denote the sequence of octets representing the UTF-8
   encoding of L, in big-endian order.

3.6 Buffers

   In these SASL mechanisms data is exchanged between the client and
   server using buffers.  A buffer acts as an envelope for the sequence
   of data elements sent by one end-point of the exchange, and expected
   by the other.

   A buffer MAY NOT contain other buffers.  It may only contain zero,
   one or more data elements.

   A buffer shall be encoded as two fields: a four-octet scalar
   quantity representing the count of following octets, and the
   concatenation of the octets of the data element(s) contained in the


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   buffer.

   We shall use the syntax {A|B|C} to denote a buffer containing A, B
   and C in that order.  For example:

      { mpi(N) | mpi(g) | utf8(L) }

   is a buffer containing, in the designated order, the encoded forms
   of an MPI N, an MPI g and a Text L.

3.7 Data Element Size Limits

   The following table details the size limit, in number of octets, for
   each of the SASL data element encodings described earlier.


       Data element type          Header       Size limit in octets
                                 (octets)       (excluding header)
       ------------------------------------------------------------
       Octet Sequence               1                  255
       MPI                          2                 65,535
       Text                         2                 65,535
       Extended Octet Sequence      4             2,147,483,383
       Buffer                       4             2,147,483,643


   An implementation SHOULD signal an exception if any size constraint
   is violated.























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4. Protocol Description

   SRP is a password-based, zero-knowledge, authentication and
   key-exchange protocol developed by Thomas Wu.  It has good
   performance, is not plaintext-equivalent and maintains perfect
   forward secrecy.  It provides authentication (optionally mutual
   authentication) and the negotiation of a session key [12].

   The mechanisms described herein are based on the optimised SRP
   protocol described at the end of section 3 in [13], since this
   reduces the total number of messages exchanged by grouping together
   pieces of information that do not depend on earlier messages.  Due
   to the design of the mechanism, mutual authentication is MANDATORY.

   This document describes the sequence of data transmitted between the
   client and server, and it adds extra control information to enable
   the client to request whether or not replay detection, integrity
   protection and/or confidentiality protection should be provided by a
   security layer.

   Mechanism data exchanges, during the authentication phase, are shown
   below:


       Client                                             Server

         -----  { utf8(U) }  -------------------------------->

         <--------------  { mpi(N) | mpi(g) | utf8(L) }  -----

         -----  { utf8(I) | mpi(A) | utf8(o) }  ------------->

         <-------------------------  { os(s) | mpi(B) }  -----

         -----  { os(M1) }  --------------------------------->

         <---------------------------------  { os(M2) }  -----


   where:

      U     is the authentication identity (username),

      N     is the safe prime modulus,

      g     is the generator,

      L     is the options list indicating available security services,



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      I     is the authorisation identity,

      A     is the client's ephemeral public key,

      o     is the options list indicating chosen security services,

      s     is the user's password salt,

      B     is the server's ephemeral public key,

      M1    is the client's evidence that the shared key K is known,

      M2    is the server's evidence that the shared key K is known.

4.1 Client sends its authentication identity

   The client determines its authentication identity U, encodes it and
   sends it to the server.

   The client sends:

      { utf8(U) }

4.2 Server sends initial protocol elements

   The server receives U, and looks up the safe prime modulus N and the
   generator g to be used for that identity.

   The server also creates an options list L, which consists of a
   comma-separated list of option strings that specify the security
   service options the server supports.  The following security service
   options strings are defined:

   o  "integrity=HMAC-<MDA-name>" indicates that the server supports
      integrity protection using the HMAC algorithm [9] with <MDA-name>
      as the underlying Message Digest Algorithm. Acceptable MDA names
      are chosen from [15] under the MessageDigest section.  A server
      SHOULD send such an option string for each HMAC algorithm it
      supports.  Note that in the interest of interoperability, if the
      server offers integrity protection it MUST, as a minimum, send
      the option string "integrity=HMAC-MD5" since support for this
      algorithm is then MANDATORY.

   o  "replay detection" indicates that the server supports replay
      detection using sequence numbers.

   o  "confidentiality=<cipher name>" indicates that the server
      supports confidentiality protection using the symmetric block
      cipher algorithm <cipher name>.  The server SHOULD send such an


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      option string for each confidentiality protection algorithm it
      supports.  Note that in the interest of interoperability, if the
      server offers confidentiality protection, it MUST send the option
      string "confidentiality=aes" since it is then MANDATORY for it to
      provide support for this algorithm.  (Rijndael [5] is synonymous
      with AES [6].)

   Additional rules:

   o  Replay detection SHALL NOT be activated without also activating
      integrity protection.  If the replay detection option is offered
      (by the server) and/or chosen (by the client) without explicitely
      specifying an integrity protection option, then the default
      integrity protection option "integrity=HMAC-MD5" is implied and
      shall be activated.

   o  The options list SHOULD NOT be interpreted in a case-sensitive
      manner, and whitespace characters SHOULD be ignored.

   For example, if the server supports integrity protection using the
   HMAC-MD5 and HMAC-SHA-160 algorithms, replay detection and no
   confidentiality protection, the options list would be:

      integrity=HMAC-MD5,integrity=HMAC-SHA-160,replay detection

   The server sends:

      { mpi(N) | mpi(g) | utf8(L) }

4.3 Client sends its ephemeral public key

   The client receives the options list L from the server that
   specifies the security service options the server supports.  The
   client selects options from this list and creates a new options list
   o that specifies the security services that will be used in the
   security layer.  At most one available integrity protection
   algorithm and one available confidentiality protection algorithm may
   be selected.

   The client determines its authorisation identity I, and generates
   its ephemeral public key A.

   The client sends:

      { utf8(I) | mpi(A) | utf8(o) }






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4.4 Server sends its ephemeral public key

   The server reads the client's salt s, calculates the shared context
   key K and generates its ephemeral public key B.

   The server sends:

      { os(s) | mpi(B) }

4.5 Client sends its evidence

   The client calculates the shared context key K, and calculates the
   evidence M1 that proves to the server that it knows the shared
   context key K, including L as part of the calculation.

   M1 is computed as:


             H(
                    bytes(H( bytes(N) )) ^ bytes( H( bytes(g) )))
                  | bytes(H( bytes(U) ))
                  | bytes(s)
                  | bytes(H( bytes(L) ))
                  | bytes(A)
                  | bytes(B)
                  | bytes(K)
              )


    where:

      H() is the result of digesting the designated input/data with the
      underlying Message Digest Algorithm function (see Section 1).

      ^ is the bitwise XOR operator.

   The client sends:

      { os(M1) }

4.6 Server sends its evidence

   The server calculates the evidence M2 that proves to the client that
   it knows the shared context key K, as well as U, I, and o.

   M2 is computed as:





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               H(
                      bytes(A)
                    | bytes(H( bytes(U) ))
                    | bytes(H( bytes(I) ))
                    | bytes(H( bytes(o) ))
                    | bytes(M1)
                    | bytes(K)
                )


    where:

      H() is the result of digesting the designated input/data with the
      underlying Message Digest Algorithm function (see Section 1)

   The server sends:

      { os(M2) }

































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5. Security Layer

   Depending on the options offered by the server and specified by the
   client, the security layer may provide integrity protection, replay
   detection, and/or confidentiality protection.

   The security layer can be thought of as a three-stage filter through
   which the data flows from the output of one stage to the input of
   the following one. The first input is the original data, while the
   last output is the data after being subject to the transformations
   of this filter.

   The data always passes through this three-stage filter, though any
   of the stages may be inactive.  Only when a stage is active would
   the output be different from the input.  In other words, if a stage
   is inactive, the octet sequence at the output side is an exact
   duplicate of the same sequence at the input side.

   Schematically, the three-stage filter security layer appears as
   follows:


                        +----------------------------+
                        |                            |     I/ p1
                p1  --->| Confidentiality protection |---+
                        |                            |   | A/ c
                        +----------------------------+   |
                                                         |
                    +------------------------------------+
                    |
                    |   +----------------------------+
                    |   |                            |     I/ p2
                p2  +-->|      Replay detection      |---+
                        |                            |   | A/ p2 | q
                        +----------------------------+   |
                                                         |
                    +------------------------------------+
                    |
                    |   +----------------------------+
                    |   |                            |     I/ p3
                p3  +-->|    Integrity protection    |--->
                        |                            |     A/ p3 | C
                        +----------------------------+



    where:

      p1, p2 and p3 are the input octet sequences at each stage,


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      I/ denotes the output at the end of one stage if/when the stage
      is inactive or disabled,

      A/ denotes the output at the end of one stage if/when the stage
      is active or enabled,

      c is the encrypted (sender-side) or decrypted (receiver-side)
      octet sequence.  c1 shall denote the value computed by the
      sender, while c2 shall denote the value computed by the receiver.

      q is a four-octet scalar quantity representing a sequence number,

      C is the Message Authentication Code. C1 shall denote the value
      of the MAC as computed by the sender, while C2 shall denote the
      value computed by the receiver.

   The following paragraphs detail each of the transformations
   mentioned above.

5.1 Confidentiality Protection

   The plaintext data octet sequence p1 is encrypted using the chosen
   confidentiality algorithm (CALG) initialised for encryption with the
   shared context key K.

      c1 = CALG(K, ENCRYPTION)( bytes(p1) )

   On the receiving side, the ciphertext data octet sequence p1 is
   decrypted using the chosen confidentiality algorithm (CALG)
   initialised for decryption, with the shared context key K.

      c2 = CALG(K, DECRYPTION)( bytes(p1) )

   The designated CALG block cipher should be used in OFB (Output
   Feedback Block) mode in the ISO variant, as described in [16],
   algorithm 7.20.

   Let k be the block size of the chosen symmetric cipher algorithm;
   e.g. for AES this is 128 bits or 16 octets.  The OFB mode used shall
   be of length/size k.

   It is recommended that Block ciphers operating in OFB mode be used
   with an Initial Vector (the mode's IV).  For the SASL mechanisms
   described in this document, the IV shall be an all-zero octet
   sequence of size k.

   In such a mode of operation - OFB with key re-use - the IV, which
   need not be secret, must be changed.  Otherwise an identical
   keystream results; and, by XORing corresponding ciphertexts, an


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   adversary may reduce cryptanalysis to that of a running-key cipher
   with one plaintext as the running key.  To counter the effect of
   fixing the IV to an all-zero octet sequence, the sender should use a
   one k-octet sequence as the value of its first block, constructed as
   follows:

   o  the first (most significant) (k-2) octets are random,

   o  the octets at position #k-1 and #k, assuming the first octet is
      at position #1, are exact copies of those at positions #1 and #2
      respectively.

   The input data to the confidentiality protection algorithm shall be
   a multiple of the symmetric cipher block size k.  When the input
   length is not a multiple of k octets, the data shall be padded
   according to the following scheme (described in [17] which itself is
   based on RFC1423 [18]):

      Assuming the length of the input is l octets, (k - (l mod k))
      octets, all having the value (k - (l mod k)), shall be appended
      to the original data.  In other words, the input is padded at the
      trailing end with one of the following sequences:


                      01 -- if l mod k = k-1
                     02 02 -- if l mod k = k-2
                               ...
                               ...
                               ...
                   k k ... k k -- if l mod k = 0


      The padding can be removed unambiguously since all input is
      padded and no padding sequence is a suffix of another.  This
      padding method is well-defined if and only if k < 256 octets,
      which is the case with symmetric block ciphers today, and in the
      forseeable future.

   The output of this stage, when it is active, is:

      at the sending side: CALG(K, ENCRYPT)( bytes(p1) )

      at the receiving side: CALG(K, DECRYPT)( bytes(p1) )

   If the receiver, after decrypting the first block, finds that the
   last two octets do not match the value of the first two, it MUST
   signal an exception and abort the exchange.




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5.2 Replay Detection

   A sequence number q is incremented every time a message is sent to
   the peer.

   The output of this stage, when it is active, is:

      p2 | q

   At the other end, the receiver increments its copy of the sequence
   number.  This new value of the sequence number is then used in the
   integrity protection transformation, which must also be active as
   described in Section 4.2.

5.3 Integrity Protection

   A message authentication code C is computed using the chosen
   integrity protection algorithm (IALG) initialised with the shared
   context key K, and applied to the sequence p3.

   The output of this stage, when it is active, is:

      IALG(K)( bytes(p3) )

   At the other end, the receiver computes its version of the MAC,
   using the same transformation, and checks if its value is equal to
   that received.  If the two values do not agree, the receiver MUST
   signal an exception and abort the exchange.

5.4 Summary of Security Layer Output

   The following table shows the data exchanged by the security layer
   peers, depending on the possible legal combinations of the three
   security services in operation:


      CP   IP   RD   Peer sends/receives

      I    I    I    { eos(p) }
      I    A    I    { eos(p) | os( IALG(K)( bytes(p) ) ) }
      I    A    A    { eos(p) | os( IALG(K)( bytes(p) | bytes(q)) ) }
      A    I    I    { eos(c) }
      A    A    I    { eos(c) | os( IALG(K)( bytes(c) ) ) }
      A    A    A    { eos(c) | os( IALG(K)((bytes(c) | bytes(q)) ) }


    where

      CP    Confidentiality protection,


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      IP    Integrity protection,

      RD    Replay detection,

      I     Security service is Inactive/disabled,

      A     Security service is Active/enabled,

      p     The original plaintext,

      q     The sequence number.

      c     The enciphered input obtained by either:

         CALG(K, ENCRYPT)( bytes(p) ) at the sender's side, or

         CALG(K, DECRYPT)( bytes(p) ) at the receiver's side, or


































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6. Example

   The example below uses SMTP authentication [19]. The base64 encoding
   of challenges and responses, as well as the reply codes preceding
   the responses are part of the SMTP authentication[19] specification,
   not part of this SASL mechanism itself.

   "C:" and "S:" indicate lines sent by the client and server
   respectively.


     S: 220 smtp.example.com ESMTP server ready

     C: EHLO zaau.example.com

     S: 250-smtp.example.com
     S: 250 AUTH SRP-SHA-160 CRAM-MD5 DIGEST-MD5

     C: AUTH SRP-SHA-160 AAAABQADZm9v

     S: AAAAqgCA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxObIlFKCH
     mONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjftawv/XLb0Brf
     t7jhr+1qJn6WunyQRfEsf5kkoZlHs5lOB//////////8AAQUAI2ludGVncml0eT1obWFj
     LW1kNSxyZXBsYXkgZGV0ZWN0aW9u

     C: AAAArAADZm9vAIBFoAAiZ7mnsz2UBmAtV4t2nW973SBNLUdL9BC3AG0CC0TCtYjjwP
     dhobc02S9ERw7G+lPcmAFXGO6KDHc7AXe33xp+WwGGkIyB49oJB8VZ+sXqCr6OBMFvV1H
     okkzIyjhogn2OZVdn89FryqG4LwuEsypCPGQ+cgxYWUGTIuAMrwAjaW50ZWdyaXR5PWht
     YWMtbWQ1LHJlcGxheSBkZXRlY3Rpb24=

     S: AAAAjgqSCwkzSOiPQ1JnAIEAmkVIho/d/xckmrzp1nMEtkWKxlOOiX0V8u+a9y9/0V
     KgzKJlcT+QI/uQH9l23tnfOOK3CfDuaZMnQgMLNCsvRy22x6YhZW07zo39QhMWLWLSjVJ
     lWXgxSQyds1JvVAQzZN+XaFdZs5lMDfSJMiC8L7MzZyw8XmHh5v1DtueK9mc=

     C: AAAAFRS0T1/zTL9Idv9R5F7tuCFMtWrCGg==

     S: AAAAFRShvobx8ubyF8fUAuupQIfWYPdu4A==

     C:

     S: 235 Authentication successful.










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7. Discussion

   The algorithms specified as mandatory were chosen for utility and
   availablity.  We felt that a mandatory confidentiality and integrity
   protection algorithm should be specified to ensure interoperability
   between implementations of these mechanisms.

   o  The HMAC-MD5 algorithm was chosen as an integrity algorithm
      because it is faster than both HMAC-SHA-160 and MAC algorithms
      based on secret key encryption algorithms [8].

   o  Rijndael was chosen as a cipher because it has undergone thorough
      scrutiny by the best cryptographers in the world and was chosen
      ahead of many other algorithms as the Advanced Encryption
      Standard.

   Since confidentiality protection is optional this mechanism should
   be usable in countries that have strict controls on the use of
   cryptography.

   It is RECOMMENDED that the server use values for the modulus (N) and
   generator (g) chosen from those listed in Appendix A so that the
   client can avoid expensive constraint checks, since these predefined
   values already meet the constraints described in [13]:

      "For maximum security, N should be a safe prime (i.e.  a number
      of the form N = 2q + 1, where q is also prime).  Also, g should
      be a generator modulo N (see [SRP] for details), which means that
      for any X where 0 < X < N, there exists a value x for which g^x %
      N == X."





















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8. Security Considerations

   These mechanisms rely on the security of SRP, which bases its
   security on the difficulty of solving the Diffie-Hellman problem in
   the multiplicative field modulo a large safe prime.  See section 4
   "Security Considerations" of [13] and section 4 "Security analysis"
   of [12].

   This mechanism also relies on the security of the HMAC algorithm and
   the underlying hash function.  Section 6 "Security" of [9] discusses
   these security issues in detail.  Weaknesses found in MD5 do not
   impact HMAC-MD5 [7].

   U, I, A and o, sent from the client to the server, and N, g, L, s
   and B, sent from the server to the client could be modified by an
   attacker before reaching the other party.  For this reason, these
   values are included in the respective calculations of evidence (M1
   and M2) to prove that each party knows the session key.  This allows
   each party to verify that these values were received unmodified.

   The use of integrity protection is RECOMMENDED to detect message
   tampering and to avoid session hijacking after authentication has
   taken place.

   Replay attacks may be avoided through the use of sequence numbers,
   because sequence numbers make each integrity protected message
   exchanged during a session different, and each session uses a
   different key.























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9. Acknowledgements

   The following people provided valuable feedback in the preparation
   of this document:

      Timothy Martin <tmartin@andrew.cmu.edu>













































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References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 0014, RFC 2119, March 1997.

   [2]   Callas, J., Donnerhacke, L., Finney, H. and R. Thayer,
         "OpenPGP Message Format", RFC 2440, November 1998.

   [3]   Yergeau, F., "UTF-8, a transformation format of Unicode and
         ISO 10646", RFC 2279, January 1998.

   [4]   "International Standard --Information technology-- Universal
         Multiple-Octet Coded Character Set (UCS) -- Part 1
         Architecture and Basic Multilingual Plane", ISO/IEC 10646-1,
         1993.

   [5]   Daemen, Joan and Vincent Rijmen, "AES Proposal: Rijndael",
         September 1999,
         <http://www.esat.kuleuven.ac.be/~rijmen/rijndael/>.

   [6]   National Institute of Standards and Technology, "Rijndael:
         NIST's Selection for the AES", December 2000,
         <http://csrc.nist.gov/encryption/aes/rijndael/Rijndael.pdf>.

   [7]   Dobbertin, H., "The Status of MD5 After a Recent Attack",
         December 1996,
         <ftp://ftp.rsasecurity.com/pub/cryptobytes/crypto2n2.pdf>.

   [8]   Eisler, M., "LIPKEY - A Low Infrastructure Public Key
         Mechanism Using SPKM", RFC 2847, June 2000.

   [9]   Krawczyk, H. et al, "HMAC: Keyed-Hashing for Message
         Authentication", RFC 2104, February 1997.

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

   [11]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June
         1999.

   [12]  Wu, T., "The Secure Remote Password Protocol", March 1998,
         <http://srp.stanford.edu/srp/ndss.html>.

   [13]  Wu, T., "The SRP Authentication and Key Exchange System", RFC
         2945, September 2000.

   [14]  Wu, T., "SRP: The Open Source Password Authentication
         Standard", March 1998,
         <http://srp.stanford.edu/srp/>.


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   [15]  Hopwood, D., "Standard Cryptographic Algorithm Naming", June
         2000,
         <http://www.eskimo.com/~weidai/scan-mirror/>.

   [16]  Menezes, A.J., van Oorschot, P.C. and S.A. Vanstone, "Handbook
         of Applied Cryptography", CRC Press, Inc., ISBN 0-8493-8523-7,
         1997,
         <http://www.cacr.math.uwaterloo.ca/hac/about/chap7.ps>.

   [17]  RSA Data Security, Inc., "PKCS #7: Cryptographic Message
         Syntax Standard", Version 1.5, November 1993,
         <ftp://ftp.rsasecurity.com/pub/pkcs/ascii/pkcs-7.asc>.

   [18]  Balenson, D., "Privacy Enhancement for Internet Electronic
         Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423,
         February 1993,
         <http://www.ietf.org/rfc/rfc1423.txt>.

   [19]  Myers, J.G., "SMTP Service Extension for Authentication", RFC
         2554, March 1999.


Authors' Addresses

   K.R.  Burdis
   Rhodes University
   Computer Science Department
   Grahamstown  6139
   ZA

   EMail: keith@rucus.ru.ac.za
   URI:   http://www.cryptix.org/~keith/


   Raif S. Naffah
   Forge Research Pty. Limited
   Suite 116, Bay 9
   Locomotive Workshop,
   Australian Technology Park
   Cornwallis Street
   Eveleigh, NSW  1430
   AU

   EMail: raif@forge.com.au
   URI:   http://www.cryptix.org/~raif/






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Appendix A. Modulus and Generator values

   Modulus (N) and generator (g) values for various modulus lengths are
   given below.  In each case the modulus is a large safe prime and the
   generator is a primitve root of GF(n) [12].  These values are taken
   from software developed by Tom Wu and Eugene Jhong for the Stanford
   SRP distribution [14].


      [264 bits]
        Modulus (base 16) =
          115B8B692E0E045692CF280B436735C77A5A9E8A9E7ED56C965F87DB5B2A2ECE
          3
        Generator = 2

      [384 bits]
        Modulus (base 16) =
          8025363296FB943FCE54BE717E0E2958A02A9672EF561953B2BAA3BAACC3ED57
          54EB764C7AB7184578C57D5949CCB41B
        Generator = 2

      [512 bits]
        Modulus (base 16) =
          D4C7F8A2B32C11B8FBA9581EC4BA4F1B04215642EF7355E37C0FC0443EF756EA
          2C6B8EEB755A1C723027663CAA265EF785B8FF6A9B35227A52D86633DBDFCA43
        Generator = 2

      [640 bits]
        Modulus (base 16) =
          C94D67EB5B1A2346E8AB422FC6A0EDAEDA8C7F894C9EEEC42F9ED250FD7F0046
          E5AF2CF73D6B2FA26BB08033DA4DE322E144E7A8E9B12A0E4637F6371F34A207
          1C4B3836CBEEAB15034460FAA7ADF483
        Generator = 2

      [768 bits]
        Modulus (base 16) =
          B344C7C4F8C495031BB4E04FF8F84EE95008163940B9558276744D91F7CC9F40
          2653BE7147F00F576B93754BCDDF71B636F2099E6FFF90E79575F3D0DE694AFF
          737D9BE9713CEF8D837ADA6380B1093E94B6A529A8C6C2BE33E0867C60C3262B
        Generator = 2

      [1024 bits]
        Modulus (base 16) =
          EEAF0AB9ADB38DD69C33F80AFA8FC5E86072618775FF3C0B9EA2314C9C256576
          D674DF7496EA81D3383B4813D692C6E0E0D5D8E250B98BE48E495C1D6089DAD1
          5DC7D7B46154D6B6CE8EF4AD69B15D4982559B297BCF1885C529F566660E57EC
          68EDBC3C05726CC02FD4CBF4976EAA9AFD5138FE8376435B9FC61D2FC0EB06E3
        Generator = 2



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      [1280 bits]
        Modulus (base 16) =
          D77946826E811914B39401D56A0A7843A8E7575D738C672A090AB1187D690DC4
          3872FC06A7B6A43F3B95BEAEC7DF04B9D242EBDC481111283216CE816E004B78
          6C5FCE856780D41837D95AD787A50BBE90BD3A9C98AC0F5FC0DE744B1CDE1891
          690894BC1F65E00DE15B4B2AA6D87100C9ECC2527E45EB849DEB14BB2049B163
          EA04187FD27C1BD9C7958CD40CE7067A9C024F9B7C5A0B4F5003686161F0605B
        Generator = 2

      [1536 bits]
        Modulus (base 16) =
          9DEF3CAFB939277AB1F12A8617A47BBBDBA51DF499AC4C80BEEEA9614B19CC4D
          5F4F5F556E27CBDE51C6A94BE4607A291558903BA0D0F84380B655BB9A22E8DC
          DF028A7CEC67F0D08134B1C8B97989149B609E0BE3BAB63D47548381DBC5B1FC
          764E3F4B53DD9DA1158BFD3E2B9C8CF56EDF019539349627DB2FD53D24B7C486
          65772E437D6C7F8CE442734AF7CCB7AE837C264AE3A9BEB87F8A2FE9B8B5292E
          5A021FFF5E91479E8CE7A28C2442C6F315180F93499A234DCF76E3FED135F9BB
        Generator = 2

      [2048 bits]
        Modulus (base 16) =
          AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050
          A37329CBB4A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50
          E8083969EDB767B0CF6095179A163AB3661A05FBD5FAAAE82918A9962F0B93B8
          55F97993EC975EEAA80D740ADBF4FF747359D041D5C33EA71D281E446B14773B
          CA97B43A23FB801676BD207A436C6481F1D2B9078717461A5B9D32E688F87748
          544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB3786160279004E57AE6
          AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8E9DBFBB6
          94B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F9E4AFF73
        Generator = 2





















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Full Copyright Statement

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Acknowledgement

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



















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