



TLS Working Group                               S. Santesson (Microsoft)
INTERNET-DRAFT                                  A. Medvinsky (Microsoft)
Intended Category: Informational                     J. Ball (Microsoft)
Expires June 2006                                          December 2005


                       TLS User Mapping Extension
                    <draft-santesson-tls-ume-00.txt>


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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Abstract

   This document specifies a TLS extension that enables clients to send
   generic user mapping data in a new handshake message. In particular
   one such mapping is defined, the UpnDomainHint, which may be used by
   a server to locate a user in a directory database.







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Table of Contents

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

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 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 extension (user_mapping) is sent in the Client Hello message.
   Per convention defined in RFC3546 [N3], 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 non-extended
   Server Hello message and the client will proceed as normal, ignoring
   the extension.

   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,
   and store it in the context for use when mapping the certificate to



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   the user's directory account.

   The reason the mapping data itself is not placed in the extension
   portion of the ClientHello 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 servers public key.

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [STDWORDS].


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 and has no data associated with it.


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


3  User mapping handshake protocol

   A new HandshakeType (user_mapping_data) is defined to accommodate
   communication of generic user mapping data.

   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
   certificate.


      enum {
             user_mapping_data(n),(255)
      } HandshakeType;



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


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

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

      struct {
             UserMappingType user_mapping_version
             select(UserMappingType) {
                   case UpnDomainHint:
                        UpnDomainHint;
             }
      } UserMappingData;


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

      user@domain

   For example the UPN 'foo@example.com' represents user 'foo' at domain
   'example.com'.

   The domain_name parameter, when specified, SHALL contain a domain
   name in the "preferred name syntax," as specified by RFC 1034 [N4]

   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.















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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.  The
   "extension_data" field of this extension SHALL be empty.

   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.  The "extension_data"
   field of this extension SHALL be empty.

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

      Client                                               Server

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

                                                      ServerHello
                                      /* with user-mapping ext */
                                                     Certificate*
                                               ServerKeyExchange*
                                              CertificateRequest*
                                   <--------      ServerHelloDone

      UserMappingData
      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                     -------->
                                               [ChangeCipherSpec]
                                   <--------             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].







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5  Security Considerations

   The UPN sent in the UserMappingData 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 safely is to use the UPN to locate
   and extract a certificate of the claimed user from a directory and
   subsequently match this certificate against the validated client
   certificate from the TLS handshake.


   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 UserMappingData handshake
        message if it is agreed upon in the Hello 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]        S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen,
               T. Wright, "Transport Layer Security (TLS) Extensions",
               RFC 3546, June 2003.

   [N4]        Mockapetris, P., "Domain Names - Concepts and
               Facilities", STD 13, RFC 1034, November 1987.



































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Appendix A. IPR Disclosure

   TBD

Authors' Addresses


   Stefan Santesson
   Microsoft
   Tuborg Boulevard 12
   2900 Hellerup
   Denmark

   EMail: stefans(at)microsoft.com


   Ari Medvinsky
   Microsoft
   One Microsoft Way
   Redmond, WA 98052-6399

   Email: arimed(at)microsoft.com


   Joshua Ball
   Microsoft
   One Microsoft Way
   Redmond, WA 98052-6399

   Email: joshball(at)microsoft.com





















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Disclaimer

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2005).

   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 June 2006






























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