TLS User Mapping Extension
A TLS extension that lets a client hint which directory account its certificate belongs to.
Santesson, S., Medvinsky, A., and J. Ball, "TLS User Mapping Extension", RFC 4681, DOI 10.17487/RFC4681, October 2006, <https://www.rfc-editor.org/info/rfc4681>.
@misc{rfc4681,
series = {Request for Comments},
number = 4681,
howpublished = {RFC 4681},
publisher = {RFC Editor},
doi = {10.17487/RFC4681},
url = {https://www.rfc-editor.org/info/rfc4681},
author = {S. Santesson and A. Medvinsky and J. Ball},
title = {{TLS User Mapping Extension}},
pagetotal = 11,
year = 2006,
month = oct,
} RFC text
Contents
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).
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 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
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",
"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(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
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 {
(255)
} UserMappingType;
struct {
UserMappingType user_mapping_version;
uint16 user_mapping_length;
select(UserMappingType) { }
} UserMappingData;
struct{
UserMappingData user_mapping_data_list<1..2^16-1>;
}UserMappingDataList;
user_mapping_length
This field is the length (in bytes) of the data selected by
UserMappingType.
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 than 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.
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.
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
* 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
requirements.
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 cases, the double handshake technique described
in [N6] can be used to provide protection for the user mapping hint
information.
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
future.
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;
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.
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
1998.
9. Acknowledgements
The authors extend a special thanks to Russ Housley, Eric Resocorla,
and Paul Leach for their substantial contributions.
Authors' Addresses
Stefan Santesson
Microsoft
Finlandsgatan 30
164 93 KISTA
Sweden
EMail: stefans@microsoft.com
Ari Medvinsky
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
USA
EMail: arimed@microsoft.com
Joshua Ball
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
USA
EMail: joshball@microsoft.com
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AI ELI5
On this page
§1Introduction
RFC 4681 defines a TLS extension, user_mapping, and a payload for the SupplementalData handshake message from RFC 4680. Together they help a server map a client to a user account when TLS client authentication is used.
The document has a normative part (Sections 2 to 5) and an informative part (Section 6). It says that no other changes to the TLS protocol are required.
This document adds a small extra step to the secure-connection handshake. The visitor can pass along a hint about who they are, so the server finds the right account sooner.
§2The user_mapping Extension
A new extension type, user_mapping(6), is added for use in both the client hello and the server hello. Its extension_data carries a UserMappingTypeList, a list of hint types.
In the client hello, the list signals the hint types the client supports. In the server hello, it signals the hint types the server prefers, and it MUST be equal to or a subset of the client's list. If the server supports none of the client's hint types, it omits the extension from its server hello.
A server that does not understand the extension simply leaves it out, and the client proceeds as normal without sending any mapping data. The hints themselves are not placed in the client hello, to avoid broadcasting them to servers that do not understand the extension.
First the visitor asks, "Can I tell you who I am?" If the guard says yes, they go on. If the guard does not understand the question, nothing is shared.
§3User Mapping Data in the Handshake
RFC 4681 adds a new supplemental data type, user_mapping_data(0). Its payload is a UserMappingDataList: a vector of UserMappingData structures, each holding one hint with a type, a user_mapping_length in bytes, and the data itself.
The hints are unauthenticated. After a successful handshake, the server MAY use a hint to locate the user's account, from which user information and credentials MAY be retrieved to support authentication based on the client certificate.
The list MAY carry several hints, and their order has no meaning. If the client sends more than one, the server SHOULD use the applicable mapping it supports. Standards-track definitions of new mapping types must include a discussion of internationalization considerations.
The hint is like a sticky note with a name on it. There can be several notes, and the guard picks one that makes sense to them.
§4Message Flow
The client MUST include a user_mapping extension in its client hello. A server MAY reply with the extension in its server hello to say it will accept mapping data. Once both hellos carry the extension, the client MAY send a SupplementalData message containing the UserMappingDataList before its Certificate message.
The server MUST expect and gracefully handle a client that sends no SupplementalData message even after successful negotiation. The client may decide at its own discretion that its hint is no longer relevant, for example because the server certificate fails to meet certain requirements.
Both sides agree first, and only then does the visitor hand over the note. The visitor may also change their mind and keep the note in their pocket.
- ClientHello (user_mapping ext)Lists the hint types the client supports.
- ServerHello (user_mapping ext)Lists preferred hint types, equal to or a subset of the client's list.
- SupplementalData (UserMappingDataList)Sent before the Certificate message; the client MAY skip it.
- Certificate, ClientKeyExchange, FinishedThe usual handshake continues.
§5The UPN Domain Hint
Section 6 is informative and describes one hint type, upn_domain_hint(64). Its UpnDomainHint structure holds a user_principal_name and a domain_name. A User Principal Name (UPN) names a user's directory entry in the form userName@domainName.
The RFC explains that relying on a UPN inside the certificate prevents using certificates without one, and requires reissuing certificates or issuing several when accounts change. With this hint, a single certificate can be mapped to one or more accounts of the user, and the certificate does not need to contain a proprietary UPN.
The domain_name field MAY be used when only domain information is needed. The user_principal_name field MAY be used to give both the user name and the domain. If both are present, the server can use whichever it chooses, and at least one MUST be non-empty. The UPN is of the form user@domain with user a UTF-8 string without "@". The domain name field is an IDN-unaware domain name slot, so names with non-ASCII characters must first be processed as described in RFC 3490.
The sticky note can say your full name and company, or just your company. The guard uses whichever part helps.
enum {
upn_domain_hint(64), (255)
} UserMappingType;
struct {
opaque user_principal_name<0..2^16-1>;
opaque domain_name<0..2^16-1>;
} UpnDomainHint;§6Security Considerations
The hint is unauthenticated data and MUST NOT be treated as a trusted identifier. Authentication of the user MUST rely solely on validation of the client certificate. One suggested approach is to use the hint to fetch the claimed user's certificate from the trusted directory and match it against the validated client certificate from the handshake.
Because the client starts this exchange, it must decide when sending a hint is appropriate. Clients SHOULD only send the information if they recognize the server as a legitimate recipient, for instance by its name and address. If the hint itself is sensitive, the double handshake technique described in RFC 4680 can protect it.
The note is only a hint, so a guard must never let someone in just because of what it says. The ID card is what counts, and you should only hand your note to guards you recognize.
§7IANA Registrations
IANA created three registrations. The first is user_mapping(6) in the ExtensionType registry. The second is user_mapping_data(0) in the SupplementalDataType registry from RFC 4680. The third is a new registry for TLS UserMappingType values, whose first entry is upn_domain_hint(64).
Values 0 to 63 are assigned by Standards Action, 64 to 223 by Specification Required, and 224 to 255 are reserved for Private Use.
A public list keeps track of the numbers used for each kind of hint so that nobody uses the same number for two different things.
| Range (decimal) | Assignment policy |
|---|---|
| 0 to 63 | Standards Action |
| 64 to 223 THIS RFC | Specification Required |
| 224 to 255 | Private Use |