TLS Working Group S. Santesson (Microsoft)
INTERNET-DRAFT A. Medvinsky (Microsoft)
Intended Category: Standards track J. Ball (Microsoft)
Expires July 2006 January 2006
TLS User Mapping Extension
<draft-santesson-tls-ume-01.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. In particular
one such mapping is defined, the UpnDomainHint, which may be used by
a server to locate a user in a directory database.
Santesson, et. all [Page 1]
INTERNET DRAFT TLS User Mapping extension January 2006
Table of Contents
1 Introduction ................................................ 2
2 User mapping extension ...................................... 3
3 User mapping handshake protocol ............................. 3
4 Message flow ................................................ 6
5 Security Considerations ..................................... 7
6 References .................................................. 8
Appendix A. IPR Disclosure ..................................... 9
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 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 extension (user_mapping) is sent in the Client Hello message.
Per convention defined in RFC3546 [N3], 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.
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,
and store it in the context for use when mapping the certificate to
Santesson, et. all [Page 2]
INTERNET DRAFT TLS User Mapping extension January 2006
the user's directory account.
The reason the mapping data itself is not placed in the extension
portion of the ClientHello 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 servers public key.
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].
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 and has no data associated with it.
enum {
user_mapping(n), (65535)
} ExtensionType;
3 User mapping handshake protocol
A new HandshakeType (user_mapping_data) is defined to accommodate
communication of generic user mapping data.
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;
Santesson, et. all [Page 3]
INTERNET DRAFT TLS User Mapping extension January 2006
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:
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 [N4]
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.
Santesson, et. all [Page 4]
INTERNET DRAFT TLS User Mapping extension January 2006
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.
Santesson, et. all [Page 5]
INTERNET DRAFT TLS User Mapping extension January 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. The
"extension_data" field of this extension SHALL be empty.
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. The "extension_data"
field of this extension SHALL be empty.
After negotiation of the use of user mapping has been successfully
completed (by exchanging hellos 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].
Santesson, et. all [Page 6]
INTERNET DRAFT TLS User Mapping extension January 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 safely is to use the UPN to
locate and extract a certificate of the claimed user from a 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 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 7]
INTERNET DRAFT TLS User Mapping extension January 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] S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen,
T. Wright, "Transport Layer Security (TLS) Extensions",
RFC 3546, June 2003.
[N4] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, November 1987.
Santesson, et. all [Page 8]
INTERNET DRAFT TLS User Mapping extension January 2006
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
Santesson, et. all [Page 9]
INTERNET DRAFT TLS User Mapping extension January 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 July 2006
Santesson, et. all [Page 10]
