Internet DRAFT - draft-santesson-tls-ume


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

                       TLS User Mapping Extension

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   This document specifies a TLS extension that enables clients to send
   generic user mapping hints in a supplemental data handshake message
   defined in RFC TBD. One such mapping hint is defined in an
   informative section, the UpnDomainHint, which may be used by a server
   to locate a user in a directory database. Other mapping hints may be
   defined in other documents in the future.

   (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 ................................................    6
   5  Security Considerations .....................................    8
   6  UPN domain hint (Informative) ...............................    9
   7  References ..................................................   10
   8  IANA Considerations .........................................   10
   Authors' Addresses .............................................   11
   Acknowledgements ...............................................   11
   Disclaimer .....................................................   12
   Copyright Statement ............................................   12

1.  Introduction

   This document has a normative part and an informative part. Sections
   2-5 are normative. Section 6 is informative.

   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 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
   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",

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   document are to be interpreted as described in RFC 2119 [N1].

   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.

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.

   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.

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

      enum {
      } UserMappingType;

      struct {
             UserMappingType user_mapping_version
             select(UserMappingType) { }
      } UserMappingData;

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

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   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.

   Implementations MAY support the UPN domain hint as specified in
   section 6 of this document.  Implementations MAY also support other
   user mapping types as they are defined.  Definitions of standards-
   track user mapping types must include a discussion of
   internationalization considerations.

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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].

   The server MUST expect and gracefully handle the case where the

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   client chooses to not send any supplementalData handshake message
   even after successful negotiation of extensions. The client MAY at
   its own discretion decide that the user mapping hint it initially
   intended to send no longer is relevant for this session. One such
   reason could be that the server certificate fails to meet certain

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5  Security Considerations

   The user mapping hint sent in the UserMappingDataList is
   unauthenticated data that MUST NOT be treated as a trusted
   identifier. Authentication of the user represented by that user
   mapping hint MUST rely solely on validation of the client
   certificate. One way to do this is to use the user mapping hint 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 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 a user mapping hint
   to just any server at any time.

   To avoid superfluously sending user mapping hints, clients SHOULD
   only send this information if it recognizes the server as a
   legitimate recipient. Recognition of the server can be done in many
   ways. One way to do this could be to recognize the name and address
   of the server.

   In some cases, the user mapping hint may itself be regarded as
   sensitive. In such case the double handshake technique described in
   [N6] can be used to provide protection for the user mapping hint

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6  UPN domain hint (Informative)

   This specification provides informative description of one user
   mapping hint type for Domain Name hints and User Principal Name
   hints. Other hint types may be defined in other documents in the

   The User Principal Name (UPN) in this hint type represents a name
   which specifies a user's entry in a directory in the form
   userName@domainName. Traditionally Microsoft has relied on such name
   form 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 in combination with the defined hint type 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 domain_name field MAY be used when only domain information is
   needed, e.g. where a user have accounts in multiple domains using the
   same username name, where that user name is known from another source
   (e.g. from the client certificate). When the user name is also
   needed, the user_principal_name field MAY be used to indicate both
   username and domain name. If both fields are present, then the server
   can make use of whichever one it chooses.

      enum {
             upn_domain_hint(64), (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;

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   The user_principal_name field, when specified, SHALL be of the form
   "user@domain", where "user" is a UTF-8 encoded Unicode string that
   does not contain the "@" character, and "domain" is a domain name
   meeting the requirements in the following paragraph.

   The domain_name field, when specified, SHALL contain a domain name in
   the usual text form: in other words, a sequence of one or more domain
   labels separated by ".", each domain label starting and ending with
   an alphanumeric character and possibly also containing "-"
   characters.  This field is an "IDN-unaware domain name slot" as
   defined in RFC 3490 [N7] and therefore, domain names containing non-
   ASCII characters have to be processed as described in RFC 3490 before
   being stored in this field.

   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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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-02,
             Date 2006.

   [N7]      P. Faltstrom, P. Hoffman, A. Costello, "Internationalizing
             Domain Names in Applications (IDNA)", RFC 3490, March 2003

   [N8]      T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
             Considerations Section in RFCs", RFC 2434, October 1998

7 IANA Considerations

   IANA needs to take the following actions:

   1) Create an entry, user_mapping(TBD), in the existing registry for
   ExtensionType (defined in RFC 4366 [N4]).

   2) Create an entry, user_mapping_data(TBD), in the new registry for
   SupplementalDataType (defined in draft-santesson-tls-supp-02).

   3) Establish a registry for TLS UserMappingType values.  The first
   entry in the registry is upn_domain_hint(64). TLS UserMappingType
   values in the inclusive range 0-63 (decimal) are assigned via RFC
   2434 [N8] 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 November 2006

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