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+INTERNET-DRAFT S. Santesson (Microsoft)
+Updates: 2246, 4346 (once approved) A. Medvinsky (Microsoft)
+Intended Category: Standards track J. Ball (Microsoft)
+Expires August 2006 February 2006
+
+
+ TLS User Mapping Extension
+ <draft-santesson-tls-ume-02.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.
+
+ This document may not be modified, and derivative works of it may not
+ be created, except to publish it as an RFC and to translate it into
+ languages other than English.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF), its areas, and its working groups. Note that
+ other groups may also distribute working documents as Internet-
+ Drafts.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than a "work in progress."
+
+ The list of current Internet-Drafts can be accessed at
+ http://www.ietf.org/1id-abstracts.html
+
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html
+
+
+Abstract
+
+ This document specifies a TLS extension that enables clients to send
+ generic user mapping data in a new handshake message. 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 future.
+
+
+
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+Santesson, et. all [Page 1]
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+Table of Contents
+
+ 1 Introduction ................................................ 2
+ 2 User mapping extension ...................................... 3
+ 3 User mapping handshake protocol ............................. 4
+ 4 Message flow ................................................ 6
+ 5 Security Considerations ..................................... 7
+ 6 References .................................................. 8
+ 7 IANA Considerations ... ...................................... 8
+ Authors' Addresses ............................................. 9
+ Disclaimer ..................................................... 10
+ Copyright Statement ............................................ 10
+
+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 TLS 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 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 message in the TLS handshake.
+
+ 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,
+
+
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+ 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",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ 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 new mapping information were to be considered
+ confidential, the addition of a new handshake message allows the data
+ to be encrypted using the server's public key.
+
+
+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.
+
+
+ enum {
+ user_mapping(n), (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
+
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+ 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 protocol
+
+ A new HandshakeType (user_mapping_data) 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 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;
+
+
+ The user_mapping_data(n) enumeration results in a new Handshake
+ Message UserMappingDataList with the following structure:
+
+
+ enum {
+ 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:
+
+
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+ UpnDomainHint;
+ }
+ } UserMappingData;
+
+ struct{
+ UserMappingData user_mapping_data_list<1..2^16-1>;
+ }UserMappingDataList;
+
+
+
+ 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 [N5]
+
+ 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
+ structures.
+
+ 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.
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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 "UserMappingDataList" 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
+
+ UserMappingDataList
+ 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] 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
+ 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 UserMappingDataList handshake
+ 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.
+
+
+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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+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 (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.
+
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+Expires August 2006
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