INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Standard Track OpenLDAP Foundation
Expires: 20 July 2001 20 January 2001
LDAP Authentication Password Attribute
<draft-zeilenga-ldap-authpasswd-04.txt>
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
This document is intended to be, after appropriate review and
revision, submitted to the RFC Editor as an Standard Track document.
Distribution of this memo is unlimited. Technical discussion of this
document will take place on the IETF LDAP Extension Working Group
mailing list <ietf-ldapext@netscape.com>. Please send editorial
comments directly to the author <Kurt@OpenLDAP.org>.
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.
Copyright 2001, The Internet Society. All Rights Reserved.
Please see the Copyright section near the end of this document for
more information.
2. Abstract
This document describes schema in support of user/password
authentication in a LDAP directory including the authPassword
attribute type. This attribute type holds values derived from the
user's password(s) (commonly using cryptographic strength one-way
hash). authPassword is intended to used instead of userPassword.
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The key words ``MUST'', ``MUST NOT'', ``REQUIRED'', ``SHALL'', ``SHALL
NOT'', ``SHOULD'', ``SHOULD NOT'', ``RECOMMENDED'', and ``MAY'' in
this document are to be interpreted as described in RFC 2119
[RFC2119].
3. Background and Intended Use
The userPassword attribute type [RFC 2256] is intended be used to used
to support the LDAP [RFC2251] "simple" bind operation. However,
values of userPassword must be clear text passwords. It is often
desirable to store values derived from the user's password(s) instead
of actual passwords.
The authPassword attribute type is intended to be used to store
information used to implement simple password based authentication.
The attribute type may be used by LDAP servers to implement the LDAP
Bind operation's "simple" authentication method.
The attribute type supports multiple storage schemes. A matching rule
is provided for use with extensible search filters to allow clients to
assert that a clear text password "matches" one of the attribute's
values.
Storage schemes often use of cryptographic strength one-way hashing.
Though the use of one-way hashing reduces the potential that exposed
values will allow unauthorized access to the Directory (unless the
hash algorithm/implementation is flawed), the hashing of passwords is
intended to be as an additional layer of protection. It is
RECOMMENDED that hashed values be protected as if they were clear text
passwords.
This attribute may be used in conjunction with server side password
generation mechanisms (such as [PW-EXOP]).
Access to this attribute may governed by administrative controls such
as those which implement password change policies.
4. Schema Definitions
The following schema definitions are described in terms of LDAPv3
Attribute Syntax Definitions [RFC2252] with specific syntax detailed
using Augmented BNF [RFC2234].
Editor's Note: object identifiers (OIDs) will be assigned before this
document is published as an RFC.
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4.1. authPasswordSyntax
( authPasswordSyntaxOID
DESC 'authentication password syntax' )
Values of this syntax are encoded according to:
authPasswordValue = w scheme s [authInfo] s authValue w
scheme = %x30-39 / %x41-5A / %x2D-2F / %x5F
; 0-9, A-Z, "-", ".", "/", or "_"
authInfo = schemeSpecificValue
authValue = schemeSpecificValue
schemeSpecificValue = *( %x21-23 / %25-7E )
; printable ascii less "$" and " "
s = w sep w
w = *sp
sep = %x24 ; dollar sign
sp = %x20 ; space
where scheme describes the mechanism and authInfo and authValue are a
scheme specific. The authInfo field is often a base64 encoded salt.
The authValue field is often a base64 encoded value derived from a
user's password(s). Values of this attribute are case sensitive.
This document describes a number of schemes, as well as requirements
for the scheme naming, in section 5.
4.2. authPasswordMatch
( authPasswordMatchOID
NAME 'authPasswordMatch'
DESC 'authentication password matching rule'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.40{128} )
This matching rule allows a client to assert that a password matches
values of authPasswordSyntax using an extensibleMatch filter
component. Each value is matched per its scheme. The assertion is
TRUE if one or more attribute values matches the asserted value, FALSE
if all values do not matches, and Undefined otherwise.
Servers which support use of this matching rule SHOULD publish
appropriate matchingRuleUse values per [RFC2252], 4.4.
Transfer of authPasswordMatch assertion values is strongly discouraged
where the underlying transport service cannot guarantee
confidentiality and may result in disclosure of the values to
unauthorized parties.
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4.3. supportedAuthPasswordSchemes
( supportedAuthPasswordSchemesOID
NAME 'supportedAuthPasswordSchemes'
DESC 'supported password storage schemes'
EQUALITY caseExactIA5Match
SYNTAX 1.3.6.1.4.1.1466.115.121.1.26{32}
USAGE dSAOperation )
The values of this attribute are names of supported authentication
password schemes which the server supports. The syntax of a scheme
name is described in section 4.1. This attribute may only be present
in the root DSE. If the server does not support any password schemes,
this attribute will not be present.
4.4. authPassword
( authPasswordOID NAME 'authPassword'
SYNTAX authPasswordSyntaxOID )
The values of this attribute are representative of the user's
password(s) and conform to the authPasswordSyntax described in 4.1.
The values of this attribute may be used for authentication purposes.
This attribute type is defined without any built-in matching rules.
The absence of an EQUALITY matching rules disallows modification of
individual values.
Transfer of authPassword values is strongly discouraged where the
underlying transport service cannot guarantee confidentiality and may
result in disclosure of the values to unauthorized parties.
4.5. authPasswordObject
( authPasswordObjectOID NAME 'authPasswordObject'
DESC 'authentication password mix in class'
MAY 'authPassword' AUXILIARY )
Entries of this object class may contain authPassword attribute types.
5. Schemes
This section describes the "MD5" and "SHA1". Other schemes may be
defined by other documents. Schemes which are not described by
standard track documents SHOULD be named with a leading "X-" to
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indicate they are a private or implementation specific scheme, or may
be named using the dotted-decimal representation [RFC2252] of an OID
assigned to the scheme.
5.1. MD5 scheme
The MD5 [RFC1321] scheme name is "MD5".
The authValue is the base64 encoding of an MD5 digest of the
concatenation the user password and salt. The base64 encoding of the
salt is provided in the authInfo field. The salt MUST be at least
64-bits long. Implementations of this scheme MUST support salts up to
128-bit in length.
Example:
Given a user "joe" who's password is "mary" and a salt of "salt",
the authInfo field would be the base64 encoding of "salt" and the
authValue field would be the base64 encoding of the MD5 digest of
"marysalt".
A match against an asserted password and an attribute value of this
scheme SHALL be true if and only if the MD5 digest of concatenation of
the asserted value and the salt is equal to the MD5 digest contained
in AuthValue. The match SHALL be undefined if the server is unable to
complete the equality test for any reason. Otherwise the match SHALL
be false.
Values of this scheme SHOULD only be used to implement simple
user/password authentication.
5.2. SHA1 scheme
The SHA1 [SHA1] scheme name is "SHA1".
The authValue is the base64 encoding of an SHA1 digest of the
concatenation the user password and the salt. The base64 encoding of
the salt is provided in the authInfo field. The salt MUST be at least
64-bits long. Implementations of this scheme MUST support salts up to
128-bit in length.
Example:
Given a user "joe" who's password is "mary" and a salt of "salt",
the authInfo field would be the base64 encoding of "salt" and the
authValue field would be the base64 encoding of the SHA1 digest of
"marysalt".
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A match against an asserted password and an attribute value of this
scheme SHALL be true if and only if the SHA1 digest of concatenation
of the asserted value and the salt is equal to the SHA1 digest
contained in AuthValue. The match SHALL be undefined if the server is
unable to complete the equality test for any reason. Otherwise the
match SHALL be false.
Values of this scheme SHOULD only be used to implement simple
user/password authentication.
6. Implementation Issues
For implementations of this specification:
Servers MAY restrict which schemes are used in conjunction with a
particular authentication process but SHOULD use all values of
selected schemes. If the asserted password matches any of the
stored values, the asserted password SHOULD be considered valid.
Servers MAY use other authentication storage mechanisms, such as
userPassword or an external password store, in conjunction with
authPassword to support the authentication process.
Servers that support simple bind MUST support the SHA1 scheme and
SHOULD support the MD5 scheme.
Servers SHOULD not publish values of authPassword nor allow
operations which expose authPassword or AuthPasswordMatch values to
unless confidentiality protection is in place.
Clients SHOULD not initiate operations which provide or request
values of authPassword or make authPasswordMatch assertions unless
confidentiality protection is in place.
Clients SHOULD not assume that a successful AuthPasswordMatch,
whether by compare or search, is sufficient to gain directory
access. The bind operation MUST be used to authentication to the
directory.
7. Security Considerations
This document describes how authentication information may be stored
in a directory. Authentication information MUST be adequately
protected as unintended disclosure will allow attackers to gain
immediate access to the directory as described by [RFC2829].
As flaws may be discovered in the hashing algorithm or with a
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particular implementation of the algorithm or may be subjected to
dictionary or other attacks if exposed, values of AuthPassword SHOULD
be protected as if they were clear text passwords. When values are
transferred, privacy protections, such as IPSEC or TLS, SHOULD be in
place.
Clients SHOULD use strong authentication mechanisms [RFC2829].
AuthPasswordMatch matching rule allows applications to test the
validity of a user password and, hence, may be used to mount an
attack. Servers SHOULD take appropriate measures to protect the
directory from such attacks.
Some password schemes may require CPU intensive operations. Servers
SHOULD take appropriate measures to protect against Denial of Service
attacks.
AuthPassword does not restrict an authentication identity to a single
password. An attacker who gains write access to this attribute may
store additional values without disabling the user's true password(s).
Use of policy aware clients and servers is RECOMMENDED.
The level of protection offered against various attacks differ from
scheme to scheme. It is RECOMMENDED that servers support scheme
selection as a configuration item. This allows for a scheme to be
easily disabled if a significant security flaw is discovered.
8. Copyright
Copyright 2001, The Internet Society. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be followed,
or as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
"AS IS" basis and THE AUTHORS, THE INTERNET SOCIETY, AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
9. Acknowledgment
This document borrows from a number of IETF documents and is based
upon input from the IETF LDAPext working group.
10. Bibliography
[RFC1321] R. Rivest, "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992
[RFC2219] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2222] J. Myers, "Simple Authentication and Security Layer (SASL)",
RFC 2222, October 1997.
[RFC2234] D. Crocker (editor), P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC2251] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC 2251, December 1997.
[RFC2252] M. Wahl, A. Coulbeck, T. Howes, S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax
Definitions", RFC 2252, December 1997.
[RFC2256] M. Wahl, "A Summary of the X.500(96) User Schema for use
with LDAPv3", RFC 2256, December 1997.
[RFC2307] L. Howard, "An Approach for Using LDAP as a Network
Information Service", RFC 2307, March 1998. (not normative)
[RFC2829] M. Wahl, H. Alvestrand, J. Hodges, RL "Bob" Morgan,
"Authentication Methods for LDAP", RFC 2829, June 2000.
[PW-EXOP] K. Zeilenga, "LDAP Password Modify Extended Operation"
draft-zeilenga-ldap-passwd-exop-xx.txt, a work in progress.
[SHA1] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
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11. Author's Address
Kurt D. Zeilenga
OpenLDAP Foundation
<Kurt@OpenLDAP.org>
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