LDAP Authentication Password Schema
A schema for storing cryptographically hashed passwords in LDAP directories
Zeilenga, K., "LDAP Authentication Password Schema", RFC 3112, DOI 10.17487/RFC3112, May 2001, <https://www.rfc-editor.org/info/rfc3112>.
@misc{rfc3112,
series = {Request for Comments},
number = 3112,
howpublished = {RFC 3112},
publisher = {RFC Editor},
doi = {10.17487/RFC3112},
url = {https://www.rfc-editor.org/info/rfc3112},
author = {K. Zeilenga},
title = {{LDAP Authentication Password Schema}},
pagetotal = 9,
year = 2001,
month = may,
} RFC text
Contents
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
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 (C) The Internet Society (2001). All Rights Reserved.
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Acknowledgement
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AI ELI5
On this page
§1Authentication Passwords in LDAP
The LDAP Authentication Password Schema provides a way to store authentication credentials securely in a Lightweight Directory Access Protocol (LDAP) directory. Instead of storing cleartext passwords like the older userPassword attribute requires, authPassword stores cryptographic hashes derived from passwords, plus additional metadata needed to verify login attempts.
The RFC defines the schema components needed for this: a special syntax for storing hashed values, matching rules that let the server compare an asserted password against stored hashes, and support for multiple hashing schemes so administrators can choose the protection level that fits their needs.
LDAP is like a phone book database. Instead of writing down actual passwords like "mary123", it scrambles them with special math (a hash function) and stores the scrambled version. That way if someone steals the phone book, they cannot use the passwords.
§2Schema Definitions and Components
The RFC defines several LDAP schema components. The authPasswordSyntax defines the format for storing hashed passwords: it holds three pieces of data: a scheme name (like "MD5" or "SHA1"), an authInfo field (usually the salt in base64 encoding), and an authValue field (the actual hash in base64 encoding). Together they look like MD5$c2FsdA==$...hash....
Two matching rules support password validation: authPasswordExactMatch allows exact comparison of stored values, and authPasswordMatch lets clients assert that a cleartext password matches a stored hash by testing it through the appropriate scheme. The authPassword attribute holds the actual hashed password values on a user entry, and the authPasswordObject class marks entries that may contain password data.
The schema is the rulebook for how to write down scrambled passwords. It says what fields go in the record (the scramble method name, the salt, the scrambled password), how to format them, and how to check if a guess matches what is stored.
§3Hashing Schemes
The RFC defines two password hashing schemes. In the MD5 scheme, the server concatenates the user's cleartext password with a salt (a random value), computes the MD5 digest of that concatenation, and stores the base64-encoded digest. The salt must be at least 64 bits long and stored in the authInfo field. To check a login attempt, the server reads the stored salt, concatenates it with the asserted password, computes MD5 again, and compares the digests.
The SHA1 scheme works the same way but uses SHA-1 hashing instead of MD5. Both schemes include the salt in the stored value so the server has it available for verification. The RFC recommends servers support SHA1 (required) and optionally support MD5. Servers can use these schemes in conjunction with other authentication methods like Kerberos.
MD5 and SHA1 are scrambling recipes. They mix the password with a random scramble-key (salt), run special math on it, and get scrambled output. When someone logs in, the server scrambles their guess the same way and checks if the results match.
§4Implementation Considerations
LDAP servers implementing this RFC face several decisions. A server may restrict which schemes apply to a particular authentication, but should check all scheme values of selected types when validating a password: if the asserted password matches any stored value of a selected scheme, authentication succeeds. Servers can also use authPassword alongside other storage methods like userPassword or external password stores, giving administrators flexibility in transition.
Both clients and servers SHOULD NOT transmit authPassword values or authPasswordMatch assertions unless the connection uses confidentiality protection like TLS, since even a hashed password should be guarded like a cleartext secret. Servers MUST also protect against abuse: the authPasswordMatch matching rule could be used to mount password-guessing attacks, so access controls and rate limiting are essential. Some schemes require CPU-intensive operations (especially salted hashing), so servers should protect against denial-of-service attacks that exploit this.
When turning on this feature, administrators need to make sure connections use encryption (like TLS), set up limits so someone cannot guess passwords by trying millions of them, and allow the system time to do the scrambling math without freezing.
§5Security Properties and Risks
The authPassword attribute offers protection against disclosure attacks but requires careful handling. If hashes are exposed, attackers cannot recover the original password immediately like they could with cleartext, but they might attack the hashes using dictionary attacks, rainbow tables, or brute force if the salt is weak. The RFC states that hashed values SHOULD be protected as if they were cleartext passwords, because flaws in the hash function or the salt's randomness could allow recovery.
One-way hashing is an additional security layer, not a complete solution. A user could have multiple passwords stored in the same attribute, creating a risk: if an attacker gains write access, they could store a new password without disabling the original. The RFC recommends using policy-aware clients and servers to prevent this. Servers should also use a scheme-selection configuration that lets them quickly disable a scheme if a vulnerability is discovered, allowing deployment of fixes without changing all stored values.
Even scrambled passwords need protection because someone might use a big guess-list (like all words in a dictionary) to find the password. If the salt is too short or predictable, attackers can pre-compute answers for many guesses. And if someone hacks in and can write new data, they could add a password of their own to a user account.