Lightweight Directory Access Protocol (LDAP) Schema for Storing Salted Challenge Response Authentication Mechanism (SCRAM) Secrets
The information below is for an old version of the document that is already published as an RFC.
This is an older version of an Internet-Draft that was ultimately published as RFC 5803.
|Last updated||2015-10-14 (Latest revision 2009-11-08)|
|RFC stream||Internet Engineering Task Force (IETF)|
|IESG||IESG state||RFC 5803 (Informational)|
|Responsible AD||Pasi Eronen|
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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 email@example.com 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. 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 May 12, 2010. Copyright Notice Melnikov Expires May 12, 2010 [Page 1] 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 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of 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 Melnikov Expires May 12, 2010 [Page 2] Internet-Draft LDAP schema for storing SCRAM secrets November 2009 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]. Melnikov Expires May 12, 2010 [Page 3] Internet-Draft LDAP schema for storing SCRAM secrets November 2009 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 Melnikov Expires May 12, 2010 [Page 4] Internet-Draft LDAP schema for storing SCRAM secrets November 2009 (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, Melnikov Expires May 12, 2010 [Page 5] Internet-Draft LDAP schema for storing SCRAM secrets November 2009 "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: firstname.lastname@example.org URI: http://www.melnikov.ca/ Melnikov Expires May 12, 2010 [Page 6]