SASL Working Group A. Melnikov
Internet-Draft Isode Limited
Intended status: Standards Track July 30, 2009
Expires: January 31, 2010
LDAP schema for storing SCRAM secrets
draft-melnikov-sasl-scram-ldap-02
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Abstract
This memo describes how authPassword LDAP attribute can be used for
storing secrets used by Salted Challenge Response (SCRAM) Simple
Authentication and Security Layer (SASL) Mechanism.
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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
ietf-sasl@imc.org.
Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Security Considerations . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Normative References . . . . . . . . . . . . . . . . . . . . 4
5.2. Informative References . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . 4
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1. Overview
This memo defines a family of schema for authPassword attribute
defined in [AUTHPASS]. Non terminal references in the following ABNF
are defined in either [AUTHPASS] or [RFC5234].
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, 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]:
scram-mech = "SCRAM-SHA-1"
;; Complies with ABNF for <scheme>
scram-authInfo = iter-count ":" salt
;; Complies with ABNF for <authInfo>
scram-authValue = stored-key ":" server-key
;; Complies with ABNF for <authValue>
iter-count = %x31-39 *DIGIT
; a positive number without leading zeros
salt = <<base-64 encoded value>>
stored-key = <<base-64 encoded value>>
server-key = <<base-64 encoded value>>
[[anchor2: Add an example.]]
Note that the authPassword attribute is multivalued. For example, it
may contain multiple SCRAM hashes for different hashing algorithms.
2. Security Considerations
Servers MUST validate format of the authPassword attribute before
using it for performing a SCRAM authentication exchange. It is
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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 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.
3. IANA Considerations
No action is required from IANA.
4. Acknowledgements
The author gratefully acknowledges the feedback provided by Chris
Newman and Kurt Zeilenga.
5. References
5.1. 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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[SCRAM] Menon-Sen, A. and C. Newman, "Salted Challenge Response
(SCRAM) SASL Mechanism", draft-newman-auth-scram-07.txt
(work in progress), July 2008.
5.2. Informative References
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
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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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