INTERNET-DRAFT                                  S. Santesson (Microsoft)
Updates: 2246, 4346 (once approved)             A. Medvinsky (Microsoft)
Intended Category: Standards track                   J. Ball (Microsoft)
Expires August 2006                                        February 2006

                       TLS User Mapping Extension

Status of this Memo

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   This document specifies a TLS extension that enables clients to send
   generic user mapping data in a new handshake message. One such
   mapping is defined, the UpnDomainHint, which may be used by a server
   to locate a user in a directory database. Other mappings may be
   defined in other documents in the future.

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Table of Contents

   1  Introduction ................................................    2
   2  User mapping extension ......................................    3
   3  User mapping handshake protocol .............................    4
   4  Message flow ................................................    6
   5  Security Considerations .....................................    7
   6  References ..................................................    8
   7 IANA Considerations ... ......................................    8
   Authors' Addresses .............................................    9
   Disclaimer .....................................................   10
   Copyright Statement ............................................   10

1.  Introduction

   This specification documents a TLS extension and a handshake message,
   which has been defined and implemented by Microsoft to accommodate
   mapping of users to their user accounts when using TLS client
   authentication as the authentication method.

   The UPN (User Principal Name) is a name form defined by Microsoft
   which specifies a user's entry in a directory in the form of
   userName@domainName.  Traditionally Microsoft has relied on such UPN
   names to be present in the client certificate when logging on to a
   domain account.

   This has several drawbacks however since it prevents the use of
   certificates with an absent UPN and also requires re-issuance of
   certificates or issuance of multiple certificates to reflect account
   changes or creation of new accounts.

   The TLS extension defined in this document provide a significant
   improvement to this situation since it allows a single certificate to
   be mapped to one or more accounts of the user and does not require
   the certificate to contain a UPN.

   The new TLS extension (user_mapping) is sent in the client hello
   message. Per convention defined in RFC 4366 [N4], the server places
   the same extension (user_mapping) in the server hello message, to
   inform the client that the server understands this extension. If the
   server does not understand the extension, it will respond with a
   server hello omitting this extension and the client will proceed as
   normal, ignoring the extension, and not include the
   UserMappingDataList message in the TLS handshake.

   If the new extension is understood, the client will inject a new
   handshake message prior to the Client's Certificate message. The
   server will then parse this message, extracting the client's domain,

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   and store it in the context for use when mapping the certificate to
   the user's directory account.

   No other modifications to the protocol are required. The messages are
   detailed in the following sections.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [STDWORDS].

   The syntax for the TLS User Mapping extension is defined using the
   TLS Presentation Language, which is specified in Section 4 of [N2].

1.2  Design considerations

   The reason the mapping data itself is not placed in the extension
   portion of the client hello is to prevent broadcasting this
   information to servers that don't understand the extension.
   Additionally, if new mapping information were to be considered
   confidential, the addition of a new handshake message allows the data
   to be encrypted using the server's public key.

2  User mapping extension

   A new extension type (user_mapping(n)) is added to the Extension used
   in both the client hello and server hello messages. The extension
   type is specified as follows.

      enum {
           user_mapping(n), (65535)
      } ExtensionType;

   The "extension_data" field of this extension SHALL contain
   "UserMappingTypeList" with a list of supported hint types where:

      struct {
            UserMappingType user_mapping_types<1..2^8-1>
      } UserMappingTypeList;

   Enumeration of hint types (user_mapping_types) defined in this
   document is provided in section 3.

   The list of user_mapping_types included in a client hello SHALL

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   signal the hint types supported by the client. The list of
   user_mapping_types included in the server hello SHALL signal the hint
   types preferred by the server.

   If none of the hint types listed by the client is supported by the
   server, the server SHALL omit the user_mapping extension in the
   server hello.

   When the user_mapping extension is included in the server hello, the
   list of hint types in "UserMappingTypeList" SHALL be either equal to,
   or a subset of, the list provided by the client.

3  User mapping handshake protocol

   A new HandshakeType (user_mapping_data) is defined to accommodate
   communication of generic user mapping data. See RFC 2246 (TLS 1.0)
   [N2] and RFC 4346 (TLS 1.1) [N3] for other handshake types.

   The information in this handshake message carries an unauthenticated
   hint, inserted by the client side. Upon receipt and successful
   completion of the TLS handshake, the server MAY use this hint to
   locate the user's account from which user information and credentials
   MAY be retrieved to support authentication based on the client

      enum {
      } HandshakeType;

   The user_mapping_data(n) enumeration results in a new Handshake
   Message UserMappingDataList with the following structure:

      enum {
             upn_domain_hint(0), (255)
      } UserMappingType;

      struct {
             opaque user_principal_name<0..2^16-1>;
             opaque domain_name<0..2^16-1>;
      } UpnDomainHint;

      struct {
             UserMappingType user_mapping_version
             select(UserMappingType) {
                   case upn_domain_hint:

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      } UserMappingData;

         UserMappingData user_mapping_data_list<1..2^16-1>;

   The user_principal_name parameter, when specified, SHALL be specified
   in the form:


   For example the UPN '' represents user 'foo' at domain

   The domain_name parameter, when specified, SHALL contain a domain
   name in the "preferred name syntax," as specified by RFC 1034 [N5]

   The UpnDomainHint MUST at least contain a non empty
   user_principal_name or a non empty domain_name. The UpnDomainHint MAY
   contain both user_principal_name and domain_name.

   The UserMappingData structure contains a single mapping of type
   UserMappingType.  This structure can be leveraged to define new types
   of user mapping hints in the future.  The UserMappingDataList MAY
   carry multiple hints; it is defined as a vector of UserMappingData

   No preference is given to the order in which hints are specified in
   this vector.  If the client sends more then one hint then the Server
   SHOULD use the applicable mapping supported by the server.

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4  Message flow

   In order to negotiate to send user mapping data to a server in
   accordance with this specification, clients MUST include an extension
   of type "user_mapping" in the (extended) client hello, which SHALL
   contain a list of supported hint types.

   Servers that receive an extended client hello containing a
   "user_mapping" extension, MAY indicate that they are willing to
   accept user mapping data by including an extension of type
   "user_mapping" in the (extended) server hello, which SHALL contain a
   list of preferred hint types.

   After negotiation of the use of user mapping has been successfully
   completed (by exchanging hello messages including "user_mapping"
   extensions), clients MAY send a "UserMappingDataList" message before
   the "Certificate" message. The message flow is illustrated in Fig. 1

      Client                                               Server

       /* with user_mapping ext */ -------->

                                      /* with user-mapping ext */
                                   <--------      ServerHelloDone

      Finished                     -------->
                                   <--------             Finished
      Application Data             <------->     Application Data

             Fig. 1 - Message flow with user mapping data

   * Indicates optional or situation-dependent messages that are not
   always sent according to RFC 2246 [N2] and RFC 4346 [N3].

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5  Security Considerations

   The UPN sent in the UserMappingDataList is unauthenticated data that
   MUST NOT be treated as a trusted identifier. Authentication of the
   user represented by that UPN MUST rely solely on validation of the
   client certificate. One way to do this in the Microsoft environment
   is to use the UPN to locate and extract a certificate of the claimed
   user from the trusted directory and subsequently match this
   certificate against the validated client certificate from the TLS

   As the client is the initiator of this TLS extension, it needs to
   determine when it is appropriate to send the User Mapping
   Information. It may not be prudent to broadcast this information to
   just any server at any time, as it can reveal network infrastructure
   the client and server are using.

   To avoid superfluously sending this information, two techniques
   SHOULD be used to control its dissemination.

      - The client SHOULD only send the UserMappingDataList handshake
        message if it is agreed upon in the hello message exchange,
        the information from being sent to a server that doesn't
        understand the User Mapping Extension.

      - The client SHOULD further only send this information if the
        server belongs to a domain to which the client intends to
        authenticate using the UPN as identifier.

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6 References

   Normative references:

   [N1]        S. Bradner, "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [N2]        T. Dierks, C. Allen, "The TLS Protocol Version 1.0",
               RFC 2246, January 1999.

   [N3]        T. Dierks, E. Rescorla, "The TLS Protocol Version 1.1",
               RFC 4346, January 2006.

   [N4]        S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen,
               T. Wright, "Transport Layer Security (TLS) Extensions",
               RFC 4366, February 2006.

   [N5]        Mockapetris, P., "Domain Names - Concepts and
               Facilities", STD 13, RFC 1034, November 1987.

7 IANA Considerations

   IANA needs to establish a registry for TLS UserMappingType values.
   The first entry in the registry is upn_domain_hint(0). TLS
   UserMappingType values in the inclusive range 0-63 (decimal) are
   assigned via RFC 2434 [IANA] Standards Action.  Values from the
   inclusive range 64-223 (decimal) are assigned via RFC 2434
   Specification Required.  Values from the inclusive range 224-255
   (decimal) are reserved for RFC 2434 Private Use.

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Appendix A. IPR Disclosure


Authors' Addresses

   Stefan Santesson
   Tuborg Boulevard 12
   2900 Hellerup

   EMail: stefans(at)

   Ari Medvinsky
   One Microsoft Way
   Redmond, WA 98052-6399

   Email: arimed(at)

   Joshua Ball
   One Microsoft Way
   Redmond, WA 98052-6399

   Email: joshball(at)

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   This document and the information contained herein are provided on an

Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

Expires August 2006

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