SASL Working Group                                           A. Melnikov
Internet-Draft                                             Isode Limited
Intended status: Informational                          November 8, 2009
Expires: May 12, 2010


                 LDAP schema for storing SCRAM secrets
                   draft-melnikov-sasl-scram-ldap-04

Abstract

   This memo describes how the "authPassword" LDAP attribute can be used
   for storing secrets used by the Salted Challenge Response (SCRAM)
   mechanism in the Simple Authentication and Security Layer (SASL)
   framework.

Note

   A revised version of this draft document will be submitted to the RFC
   editor as a Proposed Standard for the Internet Community.  Discussion
   and suggestions for improvement are requested, and should be sent to
   sasl@ietf.org mailing list.

Status of this Memo

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Copyright Notice




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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.


Table of Contents

   1.  Conventions Used in This Document . . . . . . . . . . . . . . . 3

   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4

   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5

   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5

   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5

       Author's Address  . . . . . . . . . . . . . . . . . . . . . . . 6






















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.  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 [RFC2119].


.  Overview

   This document describes how the authPassword LDAP attribute
   [AUTHPASS] can be used for storing secrets used by [SCRAM] Simple
   Authentication and Security Layer [RFC4422] Mechanisms.

      The "scheme" part of the authPassword attribute is the SCRAM
      mechanism name (always without the "-PLUS" suffix), e.g.  "SCRAM-
      SHA-1".  See [SCRAM] for the exact syntax of SCRAM mechanism
      names.

      The "authInfo" part of the authPassword attribute is the iteration
      count (iter-count in the ABNF below), followed by ":" and base-64
      [BASE64] encoded salt.

      The "authValue" part of the authPassword attribute is the base-64
      [BASE64] encoded StoredKey [SCRAM], followed by ":" and base-64
      [BASE64] encoded ServerKey [SCRAM].

   Syntax of the attribute can be expressed using ABNF [RFC5234].  Non
   terminal references in the following ABNF are defined in either
   [AUTHPASS], [RFC4422] or [RFC5234].






















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       scram-mech     = "SCRAM-SHA-1" / scram-mech-ext
                      ; Complies with ABNF for <scheme>
                      ; defined in [AUTHPASS].

       scram-authInfo = iter-count ":" salt
                      ; Complies with ABNF for <authInfo>
                      ; defined in [AUTHPASS].

       scram-authValue = stored-key ":" server-key
                      ; Complies with ABNF for <authValue>
                      ; defined in [AUTHPASS].

       iter-count   = %x31-39 *DIGIT
                      ; SCRAM iteration count.
                      ; A positive number without leading zeros

       salt         = <base-64 encoded value>

       stored-key   = <base-64 encoded value>
                      ; See definition in [SCRAM]

       server-key   = <base-64 encoded value>
                      ; See definition in [SCRAM]

       scram-mech-ext = "SCRAM-" 1*9mech-char
                      ; Other SCRAM mechanisms registered
                      ; in the IANA registry for SASL
                      ; mechanism names.

       mech-char    = <Defined in RFC 4422>

   [[anchor2: Add an example.]]

   Note that the authPassword attribute is multivalued.  For example, it
   may contain multiple SCRAM hashes for different hashing algorithms.


.  Security Considerations

   This document defines how authPassword attribute can be used to store
   SCRAM secrets.  So security considerations relevant to [SCRAM] and
   hash function used with it are also relevant to this document.

   General security considerations related to authPassword attribute (as
   specified in [AUTHPASS]) also apply to use of authPassword as
   specified in this document.  In particular values of authPassword
   SHOULD be protected as if they were clear text passwords.  A read
   operation on this attribute which is not protected by a privacy layer



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   (such as IPSec or TLS) can expose this attribute to an attacker who
   a). would be able to use the intercepted value to impersonate the
   user to all servers providing SCRAM access using the same hash
   function, password, iteration count and salt, or b) would be able to
   perform an offline dictionary or brute-force attack in order to
   recover the user's password.

   Servers MUST validate the format of the authPassword attribute before
   using it for performing a SCRAM authentication exchange.  It is
   possible that an attacker compromised the LDAP server or got access
   to the entry containing the attribute in order to exploit a
   vulnerability in the subsystem performing the SCRAM authentication
   exchange.  Big iteration counts and invalid base-64 encoding are two
   possible (but not the only) exploits in the format specified in the
   document.


.  IANA Considerations

   No action is required from IANA.


.  Acknowledgements

   The author gratefully acknowledges the feedback provided by Chris
   Newman, Kurt Zeilenga, Chris Lonvick, Peter Saint-Andre, Barry Leiba
   and Chris Ridd.


.  Normative References

   [AUTHPASS]
              Zeilenga, K., "LDAP Authentication Password Schema",
              RFC 3112, May 2001.

   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4422]  Melnikov, A. and K. Zeilenga, "Simple Authentication and
              Security Layer (SASL)", RFC 4422, June 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [SCRAM]    Menon-Sen, A., Newman, C., Melnikov, A., and N. Williams,



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              "Salted Challenge Response (SCRAM) SASL Mechanism",
              draft-ietf-sasl-scram-07.txt (work in progress),
              September 2009.


Author's Address

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

   Email: alexey.melnikov@isode.com
   URI:   http://www.melnikov.ca/



































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