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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 November 2006 May 2006
TLS User Mapping Extension
<draft-santesson-tls-ume-07.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 hints in a supplemental data handshake message
defined in RFC TBD. 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.
(NOTE TO RFC EDITOR: Replace "RFC TBD" with the RFC number assigned
to draft-santesson-tls-supp-00.txt)
Santesson, et. all [Page 1]
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Table of Contents
1 Introduction ................................................ 2
2 User mapping extension ...................................... 3
3 User mapping handshake exchange ............................. 4
4 Message flow ................................................ 6
5 Security Considerations ..................................... 8
6 UPN domain hint (Informative) ............................... 9
7 References .................................................. 10
8 IANA Considerations ......................................... 10
Authors' Addresses ............................................. 11
Acknowledgements ............................................... 11
Disclaimer ..................................................... 12
Copyright Statement ............................................ 12
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 TBD [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",
Santesson, et. all [Page 2]
INTERNET DRAFT TLS User Mapping extension May 2006
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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(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.
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.
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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(TBD) enumeration results in a new supplemental
data type UserMappingDataList with the following structure:
enum {
(255)
} UserMappingType;
struct {
UserMappingType user_mapping_version
select(UserMappingType) { }
} UserMappingData;
struct{
UserMappingData user_mapping_data_list<1..2^16-1>;
}UserMappingDataList;
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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.
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.
Santesson, et. all [Page 5]
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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 "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].
The server MUST expect and gracefully handle the case where the
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INTERNET DRAFT TLS User Mapping extension May 2006
client chooses to not 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
requirements.
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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 case the double handshake technique described in
[N6] can be used to provide protection for the user mapping hint
information.
Santesson, et. all [Page 8]
INTERNET DRAFT TLS User Mapping extension May 2006
6 UPN domain hint (Informative)
This specification provides 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
future.
The User Principal Name (UPN) in this hint type represents a name
which specifies a user's entry in a directory in the form
userName@domainName. Traditionally Microsoft has relied on such name
form 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 in combination with the defined hint type 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 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
select(UserMappingType) {
case upn_domain_hint:
UpnDomainHint;
}
} UserMappingData;
Santesson, et. all [Page 9]
INTERNET DRAFT TLS User Mapping extension May 2006
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 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.
Santesson, et. all [Page 10]
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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.
[N6] S. Santesson, "TLS Handshake Message for Supplementary
Data", RFC TBD (currently: draft-santesson-tls-supp-02,
Date 2006.
[N7] P. Faltstrom, P. Hoffman, A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003
[N8] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998
7 IANA Considerations
IANA needs to take the following actions:
1) Create an entry, user_mapping(TBD), in the existing registry for
ExtensionType (defined in RFC 4366 [N4]).
2) Create an entry, user_mapping_data(TBD), in the new registry for
SupplementalDataType (defined in draft-santesson-tls-supp-02).
3) Establish 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. all [Page 11]
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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 12]
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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.
Expires November 2006
Santesson, et. all [Page 13]
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