INTERNET-DRAFT S. Santesson (Microsoft) Updates: 2246, 4346 (once approved) A. Medvinsky (Microsoft) Intended Category: Standards track J. Ball (Microsoft) Expires September 2006 March 2006 TLS User Mapping Extension <draft-santesson-tls-ume-04.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. 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 supplemental data handshake message defined in RFC TBD. 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. (NOTE TO RFC EDITOR: Replace "RFC TBD" with the RFC number assigned to draft-santesson-tls-supp-00.txt) Santesson, et. all [Page 1]
INTERNET DRAFT TLS User Mapping extension March 2006 Table of Contents 1 Introduction ................................................ 2 2 User mapping extension ...................................... 3 3 User mapping handshake exchange ............................. 4 4 Message flow ................................................ 7 5 Security Considerations ..................................... 8 6 References .................................................. 9 7 IANA Considerations ... ...................................... 9 Authors' Addresses ............................................. 10 Acknowledgements ............................................... 10 Disclaimer ..................................................... 11 Copyright Statement ............................................ 11 1. Introduction 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 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 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 defined in this document 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 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 Santesson, et. all [Page 2]
INTERNET DRAFT TLS User Mapping extension March 2006 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", "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 mapping information were to be considered confidential, the addition of a new user mapping message type could allow the data to be encrypted using the server's public key. 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. Santesson, et. all [Page 3]
INTERNET DRAFT TLS User Mapping extension March 2006 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 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(n) enumeration results in a new supplemental data type UserMappingDataList with the following structure: enum { Santesson, et. all [Page 4]
INTERNET DRAFT TLS User Mapping extension March 2006 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: UpnDomainHint; } } UserMappingData; struct{ UserMappingData user_mapping_data_list<1..2^16-1>; }UserMappingDataList; The user_principal_name parameter, when specified, SHALL contain a Unicode UPN, encoded as a UTF-8 string in the following 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 1123. 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. This document does not specify how the server stores the user_principal_name, or how exactly it might be used to locate a Santesson, et. all [Page 5]
INTERNET DRAFT TLS User Mapping extension March 2006 certificate. For instance, it might be appropriate to do a case- insensitive lookup. It is RECOMMENDED that the server processes the user_principal_name with a stringprep profile [N7] appropriate for the identity in question, such as Nameprep [N8] for the portion domain portion of UPN, SASLprep [N9] for the user portion of the UPN and stringprep appendix B.3 [N7] as mapping table for case folding. Santesson, et. all [Page 6]
INTERNET DRAFT TLS User Mapping extension March 2006 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 ClientHello /* with user_mapping ext */ --------> ServerHello /* with user-mapping ext */ Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone SupplementalData /* with 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]. Santesson, et. all [Page 7]
INTERNET DRAFT TLS User Mapping extension March 2006 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 in the supplemental data 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. Santesson, et. all [Page 8]
INTERNET DRAFT TLS User Mapping extension March 2006 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-00, Date 2006. [N7] P. Hoffman, M. Blanchet, "Preparation of Internationalized Strings (stringprep)", RFC 3454, December 2002. [N8] P. Hoffman, M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, March 2003. [N9] K. Zeilenga, "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005. 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. Santesson, et. all [Page 9]
INTERNET DRAFT TLS User Mapping extension March 2006 Authors' Addresses Stefan Santesson Microsoft Finlandsgatan 30 164 93 KISTA Sweden EMail: stefans(at)microsoft.com Ari Medvinsky Microsoft One Microsoft Way Redmond, WA 98052-6399 USA Email: arimed(at)microsoft.com Joshua Ball Microsoft One Microsoft Way Redmond, WA 98052-6399 USA Email: joshball(at)microsoft.com Acknowledgements The authors extend a special thanks to Russ Housley, Eric Resocorla and Paul Leach for their substantial contributions. Santesson, et. all [Page 10]
INTERNET DRAFT TLS User Mapping extension March 2006 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. Expires September 2006 Santesson, et. all [Page 11]
