    Network Working Group                                        K. Zeilenga
    Request for Comments: 3112                           OpenLDAP Foundation
    Category: Informational                                         May 2001

                      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 (C) 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

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    RFC 3112          LDAP Authentication Password Schema           May 2001

       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.

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    RFC 3112          LDAP Authentication Password Schema           May 2001

       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.

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    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.

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    RFC 3112          LDAP Authentication Password Schema           May 2001

    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.

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    RFC 3112          LDAP Authentication Password Schema           May 2001

       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].

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    RFC 3112          LDAP Authentication Password Schema           May 2001

       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.

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    RFC 3112          LDAP Authentication Password Schema           May 2001

       [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

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    RFC 3112          LDAP Authentication Password Schema           May 2001

    9.  Full Copyright Statement

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

       This document and translations of it may be copied and furnished to
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       followed, or as required to translate it into languages other than
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       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
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    Acknowledgement

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

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