LDAP Authentication Password Schema

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2001). All Rights Reserved.

Abstract

This document describes schema in support of user/password authentication in a LDAP (Lightweight Directory Access Protocol) 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.

1. Background and Intended Use

The userPassword attribute type [RFC2256] is intended to be 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 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 the LDAP Password Modify [RFC3062] extended operation).

Access to this attribute may governed by administrative controls such as those which implement password change policies.

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].

2. Schema Definitions

The following schema definitions are described in terms of LDAPv3 Attribute Syntax Definitions [RFC2252] with specific syntax detailed using Augmented BNF [RFC2234].

2.1. authPasswordSyntax

      ( 1.3.6.1.4.1.4203.1.1.2
        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 / %x25-7E )
            ; printable ASCII less "$" and " "
      s = w SEP w
      w = *SP
      SEP = %x24 ; "$"
      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.

Transfer of values of this syntax is strongly discouraged where the underlying transport service cannot guarantee confidentiality and may result in disclosure of the values to unauthorized parties.

This document describes a number of schemes, as well as requirements for the scheme naming, in section 3.

2.2. authPasswordExactMatch

      ( 1.3.6.1.4.1.4203.1.2.2
        NAME 'authPasswordExactMatch'
        DESC 'authentication password exact matching rule'
        SYNTAX 1.3.6.1.4.1.4203.1.1.2 )

This matching rule allows a client to assert that an asserted authPasswordSyntax value matches authPasswordSyntax values. It is meant to be used as the EQUALITY matching rule of attributes whose SYNTAX is authPasswordSyntax.

The assertion is "TRUE" if there is an attribute value which has the same scheme, authInfo, and authValue components as the asserted value; "FALSE" if no attribute value has the same components as the asserted value; and "Undefined" otherwise.

2.3. authPasswordMatch

       ( 1.3.6.1.4.1.4203.1.2.3
         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.

2.4. supportedAuthPasswordSchemes

      ( 1.3.6.1.4.1.4203.1.3.3
      
        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 2.1. 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.

2.5. authPassword

      ( 1.3.6.1.4.1.4203.1.3.4 NAME 'authPassword'
        DESC 'password authentication information'
        EQUALITY 1.3.6.1.4.1.4203.1.2.2
        SYNTAX 1.3.6.1.4.1.4203.1.1.2 )

The values of this attribute are representative of the user's password(s) and conform to the authPasswordSyntax described in 2.1. The values of this attribute may be used for authentication purposes.

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.

2.6. authPasswordObject

      ( 1.3.6.1.4.1.4203.1.4.7 NAME 'authPasswordObject'
        DESC 'authentication password mix in class'
        MAY 'authPassword'
        AUXILIARY )

Entries of this object class may contain authPassword attribute types.

3. Schemes

This section describes the "MD5" and "SHA1" schemes. Other schemes may be defined by other documents. Schemes which are not described in an RFC SHOULD be named with a leading "X-" to 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.

3.1. 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 bits 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.

3.2. SHA1 scheme

The SHA1 [SHA1] scheme name is "SHA1".

The authValue is the base64 encoding of a 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 bits 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".

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.

4. Implementation Issues

For all 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 values or AuthPasswordMatch assertions 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 authenticate to the directory.

5. 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 particular implementation of the algorithm or values could be subject to various 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.

6. Acknowledgment

This document borrows from a number of IETF documents and is based upon input from the IETF LDAPext working group.

7. Bibliography

   [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992
   
   [RFC2219] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2234] Crocker, D., Editor, P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", RFC 2234, November 1997.
   
   [RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
             Access Protocol (v3)", RFC 2251, December 1997.
   
   [RFC2252] Wahl, M., Coulbeck, A., Howes, T., and S. Kille,
             "Lightweight Directory Access Protocol (v3): Attribute
             Syntax Definitions", RFC 2252, December 1997.
   
   [RFC2256] Wahl, A., "A Summary of the X.500(96) User Schema for use
             with LDAPv3", RFC 2256, December 1997.
   
   [RFC2307] Howard, L., "An Approach for Using LDAP as a Network
             Information Service", RFC 2307, March 1998.
   
   [RFC2829] Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
             "Authentication Methods for LDAP", RFC 2829, June 2000.
   
   [RFC3062] Zeilenga, K., "LDAP Password Modify Extended Operation",
             RFC 3062, February 2001.
   
   [SHA1]    NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.

8. Author's Address

Kurt D. Zeilenga
OpenLDAP Foundation

EMail:

          Kurt@OpenLDAP.org

9. Full Copyright Statement

Copyright © The Internet Society (2001). 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.

This document and the information contained herein is provided on an "AS IS" basis and 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.

Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society.