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'
draft-stroeder-hashed-userpassword-values-01
Abstract
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
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document is being discussed on the ldapext@ietf.org mailing
list.
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
salt.
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
request.
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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
[RFC5802].
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
attacks.
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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
[FIPS-180-4]
National Institute of Standards and Technology (NIST) ,
"Secure Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.
[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
2006.
[RFC4519] Sciberras, A., "Lightweight Directory Access Protocol
(LDAP): Schema for User Applications", RFC 4519, June
2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
7.2. Informative References
[I-D.behera-ldap-password-policy]
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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.
[I-D.hoffman-schneier-4270bis]
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
2010.
[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
DE
Email: michael@stroeder.com
URI: http://www.stroeder.com
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