INTERNET-DRAFT                                  S. Santesson (Microsoft)
Updates: 2246, 4346 (once approved)             A. Medvinsky (Microsoft)
Intended Category: Standards track                   J. Ball (Microsoft)
Expires September 2006                                        March 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 supplemental data handshake message
   defined in RFC TBD. 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

   (NOTE TO RFC EDITOR:  Replace "RFC TBD" with the RFC number assigned
   to draft-santesson-tls-supp-00.txt)

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Table of Contents

   1  Introduction ................................................    2
   2  User mapping extension ......................................    3
   3  User mapping handshake exchange .............................    4
   4  Message flow ................................................    7
   5  Security Considerations .....................................    8
   6  References ..................................................    9
   7 IANA Considerations ... ......................................    9
   Authors' Addresses .............................................   10
   Acknowledgements ...............................................   10
   Disclaimer .....................................................   11
   Copyright Statement ............................................   11

1.  Introduction

   This specification defines a TLS extension and a payload for the
   SupplementalData handshake message, defined in RFC TBD [N6], 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 however several drawbacks 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 as 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 data in the TLS handshake.

   If the new extension is understood, the client will inject
   UserMappingDataList data in the SupplementalData handshake message

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   prior to the Client's Certificate message. The server will then parse
   this message, extracting the client's domain, and store it in the
   context for use when mapping the certificate to the user's directory

   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 mapping information were to be considered
   confidential, the addition of a new user mapping message type could
   allow the data to be encrypted using the server's public key.

2  User mapping extension

   A new extension type (user_mapping(TBD)) 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(TBD), (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.

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   The list of user_mapping_types included in a client hello SHALL
   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 exchange

   The underlying structure of the SupplementalData handshake message,
   used to carry information defined in this section, is defined in RFC
   TBD [N6].

   A new SupplementalDataType [N6] 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 data type carries one or more unauthenticated
   hints, UserMappingDataList, 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 certificate.

      struct {
            SupplementalDataType supp_data_type;
            select(SupplementalDataType) {
               case user_mapping_data: UserMappingDataList;
      } SupplementalDataEntry;

      enum {
            user_mapping_data(TBD), (65535)
      } SupplementalDataType;

   The user_mapping_data(n) enumeration results in a new supplemental
   data type UserMappingDataList with the following structure:

      enum {

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             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:
      } UserMappingData;

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

   The user_principal_name parameter, when specified, SHALL contain a
   Unicode UPN, encoded as a UTF-8 string in the following 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 1123.

   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.

   This document does not specify how the server stores the
   user_principal_name, or how exactly it might be used to locate a

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   certificate.  For instance, it might be appropriate to do a case-
   insensitive lookup.  It is RECOMMENDED that the server processes the
   user_principal_name with a stringprep profile [N7] appropriate for
   the identity in question, such as Nameprep [N8] for the portion
   domain portion of UPN, SASLprep [N9] for the user portion of the UPN
   and stringprep appendix B.3 [N7] as mapping table for case folding.

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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 "SupplementalData" message containing
   the "UserMappingDataList" before the "Certificate" message. The
   message flow is illustrated in Fig. 1 below.

      Client                                               Server

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

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

       /* with UserMappingDataList */
      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 in the
        supplemental data message if it is agreed upon in the hello
        message exchange, preventing 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.

   [N6]      S. Santesson, "TLS Handshake Message for Supplementary
             Data", RFC TBD (currently: draft-santesson-tls-supp-00,
             Date 2006.

   [N7]      P. Hoffman, M. Blanchet, "Preparation of Internationalized
             Strings (stringprep)", RFC 3454, December 2002.

   [N8]      P. Hoffman, M. Blanchet, "Nameprep: A Stringprep Profile
             for Internationalized Domain Names (IDN)", RFC 3491,
             March 2003.

   [N9]      K. Zeilenga, "SASLprep: Stringprep Profile for User Names
             and Passwords", RFC 4013, February 2005.

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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Authors' Addresses

   Stefan Santesson
   Finlandsgatan 30
   164 93 KISTA

   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)


   The authors extend a special thanks to Russ Housley, Eric Resocorla
   and Paul Leach for their substantial contributions.

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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 September 2006

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