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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, Innosoft
                                                         October, 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 view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe),
ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim),
ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

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 April 1999.

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 Request

This section describes the Start TLS extended request and extended
response themselves: how to form the request, the form of the response,
and enumerates the various result codes the client MUST be prepared to

The section following this one then describes how to sequence an overall
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 {
             COMPONENTS OF LDAPResult,
             responseName     [10] LDAPOID OPTIONAL,
             response         [11] OCTET STRING OPTIONAL }

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

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3.2.  "Success" Response

If the ExtendedResponse contains a resultCode of success, this indicates
that the server is willing and able to negotiate TLS. Refer to section
4, below, for details.

3.3.  Response other than "success"

If the ExtendedResponse 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.

4.  Sequencing of the Start TLS Operation

This section describes the overall procedures clients and servers MUST
follow for TLS establishment. These procedures take into consideration
various aspects of the overall security of the LDAP association

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including discovery of resultant security level and assertion of the
client's authorization identity.

Note that the precise effects, on a client's authorzation identity, of
establishing TLS on an LDAP association are described in detail in sec-
tion 7.

4.1.  Requesting to Start TLS on an LDAP Association

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 a resultCode of
operationsError, as 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.

4.2.  Starting TLS

The server will return an extended response with the resultCode of suc-
cess if it is willing and able to negotiate TLS.  It will return other
resultCodes, documented above, if it is unable.

In the successful case, the client, which has ceased to transfer LDAP
requests on the connection, MUST either begin a TLS negotiation or close
the connection. The client will send PDUs in the TLS Record Protocol
directly over the underlying transport connection to the server to ini-
tiate TLS negotiation [TLS].

4.3.  TLS Version Negotiation

Negotiating the version of TLS or SSL to be used is a part of the TLS
Handshake Protocol, as documented in [TLS]. Please refer to that docu-
ment for details.

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4.4.  Discovery of Resultant Security Level

After a TLS connection is established on an LDAP association, both par-
ties MUST individually decide whether or not to continue based on the
privacy level achieved. Ascertaining the TLS connection's privacy level
is implementation dependent, and accomplished by communicating with
one's respective local TLS implementation.

If the client or server decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD gracefully
close the TLS connection immediately after the TLS negotiation has com-
pleted (see sections 5 and 7.2, below).

The client MAY attempt to Start TLS again, or MAY send an unbind
request, or send any other LDAP request.

4.5.  Assertion of Client's Authorization Identity

The client MAY, upon receipt of a Start TLS extended response indicating
success, assert that a specific authorization identity be utilized in
determining the client's authorization status. The client accomplishes
this via an LDAP Bind request specifying a SASL mechanism of "EXTERNAL"
[SASL]. See section 7, below.

4.6.  Server Identity Check

The client MUST check its understanding of the server's hostname against
the server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.

If a subjectAltName extension of type dNSName is present, it SHOULD be
used as the source of the server's identity.

Matching is performed according to these rules:

   - The client MUST use the server hostname it used to open
     the LDAP connection as the value to compare against the
     server name as expressed in the server's certificate.
     The client MUST NOT use the server's canonical DNS name or
     any other derived form of name.

   - If a subjectAltName extension of type dNSName is present
     in the certificate, it SHOULD be used as the source of the
     server's identity.

   - Matching is case-insensitive.

   - The "*" wildcard character is allowed.

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     - If present, it applies only to the left-most name component.

E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not bar.com.
If more than one identity of a given type is present in the certificate
(e.g. more than one dNSName name), a match in any one of the set is con-
sidered acceptable.

If the hostname does not match the dNSName-based identity in the certi-
ficate per the above check, user-oriented clients SHOULD either notify
the user (clients MAY give the user the opportunity to continue with the
connection in any case) or terminate the connection and indicate that
the server's identity is suspect. Automated clients SHOULD close the
connection, returning and/or logging an error indicating hat the
server's identity is suspect.

4.7.  Refresh of Server Capabilities Information

The client SHOULD refresh any cached server capabilities information
(e.g.  from the server's root DSE; see section 3.4 of [LDAPv3]) upon TLS
session establishment. This is necessary to protect against active-
intermediary attacks which may have altered any server capabilities
information retrieved prior to TLS establishment. The server MAY adver-
tise different capabilities after TLS establishment.

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

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association and any TLS connection established on it by dropping the
underlying 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 how various security approaches
are utilized in client authentication and authorization.

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 (section 4.2 of [1]).
The server uses these factors to determine whether and how to process
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 (e.g. a user) who
is attempting to establish an association with the other party (typi-
cally a server).  Authentication is the process of generating, transmit-
ting, and verifying these credentials and thus the identity they assert.
An authentication identity is the name presented in a credential.

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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., entity 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 a Client's Authorization Identity

This section describes the effects on a client's authorization identity
brought about by establishing TLS on an LDAP association. The default
effects are described first, and next the facilities for client asser-
tion of authorization identity are discussed including error conditions.
Lastly, the effects of closing the TLS connection are described.

7.1.  TLS Connection Establishment Effects

7.1.1.  Default 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 -- e.g. requests
the client to supply its certificate during TLS negotiation (see [TLS]).

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7.1.2.  Client Assertion of Authorization Identity

A client MAY either implicitly request that its LDAP authorization iden-
tity be derived from its authenticated TLS credentials or it MAY expli-
citly provide an authorization identity and assert that it be used in
combination with its authenticated TLS credentials. The former is known
as an implicit assertion, and the latter as an explicit assertion.  Implicit Assertion

An implicit authorization identity assertion is accomplished after TLS
establishment by invoking a Bind request of the SASL form using the
"EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include the
optional credentials octet string (found within the SaslCredentials
sequence in the Bind Request). The server will derive the client's
authorization identity from the authentication identity supplied in the
client's TLS credentials (typically a public key certificate) according
to local policy. The underlying mechanics of how this is accomplished
are implementation specific.  Explicit Assertion

An explicit authorization identity assertion is accomplished after TLS
establishment by invoking a Bind request of the SASL form using the
"EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the creden-
tials octet string. The string's syntax is described below in  section
8.  Error Conditions

For either form of assertion, the server MUST verify that the client's
authentication identity as supplied in its TLS credentials is permitted
to be mapped to the asserted authorization identity. The server MUST
reject the Bind operation with an invalidCredentials resultCode in the
Bind response if the client is not so authorized. The LDAP association's
authentication identity and authorization identity (if any) which were
in effect after TLS establishment but prior to making the Bind request,
MUST remain in force.

Additionally, with either form of assertion, if a TLS session has not
been established between the client and server prior to making the SASL
EXTERNAL Bind request and there is no other external source of authenti-
cation credentials (e.g.  IP-level security RFC 1825), or if, during the
process of establishing the TLS session, the server did not request the
client's authentication credentials, the SASL EXTERNAL bind MUST fail
with a result code of inappropriateAuthentication. Any authentication
identity and authorization identity, as well as TLS connection, which
were in effect prior to making the Bind request, MUST remain in force.

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7.2.  TLS Connection 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.  Authorization Identity

The authorization identity is carried as part of the SASL credentials
field in the LDAP Bind request and response.

When the "EXTERNAL" mechanism is being negotiated, if the credentials
field is present, it contains an authorization identity of the authzId
form described below.

The authorization identity is a string in the UTF-8 character set,
corresponding to the following ABNF [7]:

   ; Specific predefined authorization (authz) id schemes are
   ; defined below -- new schemes may be defined in the future.

   authzId    = dnAuthzId / uAuthzId

   ; distinguished-name-based authz id.
   dnAuthzId  = "dn:" dn
   dn         = utf8string    ; with syntax defined in RFC 2253

   ; unspecified userid, UTF-8 encoded.
   uAuthzId   = "u:" userid
   userid     = utf8string    ; syntax unspecified

A utf8string is defined to be the UTF-8 encoding of one or more ISO
10646 characters.

All servers which support the storage of authentication credentials,
such as passwords or certificates, in the directory MUST support the
dnAuthzId choice.

The uAuthzId choice allows for compatibility with client applications
which wish to authenticate to a local directory but do not know their
own Distinguished Name or have a directory entry.  The format of the
string is defined as only a sequence of UTF-8 encoded ISO 10646 charac-
ters, and further interpretation is subject to prior agreement between
the client and server.

For example, the userid could identify a user of a specific directory

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service, or be a login name or the local-part of an RFC 822 email
address. In general a uAuthzId MUST NOT be assumed to be globally

Additional authorization identity schemes MAY be defined in future docu-

9.  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.

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.
Nor does it secure the data from inspection by the server administra-
tors.  Once established, TLS only provides for and ensures confidential-
ity and integrity of the operations and data in transit over the LDAP
association, 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. Additionally, an active-intermediary attacker
can remove the Start TLS extended operation from the supportedExtension
attribute of the root DSE. Therefore, both parties SHOULD independently
ascertain and consent to the security level achieved once TLS is esta-
blished and before begining use of the TLS connection. For example, the
security level of the TLS connection might have been negotiated down to

Clients SHOULD either warn the user when the security level achieved
does not provide confidentiality and/or integrity protection, or be con-
figurable to refuse to proceed without an acceptable level of security.

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 and/or integrity is
required, as well as elect whether and when client authentication via
TLS is required.

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10.  Acknowledgements

The authors thank Tim Howes, Paul Hoffman, John Kristian, Shirish Rai,
Jonathan Trostle, and Harald Alvestrand for their contributions to this

11.  References

[LDAPv3]       M. Wahl, S. Kille and T. Howes. "Lightweight Directory
               Access Protocol (v3)". RFC 2251.

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

[SASL]         J. Myers. "Simple Authentication and Security Layer
               (SASL)". RFC 2222.

[TLS]          Tim Dierks, C. Allen. "The TLS Protocol Version 1.0".
               INTERNET-DRAFT, Work In Progress. draft-ietf-tls-

12.  Authors' Addresses

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

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   Innosoft International, Inc.
   8911 Capital of Texas Hwy, Suite 4140
   Austin, TX 78759

   Phone: +1 626 919 3600
   EMail:  Mark.Wahl@innosoft.com

Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an

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