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RFC 4681 - TLS User Mapping Extension

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Network Working Group                                       S. Santesson
Request for Comments: 4681                                  A. Medvinsky
Updates: 4346                                                    J. Ball
Category: Standards Track                                      Microsoft
                                                            October 2006

                       TLS User Mapping Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document specifies a TLS extension that enables clients to send
   generic user mapping hints in a supplemental data handshake message
   defined in RFC 4680.  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.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................2
      1.2. Design Considerations ......................................2
   2. User Mapping Extension ..........................................3
   3. User Mapping Handshake Exchange .................................3
   4. Message Flow ....................................................5
   5. Security Considerations .........................................6
   6. UPN Domain Hint (Informative) ...................................7
   7. IANA Considerations .............................................8
   8. Normative References ............................................9
   9. Acknowledgements ................................................9

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 4680 [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 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 [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(6)) 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(6), (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.

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
   4680 [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;
            uint16 supp_data_length;
            select(SupplementalDataType) {
               case user_mapping_data: UserMappingDataList;
      } SupplementalDataEntry;

      enum {
            user_mapping_data(0), (65535)
      } SupplementalDataType;

   The user_mapping_data(0) enumeration results in a new supplemental
   data type UserMappingDataList with the following structure:

      enum {
      } UserMappingType;

      struct {
             UserMappingType user_mapping_version;
             uint16 user_mapping_length;
             select(UserMappingType) { }
      } UserMappingData;

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

      This field is the length (in bytes) of the data selected by

   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 than 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.

4.  Message Flow

   In order to negotiate sending 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 Figure 1 below.

      Client                                               Server

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

       /* with UserMappingDataList */
      Finished                     -------->
                                   <--------             Finished
      Application Data             <------->     Application Data

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

              Figure 1.  Message Flow with User Mapping Data

   The server MUST expect and gracefully handle the case where the
   client chooses not to 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

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 cases, the double handshake technique described
   in [N6] can be used to provide protection for the user mapping hint

6.  UPN Domain Hint (Informative)

   This specification provides an 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
   that specifies a user's entry in a directory in the form
   userName@domainName.  Traditionally, Microsoft has relied on the
   presence of such a name form to be present in the client certificate
   when logging on to a domain account.  However, this has 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, provides 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
   proprietary 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;
             uint16 user_mapping_length;
             select(UserMappingType) {
                   case upn_domain_hint: UpnDomainHint;
      } UserMappingData;

   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
   [N5] 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.

7.  IANA Considerations

   IANA has taken the following actions:

   1) Created an entry, user_mapping(6), in the existing registry for
      ExtensionType (defined in RFC 4366 [N4]).

   2) Created an entry, user_mapping_data(0), in the new registry for
      SupplementalDataType (defined in RFC 4680).

   3) Established 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.

8.  Normative References

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

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

   [N3]   Dierks, T. and E. Rescorla, "The Transport Layer Security
          (TLS) Protocol Version 1.1", RFC 4346, April 2006.

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

   [N5]   Mockapetris, P., "Domain names - concepts and facilities", STD
          13, RFC 1034, November 1987.

   [N6]   Santesson, S., "TLS Handshake Message for Supplemental Data",
          RFC 4680, October 2006.

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

   [N8]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
          Considerations Section in RFCs", BCP 26, RFC 2434, October

9.  Acknowledgements

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

Authors' Addresses

   Stefan Santesson
   Finlandsgatan 30
   164 93 KISTA

   EMail: stefans@microsoft.com

   Ari Medvinsky
   One Microsoft Way
   Redmond, WA 98052-6399

   EMail: arimed@microsoft.com

   Joshua Ball
   One Microsoft Way
   Redmond, WA 98052-6399

   EMail: joshball@microsoft.com

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