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Versions: 00 01 02 03 04 RFC 5803
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
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
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Internet-Draft LDAP schema for storing SCRAM secrets November 2009
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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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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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 [RFC2119].
2. 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.
3. 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.
4. IANA Considerations
No action is required from IANA.
5. Acknowledgements
The author gratefully acknowledges the feedback provided by Chris
Newman, Kurt Zeilenga, Chris Lonvick, Peter Saint-Andre, Barry Leiba
and Chris Ridd.
6. 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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