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Network Working Group                                        M. Stroeder
Internet-Draft                                    Independent consultant
Updates: 2307 (if approved)                               March 13, 2013
Intended status: Informational
Expires: September 14, 2013

             Lightweight Directory Access Protocol (LDAP):
               Hashed Attribute values for 'userPassword'


   This document describes the widely used syntax for storing hashed
   passwords in LDAP attribute 'userPassword'.  Furthermore it points
   out some of the deficiencies of the approach.  Its purpose is solely
   to document current practice, it does not define a new standard.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 14, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Syntax for attribute 'userPassword' . . . . . . . . . . . . .   3
   3.  Implementation Issues . . . . . . . . . . . . . . . . . . . .   4
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   This document does not define a new standard.  Its purpose is solely
   to correctly document what is widely implemented in LDAP servers and
   clients to avoid interop issues in case implementors have to support
   old legacy systems using this scheme.

   Strictly speaking the 'userPassword' attribute type, intended to be
   used to support the LDAP [RFC4510] "simple" bind operation, was meant
   to only store clear text passwords [RFC4519].

   Although [RFC3112] defined a more versatile password attribute
   'authPassword' for storing hashed passwords this was not widely
   implemented in server and client implementations.  Instead current
   LDAP deployments still rely on the password hashing scheme for
   attribute 'userPassword' introduced in [RFC2307] especially since
   this attribute type is directly used in various object classes.

   The specification in [RFC2307] is missing some formal aspects
   potentially leading to interop issues.  Furthermore new hash
   algorithms are used today by various implementors which were not
   mentioned in [RFC2307].

   Therefore this document also updates [RFC2307] by fully specifying
   how to store hashed password values in attribute 'userPassword'

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   optionally using the SHA-2 hash algorithms [FIPS-180-4].  For this it
   focuses on documenting already implemented server and client
   implementations.  The password hashing scheme {crypt} was left out
   from the syntax definition because there are many platform-specific
   variants of possible values.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document is being discussed on the ldapext@ietf.org mailing

2.  Syntax for attribute 'userPassword'

   userPassword values MUST be represented by following syntax:

       userpasswordvalue  = cleartext-password / prefix b64-hashandsalt

       prefix       = "{" scheme "}"
       scheme = %x30-39 / %x41-5A / %x61-7a / %x2D-2F / %x5F
            ;0-9, A-Z, a-z, "-", ".", "/", or "_"

       b64-hashandsalt = <base64 of hashandsalt>

       hashandsalt = password-hash salt

       password-hash = <digest of cleartext-password salt>

       cleartext-password = %x00-FF

       salt = %x00-FF

   Field salt SHALL be treated as a randomely chosen sequence of bytes.
   The length of the salt SHOULD be at least 64 bits but other salt
   length MAY be recommended for specific hash algorithms.
   Implementations MUST be prepared of handling field salt of arbitrary
   length.  salt MUST be empty for non-salted hashing schemes.

   The field hashandsalt is generated by concatening the field password-
   hash and the salt.  The field password-hash is generated by
   calculating the digest over the concatenation of password followed by

   b64-hashandsalt is generated by encoding hashandsalt according to the
   base64 algorithm as specified in [RFC4648].

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   The field scheme MUST be treated case-insensitive by all server and
   client implementations.  Applications SHALL handle prefix "x-" in
   scheme just like any other scheme value without this prefix
   [RFC6648].  Other specifications MAY update this document by defining
   other values for scheme.

        |   prefix  |     Description      | Algorithm reference |
        |   {MD5}   |  MD-5 without salt   |      [RFC1321]      |
        |   {SMD5}  |     salted MD-5      |      [RFC1321]      |
        |   {SHA}   |  SHA-1 without salt  |     [FIPS-180-4]    |
        |   {SSHA}  |     salted SHA-1     |     [FIPS-180-4]    |
        |  {SHA256} | SHA-256 without salt |     [FIPS-180-4]    |
        | {SSHA256} |    salted SHA-256    |     [FIPS-180-4]    |
        |  {SHA384} | SHA-384 without salt |     [FIPS-180-4]    |
        | {SSHA384} |    salted SHA-384    |     [FIPS-180-4]    |
        |  {SHA512} | SHA-512 without salt |     [FIPS-180-4]    |
        | {SSHA512} |    salted SHA-512    |     [FIPS-180-4]    |

                   Table 1: Currently used hash schemes

3.  Implementation Issues

   All implementations SHOULD prepare textual strings used as field
   password like described for clear-text storage in section 2.41 in
   [RFC4519] before deriving a hash value from them.  It is up to the
   implementation to determine what a textual password is.

   Hashed 'userPassword' values are only suitable for directly comparing
   it to a clear-text password.  They SHOULD NOT be used in challenge-
   response authentication schemes.

   It is clear from the syntax specification that distinguishing clear-
   text passwords and hashed passwords is somewhat ambigous which is a
   well-known deficiency of this approach.  Therefore implementing
   [RFC3112] is RECOMMENDED if a better solution is strictly required.

   Implementations SHALL first check whether a value stored in attribute
   'userPassword' adheres to the syntax specified above.  Syntax
   checking SHALL be implemented by checking hash and optional salt
   following the algorithm-specific rules.  Any value which do not
   adhere to this syntax MAY be treated as clear-text password by the
   DSA when processing a LDAP simple bind request or LDAP compare

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   Servers MAY provide local configuration items to limit the set of
   hash schemes to be processed and for completely disabling use of
   clear-text passwords in attribute 'userPassword'.

4.  Acknowledgements

   The syntax definition for 'userPassword' values in this document is
   based on and supersedes the specification in section 5.3 of [RFC2307]
   by L.  Howard.

   Some basics of the formal specification and security considerations
   are based on text in [RFC3112] by K.  Zeilenga.

5.  IANA Considerations

   There are no identifiers defined herein to be reserved by IANA.

6.  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].

   Hashed values in attribute 'userPassword' SHOULD be protected as if
   they were clear text passwords because they are subject to dictionary
   or other attacks and flaws may be discovered in the hashing algorithm
   or with a particular implementation of the algorithm.

   Especially it is RECOMMENDED to avoid using hashing schemes based on
   MD-5 because of known weaknesses of this digest algorithm [RFC6151].

   Applications SHOULD NOT use the non-salted password hash schemes and
   SHOULD use a sufficiently long salt value.

   When values are transferred, privacy protections, such as IPSEC or
   TLS, SHOULD be in place.

   Clients SHOULD use stronger authentication mechanisms like defined in

   Servers SHOULD use stronger credential storage mechanisms like
   defined in [RFC5803].

   Some password schemes may require CPU intensive operations.  Servers
   SHOULD take appropriate measures to protect against Denial of Service

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   Using 'userPassword' attribute type description as defined in
   [RFC4519] 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).  If a server supports defining additional local
   constraints to limit the count of attribute values it is RECOMMENDED
   to define such a constraint for 'userPassword' values to be limited
   to one without having to change the standard schema.

   Use of policy aware clients and servers is RECOMMENDED (see example
   in [I-D.behera-ldap-password-policy]).

   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.

7.  References

7.1.  Normative References

              National Institute of Standards and Technology (NIST) ,
              "Secure Hash Standard (SHS)", FIPS PUB 180-4, March 2012,

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510, June

   [RFC4519]  Sciberras, A., "Lightweight Directory Access Protocol
              (LDAP): Schema for User Applications", RFC 4519, June

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

7.2.  Informative References


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              Sermersheim, J., Poitou, L., and H. Chu, "Password Policy
              for LDAP Directories", draft-behera-ldap-password-
              policy-10 (work in progress), August 2009.

              Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", draft-hoffman-schneier-
              4270bis-01 (work in progress), November 2012.

   [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, May 2000.

   [RFC3112]  Zeilenga, K., "LDAP Authentication Password Schema", RFC
              3112, May 2001.

   [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270, November 2005.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.

   [RFC5803]  Melnikov, A., "Lightweight Directory Access Protocol
              (LDAP) Schema for Storing Salted Challenge Response
              Authentication Mechanism (SCRAM) Secrets", RFC 5803, July

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011.

   [RFC6648]  Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012.

Author's Address

   Michael Stroeder
   Independent consultant
   Klauprechtstr. 11
   Karlsruhe  76137

   Email: michael@stroeder.com
   URI:   http://www.stroeder.com

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