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Network Working Group                                        D. Cridland
Internet-Draft                                               A. Melnikov
Expires: August 30, 2007                                   Isode Limited
                                                       February 26, 2007


            The Hash Exchange Authentication SASL Mechanism
                        draft-cridland-sasl-hexa-00

Status of this Memo

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   This Internet-Draft will expire on August 30, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo defines and discusses a SASL mechanism that is based on the
   exchange of hashes.  It does not require the storage of a plaintext
   equivalent on the server, is simple to implement, and provides a
   reasonable level of security.







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Table of Contents

   1.  Conventions used in this document  . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . .  3
       2.1.1.  Deployability  . . . . . . . . . . . . . . . . . . . .  3
       2.1.2.  Hash Agility . . . . . . . . . . . . . . . . . . . . .  3
       2.1.3.  Ease of Implementation . . . . . . . . . . . . . . . .  4
   3.  Notations and Definitions  . . . . . . . . . . . . . . . . . .  4
     3.1.  HMAC and Hash functions  . . . . . . . . . . . . . . . . .  4
       3.1.1.  Notation . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Wire Message Format  . . . . . . . . . . . . . . . . . . .  5
     3.3.  Prior Setup  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.4.  Authentication Process . . . . . . . . . . . . . . . . . .  5
       3.4.1.  Initial client message . . . . . . . . . . . . . . . .  5
       3.4.2.  Server challenge message . . . . . . . . . . . . . . .  6
       3.4.3.  Client response message  . . . . . . . . . . . . . . .  6
       3.4.4.  Server Authentication and Mutual Auth  . . . . . . . .  6
   4.  Mandatory to Implement . . . . . . . . . . . . . . . . . . . .  7
   5.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
     6.1.  Plaintext Equivalents  . . . . . . . . . . . . . . . . . .  8
     6.2.  Hash algorithm usage . . . . . . . . . . . . . . . . . . .  8
     6.3.  Resistance to attacks  . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 12






















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1.  Conventions used in this document

   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 [KEYWORDS].


2.  Introduction

   Although other hash-based [SASL] mechanisms exist, they are being
   rapidly outdated by advances in computing speed and the discovery of
   weaknesses in hash functions.  Moreover, both [CRAM-MD5] and
   [DIGEST-MD5] involve the server having plaintext equivalents in the
   shared secret store.

   This mechanism was borne out of a percieved need within the system
   administration community to have a mechanism which was both easier to
   implement and also safer than [DIGEST-MD5].

2.1.  Rationale

   HEXA is specifically aimed at providing three key features.

2.1.1.  Deployability

   HEXA is designed to be acceptable to deploy in the real world.  Its
   authentication database is designed such that it may be used directly
   for local logins, effectively having the same properties as a typical
   /etc/shadow file on UNIX systems.  This allows HEXA to co-exist with
   very well deployed mechanisms such as [PLAIN], freeing the need for
   transitioning.

2.1.2.  Hash Agility

   Hash algorithms have an alarming tendancy to age.  It is therefore
   beneficial to allow a server administrator to switch hash algorithms.
   This is only practical, however, when it is known that the new hash
   algorithm can be well supported by the clients in use.  Where the
   clients are out of the control of the administrator, for example in
   typical commercial settings, it is useful for the administrator to be
   aware of what the current deployed base is able to use.

   Therefore, in HEXA, clients advertise their capability to the server,
   allowing a server administrator to upgrade the hash algorithm as
   required.






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2.1.3.  Ease of Implementation

   In general, [DIGEST-MD5] has been found to be difficult to implement
   interoperably.  Even well known implementations have been found to
   have interoperability problems under some circumstances.  HEXA
   attempts to tackle this by using a small number of operation types,
   and simple parsing.  This allows for simple scripting languages to
   implement, as well as using hash algorithms and related functions
   that are known to be well deployed.

   Since no mechanism can be considered secure in practise if no
   implementations exist, this specification chooses easily available
   pre-existing source code above stronger, less well implemented
   algorithms.


3.  Notations and Definitions

3.1.  HMAC and Hash functions

   This specification requires the use of [HMAC], based on hash
   functions such as [MD5], SHA1, or SHA-256.

   Messages are shown in plain text, with the CR LF pair shown as a line
   ending.

   Mandatory to implement hashes are discussed in Section 4, and are
   considered volatile parts of this specification, very likely to
   change in future revisions of this specification.

3.1.1.  Notation

   We use a relatively simple notation to show the calculations
   involved:
   HASH(T)
      An agreed hash algorithm, used to produce a cryptographically
      secure hash of the input data T.
   HMAC(K,T)
      An [HMAC] function used with an agreed hash function, used to
      produce a MAC for T with a key of K.
   HMAC[n](K,T)
      Where n is an integer, expands to HMAC(HMAC[n-1](K,T),T).
      HMAC[1](K,T) is equivalent to HMAC(K,T).
   Q + W
      Where Q and W are strings, represents simple concatenation.






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   Q ^ W
      Where Q and W are strings, represents an octet by octet XOR.
   SASLprep(X)
      Where X is a string, represents the application of the [SASLPREP]
      algorithm to the string.

3.2.  Wire Message Format

   HEXA uses a wire format based on a simplified variant of email
   message header formats, and only transfers text.  No folding or
   encoding is required or allowed.

   The message contains keys and values, where each key appears a
   maximum of once.  Keys consist of ASCII letters, numbers, and the
   hyphen character.  Values contain UTF-8, and begin with a non-space
   character.  They never contain NUL, CR, or LF.  Each Key Value pair
   ends with a CR LF pair.

   Keys appear first, followed by a single colon (":"), followed by the
   value.  Any surrounding spaces are considered part of the value.

3.3.  Prior Setup

   The client is assumed to have an Authcid, an optional Authzid, and a
   Password.

   The server has a Realm, and a database keyed against Authcid
   containing a Salt, and hash output known as the Verifier.

       Verifier := HMAC[n](Intermediate, Salt)
       Intermediate := HMAC[n](Realm + SASLprep(Authcid)
           + SASLprep(Password), Salt)

   The server's database is initially populated before authentication by
   temporarily calculating the Intermediate from the supplied Password,
   and choosing a new Salt.  A new Salt SHOULD be used whenever the
   Password is changed.  The hash algorithm used is also remembered by
   the server - servers MAY use multiple hash algorithms.

   After calculating and storing the Verifier, the Intermediate MUST be
   discarded.

3.4.  Authentication Process

3.4.1.  Initial client message

   Initially, the client sends a message containing Authcid, optionally
   Authzid, a list of hash algorithm names it supports, and some random



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   data to use as a client nonce, this message is ClientMessage:

       Authcid:mel
       Hashes:MD5 SHA1 SHA-256
       Client-Nonce:laksjdoijcosijdv
       Channel-Bindings:TLS

3.4.2.  Server challenge message

   The server looks up Authcid in its database, selects the strongest
   hash algorithm mutually supported, and returns the hash algorithm,
   the number of cycles it uses, and the value of Salt.  It also creates
   some random data for use as a server nonce.  Because this MUST be
   textual, servers MAY base64 encode this data, however, this is an
   implementation detail.  The server sends the server nonce, Salt, and
   Realm to the client, along with an indication of which channel
   binding the server will use, if any:

       Realm:example.net
       Salt:aajvskjhvslkjdnvcn
       Hash:MD5
       Hash-Cycles:5
       Server-Nonce:ksjdnclksdhufdh
       Channel-Binding:TLS

3.4.3.  Client response message

   The client stores this message precisely as received, as
   ServerMessage.

   The client now calculates Intermediate and Verifier as above, and in
   addition a hash Key, and a value Exchange, such that:

       Key := HMAC[n](Verifier, ClientMessage
           + ServerMessage + ChannelBinding)
       Exchange := Key ^ Intermediate

   If there is no channel binding available that the server supports,
   then ChannelBinding will be an empty string.  Exchange is represented
   as hex, and the result is sent to the server:

       Hash-Exchange:1f2d...

3.4.4.  Server Authentication and Mutual Auth

   The server can now construct Key, extract Intermediate from Exchange,
   and verify Intermediate against the stored hash output Verifier.  In
   order to prove to the client that it can do so, it sends the client a



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   final message containing a hash output Authentication, such that:

       Intermediate := Exchange ^ Key
       Authentication := HMAC[n](Intermediate, ChannelBinding
           + ServerMessage + Salt + ClientMessage)

   This has is sent to the client as:

       Server-Auth:3f4d5a...


4.  Mandatory to Implement

   The rationale behind this mechanism is ease of deployment and
   implementation, thus implementations MUST provide a configuration
   which supports the [MD5] hash algorithm using a minimal number of
   cycles of 16.  Implementations SHOULD also support SHA-256.

   This is because both [MD5], and [HMAC] implementations which are
   hardcoded to use [MD5], are easily available in many languages and
   environments.


5.  Formal Syntax

   Insert boilerplate about [ABNF] here.

   wire-message = *key-value
   key-value = key ":" value CRLF
   key = ALPHA *( ALPHA / "-" / DIGIT )
   value = utf8-text
   utf8-text = 1*( VCHAR / SP / UTF-2 / UTF8-3 / UTF8-4 )
   ; visible UTF-8 or space.
   hash-output = 1*( HEXDIG )
   ; Output of hash algorithm, generally 32 or more has digits.

   ; Following productions all conform to wire-message:

   client-init-message = authcid [ authzid ] hashes client-nonce
       [ channel-bindings ] *( extension )
   server-challenge-message = realm hash hash-cycles server-nonce
       [ channel-binding ] *( extension )
   client-response-message = hash-exchange *( extension )
   server-auth-message = server-auth *( extension )

   ; Following productions all conform to key-value:

   ; client-init-message:



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   authcid = "Authcid" ":" utf8text CRLF
   authzid = "Authzid" ":" utf8text CRLF
   ; SASLPrepped authcid/authzid.
   hashes = "Hashes" ":" hash-name *( SP hash-name ) CRLF
   client-nonce = "Client-Nonce" ":" utf8text CRLF
   ; MUST be generated afresh with reasonable entropy.
   channel-bindings = "Channel-Bindings" ":" channel-binding-name
       *( SP channel-binding-name ) CRLF

   ; server-challenge-message:
   realm = "Realm" ":" utf8text CRLF
   hash = "Hash" ":" hash-name CRLF
   hash-cycles = "Cycles" ":" 1*( DIGIT ) CRLF
   server-nonce = "Server-Nonce" ":" utf8text CRLF
   ; MUST be generated afresh with reasonable entropy.
   channel-binding = "Channel-Binding:" ":" channel-binding-name CRLF

   ; client-response-message:
   hash-exchange = "Hash-Exchange" ":" hash-output CRLF

   ; server-auth-message:
   server-auth = "Server-Auth" ":" hash-output CRLF

   ; Values:
   channel-binding-name = ALPHA *( ALPHA / DIGIT / "-" )
   hash-name = ALPHA *( ALPHA / DIGIT / "-" )

   ; Extensions:
   extension = key-value


6.  Security Considerations

6.1.  Plaintext Equivalents

   The intermediate hash B is a plaintext equivalent.  Clients SHOULD
   NOT store this, and MUST NOT store the original plaintext password.
   Servers MUST NOT store B.

6.2.  Hash algorithm usage

   In general, it is thought that the recursive application of hash
   functions increases the strength of a hash.  In particular, if the
   hash has no weaknesses at all, merely doubling the number of
   iterations will cause an offline dictionary attack to take twice as
   much CPU resource.  Making this a variable, negotiated, factor allows
   very simple increases in security, as long as the hash algorithm
   itself is not compromised sufficiently that a non-brute-force attack



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   becomes practical.

   The exact hash algorithm used may be changed by live deployments.
   HEXA provides a simple method for server administrators to discover
   actual availability of new hash algorithms in clients, simplifying a
   hash algorithm change.

6.3.  Resistance to attacks

   HEXA is thought to be resistent to slightly more attacks than
   [DIGEST-MD5]:
   Downgrade
      Assuming that HEXA can be negotiated at all, a downgrade attack
      inside HEXA cannot be mounted, as complete messages are used as
      input to the hashing functions - a man-in-the-middle attack will
      cause the authentication to fail.
   Server based attack
      Merely obtaining the authentication database will not directly
      allow an attcker to authenticate masquerading as a legitimate user
      without substantial offline dictionary attacks.  However, if an
      attacker can both obtain the authentication database and observe
      traffic on the wire, then the attacker can obtain B. As with
      [DIGEST-MD5], this will not yield the password without an
      expensive offline-dictionary attack.
   Client based attack
      Clients typically store sufficient data to reauthenticate later
      without interactively requesting passwords from the user.  Like
      [DIGEST-MD5], clients need not store the actual password, but can
      merely store B for this purpose.  This practise is not
      recommended, as an attacker obtaining B can authenticate as the
      user.


7.  References

7.1.  Normative References

   [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

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

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




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

   [SASL]     Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [SASLPREP]
              Zeilenga, K., "SASLprep: Stringprep Profile for User Names
              and Passwords", RFC 4013, February 2005.

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

7.2.  Informative References

   [CRAM-MD5]
              Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
              AUTHorize Extension for Simple Challenge/Response",
              RFC 2195, September 1997.

   [DIGEST-MD5]
              Leach, P. and C. Newman, "Using Digest Authentication as a
              SASL Mechanism", RFC 2831, May 2000.

   [PLAIN]    Zeilenga, K., "The PLAIN Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4616, August 2006.


Authors' Addresses

   Dave Cridland
   Isode Limited
   5 Castle Business Village
   36, Station Road
   Hampton, Middlesex  TW12 2BX
   GB

   Email: dave.cridland@isode.com













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   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36, Station Road
   Hampton, Middlesex  TW12 2BX
   GB

   Email: alexey.melnikov@isode.com











































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