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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

Santesson, et al. Standards Track [Page 1] RFC 4681 TLS User Mapping Extension October 2006

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.

Santesson, et al. Standards Track [Page 2] RFC 4681 TLS User Mapping Extension October 2006

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

Santesson, et al. Standards Track [Page 3] RFC 4681 TLS User Mapping Extension October 2006

 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.

Santesson, et al. Standards Track [Page 4] RFC 4681 TLS User Mapping Extension October 2006

 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.

Santesson, et al. Standards Track [Page 5] RFC 4681 TLS User Mapping Extension October 2006

    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.

Santesson, et al. Standards Track [Page 6] RFC 4681 TLS User Mapping Extension October 2006

 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;

Santesson, et al. Standards Track [Page 7] RFC 4681 TLS User Mapping Extension October 2006

    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.

Santesson, et al. Standards Track [Page 8] RFC 4681 TLS User Mapping Extension October 2006

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.

Santesson, et al. Standards Track [Page 9] RFC 4681 TLS User Mapping Extension October 2006

Authors' Addresses

 Stefan Santesson
 Finlandsgatan 30
 164 93 KISTA
 Ari Medvinsky
 One Microsoft Way
 Redmond, WA 98052-6399
 Joshua Ball
 One Microsoft Way
 Redmond, WA 98052-6399

Santesson, et al. Standards Track [Page 10] RFC 4681 TLS User Mapping Extension October 2006

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Santesson, et al. Standards Track [Page 11]

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