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Versions: 00 01 02 03 04 05 rfc2830                                     
LDAPExt Working Group                            Jeff Hodges, Stanford
INTERNET-DRAFT                               RL "Bob" Morgan, Stanford
Category: Standards Track               Mark Wahl, Critical Angle Inc.
                                                           March, 1998

              Lightweight Directory Access Protocol (v3):
                 Extension for Transport Layer Security

                        Status of this Document

This document is an Internet-Draft.  Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its
working groups.  Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as ``work in progress.''

To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

Comments and suggestions on this document are encouraged.  Comments on
this document should be sent to the LDAPEXT working group discussion

This document expires in September 1998.

1.  Abstract

This document defines the "Start Transport Layer Security (TLS) Opera-
tion" for LDAP [LDAPv3, TLS]. This operation provides for TLS establish-
ment in an LDAP association and is defined in terms of an LDAP extended

2.  Conventions Used in this Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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document are to be interpreted as described in [ReqsKeywords].

3.  The Start TLS Operation

3.1.  Requesting TLS Establishment

A client may perform a Start TLS operation by transmitting an LDAP PDU
containing an ExtendedRequest [LDAPv3] specifying the OID for the Start
TLS operation:

An LDAP ExtendedRequest is defined as follows:

     ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
             requestName             [0] LDAPOID,
             requestValue            [1] OCTET STRING OPTIONAL }

A Start TLS extended request is formed by setting the requestName field
to the OID string given above.  The requestValue field is absent.  The
client MUST NOT send any PDUs on this connection following this request
until it receives a Start TLS extended response.

When a Start TLS extended request is made, the server MUST return an
LDAP PDU containing a Start TLS extended response.  An LDAP Exten-
dedResponse is defined as follows:

     ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
             responseName            [0] LDAPOID OPTIONAL,
             response                [1] OCTET STRING OPTIONAL,
             standardResponse        [2] LDAPResult }

A Start TLS extended response MUST contain a responseName field which
MUST be set to the same string as that present in the Start TLS extended
request. The response field is absent. The server MUST set the
resultCode of the standardResponse field to either success or one of the
other values outlined in section 3.3.

3.2.  "Success" Response

If the standardResponse field contains a resultCode of success, this
indicates that the server is willing and able to negotiate TLS. At this
point the client, which has ceased to transfer LDAP requests on the con-
nection, MUST either begin a TLS negotiation, or close the connection.
In the former case, the client will send PDUs in the TLS Record Protocol
directly over the underlying TCP bytestream to the server.

After the TLS connection is established, both parties MUST individually

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decide whether or not to continue based on the privacy level achieved.
Ascertaining the TLS connection's privacy level is implementation depen-
dent, and accomplished by communicating with one's respective local TLS

If the client or server decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD close the TLS
connection immediately after the TLS negotiation has completed, to
disconnect the TLS service and return to an LDAP state (see section 5,
below). This will cause the client's  authorization identity to be reset
to anonymous. The client MAY attempt to Start TLS again, or MAY send an
unbind request, or send any other LDAP request.

3.3.  Response other than "success"

If the standardResponse field contains a resultCode other than success,
this indicates that the server is unwilling or unable to negotiate TLS.

If the Start TLS extended request was not successful, the resultCode
will be one of:

     - operationsError (operations sequencing incorrect; e.g. TLS already
     - protocolError (TLS not supported or incorrect PDU structure)
     - referral (this server doesn't do TLS, try this one)
     - unavailable (e.g. some major problem with TLS, or server is
                    shutting down)

The server MUST return operationsError if the client violates any of the
Start TLS extended operation sequencing requirements described in sec-
tion 4, below.

If the server does not support TLS (whether by design or by current con-
figuration), it MUST set the resultCode to protocolError (see section
4.1.1 of [LDAPv3]), or to referral. The server MUST include an actual
referral value in the LDAP Result if it returns a resultCode of refer-
ral. The client's current session is unaffected if the server does not
support TLS. The client MAY proceed with any LDAP operation, or it MAY
close the connection.

The server MUST return unavailable if it supports TLS but cannot estab-
lish a TLS connection for some reason, e.g. the certificate server not
responding, it cannot contact its TLS implementation, or if the server
is in process of shutting down. The client MAY retry the StartTLS opera-
tion, or it MAY proceed with any other LDAP operation, or it MAY close
the connection.

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4.  Sequencing of the Start TLS Operation

The client MAY send the Start TLS extended request at any time after
establishing an LDAP association, except that in the following cases the
client MUST NOT send a Start TLS extended request:

     - if TLS is currently established on the connection, or
     - during a multi-stage SASL negotiation, or
     - if there are any LDAP operations outstanding on the connection.

The result of violating any of these requirements is described above in
section 3.3.

The client MAY have already perfomed a Bind operation when it sends a
Start TLS request, or the client might have not yet bound.

If the client did not establish a TLS connection before sending any
other requests, and the server requires the client to establish a TLS
connection before performing a particular request, the server MUST
reject that request with a confidentialityRequired or strongAuthRequired
result. The client MAY send a Start TLS extended request, or it MAY
choose to close the connection.

5.  Closing a TLS Connection

5.1.  Graceful Closure

Either the client or server MAY terminate the TLS connection on an LDAP
association by sending a TLS closure alert. This will leave the LDAP
association intact.

Before closing a TLS connection, the client MUST either wait for any
outstanding LDAP operations to complete, or explicitly abandon them

After the initiator of a close has sent a closure alert, it MUST discard
any TLS messages until it has received an alert from the other party.
It will cease to send TLS Record Protocol PDUs, and following the
reciept of the alert, MAY send and receive LDAP PDUs.

The other party, if it receives a closure alert, MUST immediately
transmit a TLS closure alert.  It will subequently cease to send TLS
Record Protocol PDUs, and MAY send and receive LDAP PDUs.

5.2.  Abrupt Closure

Either the client or server MAY abruptly close the entire LDAP associa-
tion and any TLS connection established on it by dropping the underlying

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TCP connection. A server MAY beforehand send the client a Notice of
Disconnection [LDAPv3] in this case.

6.  Authentication and Authorization:  Definitions and Concepts

This section defines basic terms, concepts, and interrelationships
regarding authentication, authorization, credentials, and identity.
These concepts are used in describing the use of TLS in client authenti-
cation and authorization in section 7.

6.1.  Access Control Policy

An access control policy is a set of rules defining the protection of
resources, generally in terms of the capabilities of persons or other
entities accessing those resources.  A common expression of an access
control policy is an access control list.  Security objects and mechan-
isms, such as those described here, enable the expression of access con-
trol policies and their enforcement.  Access control policies are typi-
cally expressed in terms of access control attributes as described

6.2.  Access Control Factors

A request, when it is being processed by a server, may be associated
with a wide variety of security-related factors (see [LDAPv3] section
4.2). The server uses these factors to determine whether and how to pro-
cess the request.  These are called access control factors (ACFs).  They
might include source IP address, encryption strength, the type of opera-
tion being requested, time of day, etc.  Some factors may be specific to
the request itself, others may be associated with the connection via
which the request is transmitted, others (e.g. time of day) may be

Access control policies are expressed in terms of access control fac-
tors.  E.g., a request having ACFs i,j,k can perform operation Y on
resource Z. The set of ACFs that a server makes available for such
expressions is implementation-specific.

6.3.  Authentication, Credentials, Identity

Authentication credentials are the evidence supplied by one party to
another, asserting the identity of the supplying party (typically a
user) who is attempting to establish an association with the other party
(typically a server).  Authentication is the process of generating,
transmitting, and verifying these credentials and thus the identity they
assert. An authentication identity is the name presented in a creden-

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There are many forms of authentication credentials -- the form used
depends upon the particular authentication mechanism negotiated by the
parties.  For example: X.509 certificates, Kerberos tickets, simple
identity and password pairs.  Note that an authentication mechanism may
constrain the form of authentication identities used with it.

6.4.  Authorization Identity

An authorization identity is one kind of access control factor.  It is
the name of the user or other entity that requests that operations be
performed.  Access control policies are often expressed in terms of
authorization identities; e.g., user X can perform operation Y on
resource Z.

The authorization identity bound to an association is often exactly the
same as the authentication identity presented by the client, but it may
be different.  SASL allows clients to specify an authorization identity
distinct from the authentication identity asserted by the client's
credentials.  This permits agents such as proxy servers to authenticate
using their own credentials, yet request the access privileges of the
identity for which they are proxying [SASL].  Also, the form of authen-
tication identity supplied by a service like TLS may not correspond to
the authorization identities used to express a server's access control
policy, requiring a server-specific mapping to be done.  The method by
which a server composes and validates an authorization identity from the
authentication credentials supplied by a client is implementation-

7.  Effects of TLS on the Client's Authorization Identity

7.1.  Session Establishment Effects

Upon establishment of the TLS connection onto the LDAP association, any
previously established authentication and authorization identities MUST
remain in force, including anonymous state. This holds even in the case
where the server requests client authentication via TLS (i.e. requests
the client to supply its certificate during TLS negotiation).

A client MAY explicitly request that its authenticated TLS credentials
be used to establish its LDAP authorization identity. This is accom-
plished after TLS establishment by invoking a Bind request of the SASL
form using the "EXTERNAL" mechanism name [SASL].

The credentials field (within the SaslCredentials sequence in the Bind
Request) MAY contain an authorization identity, or it MAY be empty.  If
it does contain an identity, the server MUST verify that the client's
authenticated TLS credentials are permitted to use that authorization
identity.  The server MUST reject the Bind operation with an

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invalidAuthorizationId resultCode in the Bind response if the client is
not so authorized.  If the credentials field is empty, the server bases
the client's authorization identity on the authentication identity sup-
plied in the client's TLS credentials (typically a public-key certifi-

If a TLS session has not been established between the client and server
(and there is no other external source of authentication credentials),
or if, during the process of establishing the TLS session, the server
did not request the client's authentication credentials, the SASL EXTER-
NAL bind MUST fail, with a result code of inappropriateAuthentication.

7.2.  Session Closure Effects

Closure of the TLS connection MUST cause the LDAP association to move to
an anonymous authentication and authorization state regardless of the
state established over TLS and regardless of the authentication and
authorization state prior to TLS connection establishment.

8.  invalidAuthorizationId Error Code

A value of the resultCode field of the LDAPResult construct is defined:

                             invalidAuthorizationId       (55)

   - invalidAuthorizationId: the authorization identity requested
     is invalid or is not consistent with the supplied
     authentication credentials.

9.  Conformance Requirements

The TLS standard [TLS] does not mandate that the client must have a cer-
tificate -- i.e. client-side authentication is optional within the
bounds of the TLS specification.  However, clients conformant to this
specification MUST have the capability to supply a client side certifi-
cate to the server. Additionally, they MUST implement the mandatory
cipher suite specified in [TLS].

10.  Security Considerations

The goals of using the TLS protocol with LDAP are to ensure connection
confidentiality and integrity, and to optionally provide for authentica-
tion. TLS expressly provides these capabilities, as described in [TLS].

All security gained via use of the Start TLS operation is gained by the
use of TLS itself. The Start TLS operation, on its own, does not provide
any additional security.

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The use of TLS does not provide or ensure for confidentiality and/or
non-repudiation of the data housed by an LDAP-based directory server.
Once established, TLS only provides for and ensures confidentiality and
integrity of the operations and data in transit over the LDAP associa-
tion, and only if the implementations on the client and server support
and negotiate it.

The level of security provided though the use of TLS depends directly on
both the quality of the TLS implementation used and the style of usage
of that implementation. Both parties SHOULD independently ascertain and
consent to the privacy level achieved once TLS is established and before
begining use of the TLS connection. For example, the privacy level of
the TLS connection might have been negotiated down to plaintext.

Client and server implementors SHOULD take measures to ensure proper
protection of credentials and other confidential data where such meas-
ures are not otherwise provided by the TLS implementation.

Server implementors SHOULD allow for server administrators to elect
whether and when connection confidentiality is required.

11.  Acknowledgements

The authors thank Tim Howes, Paul Hoffman, John Kristian, and Harald
Alvestrand for their contributions to this document.

12.  References

     M. Wahl, S. Kille and T. Howes, "Lightweight Directory Access Pro-
     tocol (v3)", RFC 2251.

     Scott Bradner, "Key Words for use in RFCs to Indicate Requirement
     Levels", RFC 2119.

[SASL]J. Myers, "Simple Authentication and Security Layer (SASL)", RFC

[TLS]Tim Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC 22??.

13.  Authors' Addresses

   Jeff Hodges
   Computing & Communication Services
   Stanford University
   Pine Hall
   241 Panama Street

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   Stanford, CA 94305-4122

   Phone: +1-650-723-2452
   EMail: Jeff.Hodges@Stanford.edu

   RL "Bob" Morgan
   Computing & Communication Services
   Stanford University
   Pine Hall
   241 Panama Street
   Stanford, CA 94305-4122

   Phone: +1-650-723-9711
   EMail: Bob.Morgan@Stanford.edu

   Mark Wahl
   Critical Angle Inc.
   4815 W. Braker Lane #502-385
   Austin, TX 78759

   EMail:  M.Wahl@critical-angle.com


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   This document and the information contained herein is provided on an

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