As per Relevance of the word mechanism, we have this rfc below:
Network Working Group M.
Request for Comments: 2829 Sun Microsystems, Inc
Category: Standards Track H.
EDB
J.
Oblix, Inc
R.
University of
May 2000
Authentication Methods for
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document specifies particular combinations of
mechanisms which are required and recommended in LDAP [1]
implementations
1.
LDAP version 3 is a powerful access protocol for directories
It offers means of searching, fetching and manipulating
content, and ways to access a rich set of security functions
In order to function for the best of the Internet, it is vital
these security functions be interoperable; therefore there has to
a minimum subset of security functions that is common to
implementations that claim LDAPv3 conformance
Basic threats to an LDAP directory service include
(1) Unauthorized access to data via data-fetching operations
Wahl, et al. Standards Track [Page 1]
RFC 2829 Authentication Methods for LDAP May 2000
(2) Unauthorized access to reusable client
information by monitoring others' access
(3) Unauthorized access to data by monitoring others' access
(4) Unauthorized modification of data
(5) Unauthorized modification of configuration
(6) Unauthorized or excessive use of resources (denial
service),
(7) Spoofing of directory: Tricking a client into believing
information came from the directory when in fact it did not
either by modifying data in transit or misdirecting
client's connection
Threats (1), (4), (5) and (6) are due to hostile clients.
(2), (3) and (7) are due to hostile agents on the path between
and server, or posing as a server
The LDAP protocol suite can be protected with the following
mechanisms
(1) Client authentication by means of the SASL [2]
set, possibly backed by the TLS credentials
mechanism
(2) Client authorization by means of access control based on
requestor's authenticated identity
(3) Data integrity protection by means of the TLS protocol
data-integrity SASL mechanisms
(4) Protection against snooping by means of the TLS protocol
data-encrypting SASL mechanisms
(5) Resource limitation by means of administrative limits
service controls,
(6) Server authentication by means of the TLS protocol or
mechanism
At the moment, imposition of access controls is done by means
the scope of the LDAP protocol
In this document, the term "user" represents any application which
an LDAP client using the directory to retrieve or store information
Wahl, et al. Standards Track [Page 2]
RFC 2829 Authentication Methods for LDAP May 2000
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
document are to be interpreted as described in RFC 2119 [3].
2. Example deployment
The following scenarios are typical for LDAP directories on
Internet, and have different security requirements. (In
following, "sensitive" means data that will cause real damage to
owner if revealed; there may be data that is protected but
sensitive). This is not intended to be a comprehensive list,
scenarios are possible, especially on physically protected networks
(1) A read-only directory, containing no sensitive data
accessible to "anyone", and TCP connection hijacking or
spoofing is not a problem. This directory requires
security functions except administrative service limits
(2) A read-only directory containing no sensitive data;
access is granted based on identity. TCP
hijacking is not currently a problem. This scenario
a secure authentication function
(3) A read-only directory containing no sensitive data; and
client needs to ensure that the directory data
authenticated by the server and not modified while
returned from the server
(4) A read-write directory, containing no sensitive data;
access is available to "anyone", update access to
authorized persons. TCP connection hijacking is
currently a problem. This scenario requires a
authentication function
(5) A directory containing sensitive data. This
requires session confidentiality protection AND
authentication
3. Authentication and Authorization: Definitions and
This section defines basic terms, concepts, and
regarding authentication, authorization, credentials, and identity
These concepts are used in describing how various security
are utilized in client authentication and authorization
Wahl, et al. Standards Track [Page 3]
RFC 2829 Authentication Methods for LDAP May 2000
3.1. Access Control
An access control policy is a set of rules defining the protection
resources, generally in terms of the capabilities of persons or
entities accessing those resources. A common expression of an
control policy is an access control list. Security objects
mechanisms, such as those described here, enable the expression
access control policies and their enforcement. Access
policies are typically expressed in terms of access
attributes as described below
3.2. Access Control
A request, when it is being processed by a server, may be
with a wide variety of security-related factors (section 4.2 of [1]).
The server uses these factors to determine whether and how to
the request. These are called access control factors (ACFs).
might include source IP address, encryption strength, the type
operation being requested, time of day, etc. Some factors may
specific to the request itself, others may be associated with
connection via which the request is transmitted, others (e.g. time
day) may be "environmental".
Access control policies are expressed in terms of access
factors. E.g., a request having ACFs i,j,k can perform operation
on resource Z. The set of ACFs that a server makes available for
expressions is implementation-specific
3.3. Authentication, Credentials,
Authentication credentials are the evidence supplied by one party
another, asserting the identity of the supplying party (e.g. a user
who is attempting to establish an association with the other
(typically a server). Authentication is the process of generating
transmitting, and verifying these credentials and thus the
they assert. An authentication identity is the name presented in
credential
There are many forms of authentication credentials -- the form
depends upon the particular authentication mechanism negotiated
the parties. For example: X.509 certificates, Kerberos tickets
simple identity and password pairs. Note that an
mechanism may constrain the form of authentication identities
with it
Wahl, et al. Standards Track [Page 4]
RFC 2829 Authentication Methods for LDAP May 2000
3.4. Authorization
An authorization identity is one kind of access control factor.
is the name of the user or other entity that requests that
be performed. Access control policies are often expressed in
of authorization identities; e.g., entity X can perform operation
on resource Z
The authorization identity bound to an association is often
the same as the authentication identity presented by the client,
it may be different. SASL allows clients to specify an
identity distinct from the authentication identity asserted by
client's credentials. This permits agents such as proxy servers
authenticate using their own credentials, yet request the
privileges of the identity for which they are proxying [2]. Also
the form of authentication identity supplied by a service like
may not correspond to the authorization identities used to express
server's access control policy, requiring a server-specific
to be done. The method by which a server composes and validates
authorization identity from the authentication credentials
by a client is implementation-specific
4. Required security
It is clear that allowing any implementation, faced with the
requirements, to pick and choose among the possible alternatives
not a strategy that is likely to lead to interoperability. In
absence of mandates, clients will be written that do not support
security function supported by the server, or worse, support
mechanisms like cleartext passwords that provide clearly
security
Active intermediary attacks are the most difficult for an attacker
perform, and for an implementation to protect against. Methods
protect only against hostile client and passive eavesdropping
are useful in situations where the cost of protection against
intermediary attacks is not justified based on the perceived risk
active intermediary attacks
Given the presence of the Directory, there is a strong desire to
mechanisms where identities take the form of a Distinguished Name
authentication data can be stored in the directory; this means
either this data is useless for faking authentication (like the
"/etc/passwd" file format used to be), or its content is never
across the wire unprotected - that is, it's either updated
the protocol or it is only updated in sessions well protected
snooping. It is also desirable to allow authentication methods
Wahl, et al. Standards Track [Page 5]
RFC 2829 Authentication Methods for LDAP May 2000
carry authorization identities based on existing forms of
identities for backwards compatibility with non-LDAP-
authentication services
Therefore, the following implementation conformance requirements
in place
(1) For a read-only, public directory, anonymous authentication
described in section 5, can be used
(2) Implementations providing password-based
access MUST support authentication using the DIGEST-MD5
mechanism [4], as described in section 6.1. This
client authentication with protection against
eavesdropping attacks, but does not provide
against active intermediary attacks
(3) For a directory needing session protection
authentication, the Start TLS extended operation [5],
either the simple authentication choice or the SASL
mechanism, are to be used together. Implementations
support authentication with a password as described
section 6.2, and SHOULD support authentication with
certificate as described in section 7.1. Together,
can provide integrity and disclosure protection
transmitted data, and authentication of client and server
including protection against active intermediary attacks
If TLS is negotiated, the client MUST discard all information
the server fetched prior to the TLS negotiation. In particular,
value of supportedSASLMechanisms MAY be different after TLS has
negotiated (specifically, the EXTERNAL mechanism or the
PLAIN mechanism are likely to only be listed after a TLS
has been performed).
If a SASL security layer is negotiated, the client MUST discard
information about the server fetched prior to SASL. In particular
if the client is configured to support multiple SASL mechanisms,
SHOULD fetch supportedSASLMechanisms both before and after the
security layer is negotiated and verify that the value has
changed after the SASL security layer was negotiated. This
active attacks which remove supported SASL mechanisms from
supportedSASLMechanisms list, and allows the client to ensure that
is using the best mechanism supported by both client and
(additionally, this is a SHOULD to allow for environments where
supported SASL mechanisms list is provided to the client through
different trusted source, e.g. as part of a digitally signed object).
Wahl, et al. Standards Track [Page 6]
RFC 2829 Authentication Methods for LDAP May 2000
5. Anonymous
Directory operations which modify entries or access
attributes or entries generally require client authentication
Clients which do not intend to perform any of these
typically use anonymous authentication
LDAP implementations MUST support anonymous authentication,
defined in section 5.1.
LDAP implementations MAY support anonymous authentication with TLS
as defined in section 5.2.
While there MAY be access control restrictions to prevent access
directory entries, an LDAP server SHOULD allow an anonymously-
client to retrieve the supportedSASLMechanisms attribute of the
DSE
An LDAP server MAY use other information about the client provided
the lower layers or external means to grant or deny access even
anonymously authenticated clients
5.1. Anonymous authentication
An LDAP client which has not successfully completed a bind
on a connection is anonymously authenticated
An LDAP client MAY also specify anonymous authentication in a
request by using a zero-length OCTET STRING with the
authentication choice
5.2. Anonymous authentication and
An LDAP client MAY use the Start TLS operation [5] to negotiate
use of TLS security [6]. If the client has not bound beforehand
then until the client uses the EXTERNAL SASL mechanism to
the recognition of the client's certificate, the client
anonymously authenticated
Recommendations on TLS ciphersuites are given in section 10.
An LDAP server which requests that clients provide their
during TLS negotiation MAY use a local security policy to
whether to successfully complete TLS negotiation if the client
not present a certificate which could be validated
Wahl, et al. Standards Track [Page 7]
RFC 2829 Authentication Methods for LDAP May 2000
6. Password-based
LDAP implementations MUST support authentication with a
using the DIGEST-MD5 SASL mechanism for password protection,
defined in section 6.1.
LDAP implementations SHOULD support authentication with the "simple
password choice when the connection is protected
eavesdropping using TLS, as defined in section 6.2.
6.1. Digest
An LDAP client MAY determine whether the server supports
mechanism by performing a search request on the root DSE,
the supportedSASLMechanisms attribute, and checking whether
string "DIGEST-MD5" is present as a value of this attribute
In the first stage of authentication, when the client is
an "initial authentication" as defined in section 2.1 of [4],
client sends a bind request in which the version number is 3,
authentication choice is sasl, the sasl mechanism name is "DIGEST
MD5", and the credentials are absent. The client then waits for
response from the server to this request
The server will respond with a bind response in which the
is saslBindInProgress, and the serverSaslCreds field is present.
contents of this field is a string defined by "digest-challenge"
section 2.1.1 of [4]. The server SHOULD include a realm
and MUST indicate support for UTF-8.
The client will send a bind request with a distinct message id,
which the version number is 3, the authentication choice is sasl,
sasl mechanism name is "DIGEST-MD5", and the credentials contain
string defined by "digest-response" in section 2.1.2 of [4].
serv-type is "ldap".
The server will respond with a bind response in which the
is either success, or an error indication. If the authentication
successful and the server does not support subsequent authentication
then the credentials field is absent. If the authentication
successful and the server supports subsequent authentication,
the credentials field contains the string defined by "response-auth
in section 2.1.3 of [4]. Support for subsequent authentication
OPTIONAL in clients and servers
Wahl, et al. Standards Track [Page 8]
RFC 2829 Authentication Methods for LDAP May 2000
6.2. "simple" authentication choice under TLS
A user who has a directory entry containing a userPassword
MAY authenticate to the directory by performing a simple
bind sequence following the negotiation of a TLS
providing connection confidentiality [6].
The client will use the Start TLS operation [5] to negotiate the
of TLS security [6] on the connection to the LDAP server. The
need not have bound to the directory beforehand
For this authentication procedure to be successful, the client
server MUST negotiate a ciphersuite which contains a bulk
algorithm of appropriate strength. Recommendations on cipher
are given in section 10.
Following the successful completion of TLS negotiation, the
MUST send an LDAP bind request with the version number of 3, the
field containing the name of the user's entry, and the "simple
authentication choice, containing a password
The server will, for each value of the userPassword attribute in
named user's entry, compare these for case-sensitive equality
the client's presented password. If there is a match, then
server will respond with resultCode success, otherwise the
will respond with resultCode invalidCredentials
6.3. Other authentication choices with
It is also possible, following the negotiation of TLS, to perform
SASL authentication which does not involve the exchange of
reusable passwords. In this case the client and server need
negotiate a ciphersuite which provides confidentiality if the
service required is data integrity
7. Certificate-based
LDAP implementations SHOULD support authentication via a
certificate in TLS, as defined in section 7.1.
7.1. Certificate-based authentication with
A user who has a public/private key pair in which the public key
been signed by a Certification Authority may use this key pair
authenticate to the directory server if the user's certificate
requested by the server. The user's certificate subject field
be the name of the user's directory entry, and the
Authority must be sufficiently trusted by the directory server
Wahl, et al. Standards Track [Page 9]
RFC 2829 Authentication Methods for LDAP May 2000
have issued the certificate in order that the server can process
certificate. The means by which servers validate certificate
is outside the scope of this document
A server MAY support mappings for certificates in which the
field name is different from the name of the user's directory entry
A server which supports mappings of names MUST be capable of
configured to support certificates for which no mapping is required
The client will use the Start TLS operation [5] to negotiate the
of TLS security [6] on the connection to the LDAP server. The
need not have bound to the directory beforehand
In the TLS negotiation, the server MUST request a certificate.
client will provide its certificate to the server, and MUST perform
private key-based encryption, proving it has the private
associated with the certificate
As deployments will require protection of sensitive data in transit
the client and server MUST negotiate a ciphersuite which contains
bulk encryption algorithm of appropriate strength.
of cipher suites are given in section 10.
The server MUST verify that the client's certificate is valid.
server will normally check that the certificate is issued by a
CA, and that none of the certificates on the client's
chain are invalid or revoked. There are several procedures by
the server can perform these checks
Following the successful completion of TLS negotiation, the
will send an LDAP bind request with the SASL "EXTERNAL" mechanism
8. Other
The LDAP "simple" authentication choice is not suitable
authentication on the Internet where there is no network or
layer confidentiality
As LDAP includes native anonymous and plaintext
methods, the "ANONYMOUS" and "PLAIN" SASL mechanisms are not
with LDAP. If an authorization identity of a form different from
DN is requested by the client, a mechanism that protects the
in transit SHOULD be used
The following SASL-based mechanisms are not considered in
document: KERBEROS_V4, GSSAPI and SKEY
Wahl, et al. Standards Track [Page 10]
RFC 2829 Authentication Methods for LDAP May 2000
The "EXTERNAL" SASL mechanism can be used to request the LDAP
make use of security credentials exchanged by a lower layer. If a
session has not been established between the client and server
to making the SASL EXTERNAL Bind request and there is no
external source of authentication credentials (e.g. IP-
security [8]), or if, during the process of establishing the
session, the server did not request the client's
credentials, the SASL EXTERNAL bind MUST fail with a result code
inappropriateAuthentication. Any client authentication
authorization state of the LDAP association is lost, so the
association is in an anonymous state after the failure
9. Authorization
The authorization identity is carried as part of the SASL
field in the LDAP Bind request and response
When the "EXTERNAL" mechanism is being negotiated, if the
field is present, it contains an authorization identity of
authzId form described below
Other mechanisms define the location of the authorization identity
the credentials field
The authorization identity is a string in the UTF-8 character set
corresponding to the following ABNF [7]:
; Specific predefined authorization (authz) id schemes
; defined below -- new schemes may be defined in the future
authzId = dnAuthzId /
; distinguished-name-based authz id
dnAuthzId = "dn:"
dn = utf8string ; with syntax defined in RFC 2253
; unspecified userid, UTF-8 encoded
uAuthzId = "u:"
userid = utf8string ; syntax
A utf8string is defined to be the UTF-8 encoding of one or more
10646 characters
All servers which support the storage of authentication credentials
such as passwords or certificates, in the directory MUST support
dnAuthzId choice
Wahl, et al. Standards Track [Page 11]
RFC 2829 Authentication Methods for LDAP May 2000
The uAuthzId choice allows for compatibility with client
which wish to authenticate to a local directory but do not know
own Distinguished Name or have a directory entry. The format of
string is defined as only a sequence of UTF-8 encoded ISO 10646
characters, and further interpretation is subject to prior
between the client and server
For example, the userid could identify a user of a specific
service, or be a login name or the local-part of an RFC 822
address. In general a uAuthzId MUST NOT be assumed to be
unique
Additional authorization identity schemes MAY be defined in
versions of this document
10. TLS
The following ciphersuites defined in [6] MUST NOT be used
confidentiality protection of passwords or data
TLS_NULL_WITH_NULL_
TLS_RSA_WITH_NULL_MD
TLS_RSA_WITH_NULL_
The following ciphersuites defined in [6] can be cracked easily (
than a week of CPU time on a standard CPU in 1997). The client
server SHOULD carefully consider the value of the password or
being protected before using these ciphersuites
TLS_RSA_EXPORT_WITH_RC4_40_MD
TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD
TLS_RSA_EXPORT_WITH_DES40_CBC_
TLS_DH_DSS_EXPORT_WITH_DES40_CBC_
TLS_DH_RSA_EXPORT_WITH_DES40_CBC_
TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_
TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_
TLS_DH_anon_EXPORT_WITH_RC4_40_MD
TLS_DH_anon_EXPORT_WITH_DES40_CBC_
The following ciphersuites are vulnerable to man-in-the-
attacks, and SHOULD NOT be used to protect passwords or
data, unless the network configuration is such that the danger of
man-in-the-middle attack is tolerable
Wahl, et al. Standards Track [Page 12]
RFC 2829 Authentication Methods for LDAP May 2000
TLS_DH_anon_EXPORT_WITH_RC4_40_MD
TLS_DH_anon_WITH_RC4_128_MD
TLS_DH_anon_EXPORT_WITH_DES40_CBC_
TLS_DH_anon_WITH_DES_CBC_
TLS_DH_anon_WITH_3DES_EDE_CBC_
A client or server that supports TLS MUST support at
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA
11. SASL service name for
For use with SASL [2], a protocol must specify a service name to
used with various SASL mechanisms, such as GSSAPI. For LDAP,
service name is "ldap", which has been registered with the IANA as
GSSAPI service name
12. Security
Security issues are discussed throughout this memo;
(unsurprising) conclusion is that mandatory security is important
and that session encryption is required when snooping is a problem
Servers are encouraged to prevent modifications by anonymous users
Servers may also wish to minimize denial of service attacks by
out idle connections, and returning the unwillingToPerform
code rather than performing computationally expensive
requested by unauthorized clients
A connection on which the client has not performed the Start
operation or negotiated a suitable SASL mechanism for
integrity and encryption services is subject to man-in-the-
attacks to view and modify information in transit
Additional security considerations relating to the EXTERNAL
to negotiate TLS can be found in [2], [5] and [6].
13.
This document is a product of the LDAPEXT Working Group of the IETF
The contributions of its members is greatly appreciated
Wahl, et al. Standards Track [Page 13]
RFC 2829 Authentication Methods for LDAP May 2000
14.
[1] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Protocol (v3)", RFC 2251, December 1997.
[2] Myers, J., "Simple Authentication and Security Layer (SASL)",
2222, October 1997.
[3] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.
[4] Leach, P. and C. Newman, "Using Digest Authentication as a
Mechanism", RFC 2831, May 2000.
[5] Hodges, J., Morgan, R. and M. Wahl, "Lightweight Directory
Protocol (v3): Extension for Transport Layer Security", RFC 2830,
May 2000.
[6] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
2246, January 1999.
[7] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Specifications: ABNF", RFC 2234, November 1997.
[8] Kent, S. and R. Atkinson, "Security Architecture for the
Protocol", RFC 2401, November 1998.
Wahl, et al. Standards Track [Page 14]
RFC 2829 Authentication Methods for LDAP May 2000
15. Authors'
Mark
Sun Microsystems, Inc
8911 Capital of Texas Hwy #4140
Austin TX 78759
EMail: M.Wahl@innosoft.
Harald Tveit
EDB
N-7462 Trondheim,
Phone: +47 73 54 57 97
EMail: Harald@Alvestrand.
Jeff
Oblix, Inc
18922 Forge
Cupertino, CA 95014
Phone: +1-408-861-6656
EMail: JHodges@oblix.
RL "Bob"
Computing and
University of
Seattle, WA 98105
Phone: +1-206-221-3307
EMail: rlmorgan@washington.
Wahl, et al. Standards Track [Page 15]
RFC 2829 Authentication Methods for LDAP May 2000
16. Full Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Wahl, et al. Standards Track [Page 16]
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