Internet-Draft M. Brown
May 2006 RedPhone Security
Expires: November 2006 R. Housley
Vigil Security
Transport Layer Security (TLS) Authorization Extensions
<draft-housley-tls-authz-extns-06.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 as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (2006). All Rights Reserved.
Abstract
This document specifies authorization extensions to the Transport
Layer Security (TLS) Handshake Protocol. Extensions carried in the
client and server hello messages to confirm that both parties support
the desired authorization data types. Then, if supported by both the
client and the server, authorization information is exchanged in the
supplemental data handshake message.
Brown & Housley [Page 1]
Internet-Draft May 2006
1. Introduction
Transport Layer Security (TLS) protocol [TLS1.0][TLS1.1] is being
used in an increasing variety of operational environments, including
ones that were not envisioned at the time of the original design for
TLS. The extensions introduced in this document are designed to
enable TLS to operate in environments where authorization information
needs to be exchanged between the client and the server before any
protected data is exchanged.
The use of these TLS authorization extensions is especially
attractive when more than one application protocol can make use of
the same authorization information. Straightforward binding of
identification, authentication, and authorization information is
possible when all of these are handled within TLS. If each
application requires unique authorization information, then it might
best be carried within the TLS-protected application protocol.
However, care must be taken to ensure appropriate bindings when
identification, authentication, and authorization information are
handled at different protocol layers.
This document describes authorization extensions for the TLS
Handshake Protocol in both TLS 1.0 and TLS 1.1. These extensions
observe the conventions defined for TLS Extensions [TLSEXT] that make
use of the general extension mechanisms for the client hello message
and the server hello message. The extensions described in this
document confirm that both the client and the server support the
desired authorization data types. Then, if supported, authorization
information is exchanged in the supplemental data handshake message
[TLSSUPP].
The authorization extensions may be used in conjunction with TLS 1.0
and TLS 1.1. The extensions are designed to be backwards compatible,
meaning that the Handshake Protocol Supplemental Data messages will
only contain authorization information of a particular type if the
client indicates support for them in the client hello message and the
server indicates support for them in the server hello message.
Clients typically know the context of the TLS session that is being
setup, thus the client can use the authorization extensions when they
are needed. Servers must accept extended client hello messages, even
if the server does not "understand" the all of the listed extensions.
However, the server will not indicate support for these "not
understood" extensions. Then, clients may reject communications with
servers that do not support the authorization extensions.
Brown & Housley [Page 2]
Internet-Draft May 2006
1.1. Conventions
The syntax for the authorization messages is defined using the TLS
Presentation Language, which is specified in Section 4 of [TLS1.0].
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].
1.2. Overview
Figure 1 illustrates the placement of the authorization extensions
and supplemental data messages in the full TLS handshake.
Client Server
ClientHello (w/ extensions) -------->
ServerHello (w/ extensions)
SupplementalData*
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
SupplementalData*
Certificate*
ClientKeyExchange
CertificateVerify*
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Application Data <-------> Application Data
* Indicates optional or situation-dependent messages that
are not always sent.
[] Indicates that ChangeCipherSpec is an independent TLS
Protocol content type; it is not actually a TLS
handshake message.
Figure 1. Authorization data exchange in full TLS handshake
The ClientHello message includes an indication of the client
authorization data formats that are supported and an indication of
the server authorization data formats that are supported. The
Brown & Housley [Page 3]
Internet-Draft May 2006
ServerHello message contains similar indications, but any
authorization data formats that are not supported by the server are
not included. Both the client and the server MUST indicate support
for the authorization data types. If the list of mutually supported
authorization data formats is empty, then the ServerHello message
MUST NOT carry the affected extension at all.
2. Authorization Extension Types
The general extension mechanisms enable clients and servers to
negotiate whether to use specific extensions, and how to use specific
extensions. As specified in [TLSEXT], the extension format used in
the extended client hello message and extended server hello message
is repeated here for convenience:
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
The extension_type identifies a particular extension type, and the
extension_data contains information specific to the particular
extension type.
As specified in [TLSEXT], for all extension types, the extension type
MUST NOT appear in the extended server hello message unless the same
extension type appeared in the corresponding client hello message.
Clients MUST abort the handshake if they receive an extension type in
the extended server hello message that they did not request in the
associated extended client hello message.
When multiple extensions of different types are present in the
extended client hello message or the extended server hello message,
the extensions can appear in any order, but there MUST NOT be more
than one extension of the same type.
This document specifies the use of two new extension types:
client_authz and server_authz. These extension types are described
in Section 2.1 and Section 2.2, respectively. This specification
adds two new types to ExtensionType:
enum {
client_authz(TBD), server_authz(TBD), (65535)
} ExtensionType;
The authorization extensions are relevant when a session is initiated
and any subsequent session resumption. However, a client that
requests resumption of a session does not know whether the server
Brown & Housley [Page 4]
Internet-Draft May 2006
will have all of the context necessary to accept this request, and
therefore the client SHOULD send an extended client hello message
that includes the extension types associated with the authorization
extensions. This way, if the resumption request is denied, then the
authorization extensions will be negotiated as normal.
2.1. The client_authz Extension Type
Clients MUST include the client_authz extension type in the extended
client hello message to indicate their desire to send authorization
data to the server. The extension_data field indicates the format of
the authorization data that will be sent in the supplemental data
handshake message. The syntax of the client_authz extension_data
field is described in Section 2.3.
Servers that receive an extended client hello message containing the
client_authz extension MUST respond with the same client_authz
extension in the extended server hello message if the server is
willing to receive authorization data in the indicated format. Any
unacceptable formats must be removed from the list provided by the
client. The client_authz extension MUST be omitted from the extended
server hello message if the server is not willing to receive
authorization data in any of the indicated formats.
2.2. The server_authz Extension Type
Clients MUST include the server_authz extension type in the extended
client hello message to indicate their desire to receive
authorization data from the server. The extension_data field
indicates the format of the authorization data that will be sent in
the supplemental data handshake message. The syntax of the
server_authz extension_data field as described in Section 2.3.
Servers that receive an extended client hello message containing the
server_authz extension MUST respond with the same server_authz
extension in the extended server hello message if the server is
willing to provide authorization data in the requested format. Any
unacceptable formats must be removed from the list provided by the
client. The server_authz extension MUST be omitted from the extended
server hello message if the server is not able to provide
authorization data in any of the indicated formats.
Brown & Housley [Page 5]
Internet-Draft May 2006
2.3. AuthzDataFormat Type
The AuthzDataFormat type is used in both the client_authz and the
server_authz extensions. It indicates the format of the
authorization data that will be transferred. The AuthzDataFormats
type definition is:
enum {
x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2),
saml_assertion_url(3), (255)
} AuthzDataFormat;
AuthzDataFormats authz_format_list<1..2^8-1>;
When the x509_attr_cert value is present, the authorization data is
an X.509 Attribute Certificate (AC) that conforms to the profile in
RFC 3281 [ATTRCERT].
When the saml_assertion value is present, the authorization data is
an assertion composed using the Security Assertion Markup Language
(SAML) [SAML1.1][SAML2.0].
When the x509_attr_cert_url value is present, the authorization data
is an X.509 AC that conforms to the profile in RFC 3281 [ATTRCERT];
however, the AC is fetched with the supplied URL. A one-way hash
value is provided to ensure that the intended AC is obtained.
When the saml_assertion_url value is present, the authorization data
is a SAML Assertion; however, the SAML Assertion is fetched with the
supplied URL. A one-way hash value is provided to ensure that the
intended SAML Assertion is obtained.
3. Supplemental Data Handshake Message Usage
As shown in Figure 1, supplemental data can be exchanges in two
places in the handshake protocol. The client_authz extension
determines what authorization data formats are acceptable for
transfer from the client to the server, and the server_authz
extension determines what authorization data formats are acceptable
for transfer from the server to the client. In both cases, the
syntax specified in [TLSSUPP] is used along with the authz_data type
defined in this document.
Brown & Housley [Page 6]
Internet-Draft May 2006
enum {
authz_data(TBD), (65535)
} SupplementalDataType;
struct {
SupplementalDataType supplemental_data_type;
select(SupplementalDataType) {
case authz_data: AuthorizationData;
}
} SupplementalData;
3.1. Client Authorization Data
The SupplementalData message sent from the client to the server
contains authorization data associated with the TLS client.
Following the principle of least privilege, the client ought to send
the minimal set of authorization information necessary to accomplish
the task at hand. That is, only those authorizations that are
expected to be required by the server in order to gain access to the
needed server resources ought to be included. The format of the
authorization data depends on the format negotiated in the
client_authz hello message extension. The AuthorizationData
structure is described in Section 3.3.
In some systems, clients present authorization information to the
server, and then the server provides new authorization information.
This type of transaction is not supported by SupplementalData
messages. In cases where the client intends to request the TLS
server to perform authorization translation or expansion services,
such translation services ought to occur within the ApplicationData
messages, not within the TLS Handshake protocol.
3.2. Server Authorization Data
The SupplementalData message sent from the server to the client
contains authorization data associated with the TLS server. This
authorization information is expected to include statements about the
server's qualifications, reputation, accreditation, and so on.
Wherever possible, authorizations that can be misappropriated for
fraudulent use ought to be avoided. The format of the authorization
data depends on the format negotiated in the server_authz hello
message extensions. The AuthorizationData structure is described in
Section 3.3.
Brown & Housley [Page 7]
Internet-Draft May 2006
3.3. AuthorizationData Type
The AuthorizationData structure carried authorization information for
either the client or the server. The AuthzDataFormat specified in
Section 2.3 for use in the hello extensions is also used in this
structure.
All of the entries in the authz_data_list MUST employ authorization
data formats that were negotiated in the relevant hello message
extension.
struct{
AuthorizationDataEntry authz_data_list<1..2^16-1>;
} AuthorizationData;
struct {
AuthzDataFormat authz_format;
select (AuthzDataFormat) {
case x509_attr_cert: X509AttrCert;
case saml_assertion: SAMLAssertion;
case x509_attr_cert_url: URLandHash;
case saml_assertion_url: URLandHash;
}
} AuthorizationDataEntry;
enum {
x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2),
saml_assertion_url(3), (255)
} AuthzDataFormat;
opaque X509AttrCert<1..2^16-1>;
opaque SAMLAssertion<1..2^16-1>;
struct {
opaque url<1..2^16-1>;
HashType hash_type;
select (hash_type) {
case sha1: SHA1Hash;
case sha256: SHA256Hash;
} hash;
} URLandHash;
Brown & Housley [Page 8]
Internet-Draft May 2006
enum {
sha1(0), sha256(1), (255)
} HashType;
opaque SHA1Hash[20];
opaque SHA256Hash[32];
3.3.1. X.509 Attribute Certificate
When X509AttrCert is used, the field contains an ASN.1 DER-encoded
X.509 Attribute Certificate (AC) that follows the profile in RFC 3281
[ATTRCERT]. An AC is a structure similar to a public key certificate
(PKC) [PKIX1]; the main difference being that the AC contains no
public key. An AC may contain attributes that specify group
membership, role, security clearance, or other authorization
information associated with the AC holder.
When making an authorization decision based on an AC, proper linkage
between the AC holder and the public key certificate that is
transferred in the TLS Certificate message is needed. The AC holder
field provides this linkage. The holder field is a SEQUENCE allowing
three different (optional) syntaxes: baseCertificateID, entityName
and objectDigestInfo. In the TLS authorization context, the holder
field MUST use the either baseCertificateID or entityName. In the
baseCertificateID case, the baseCertificateID field MUST match the
issuer and serialNumber fields in the certificate. In the entityName
case, the entityName MUST be the same as the subject field in the
certificate or one of the subjectAltName extension values in the
certificate. Note that [PKIX1] mandates that the subjectAltName
extension be present if the subject field contains an empty
distinguished name.
3.3.2. SAML Assertion
When SAMLAssertion is used, the field contains XML constructs with a
nested structure defined in [SAML1.1][SAML2.0]. SAML is an XML-based
framework for exchanging security information. This security
information is expressed in the form of assertions about subjects,
where a subject is either human or computer with an identity. In
this context, the SAML assertions are most likely to convey
authentication or attribute statements to be used as input to
authorization policy governing whether subjects are allowed to access
certain resources. Assertions are issued by SAML authorities.
When making an authorization decision based on a SAML assertion,
proper linkage between the SAML assertion and the public key
certificate that is transferred in the TLS Certificate message may be
Brown & Housley [Page 9]
Internet-Draft May 2006
needed. A "Holder of Key" subject confirmation method in the SAML
assertion can provide this linkage. In other scenarios, it may be
acceptable to use alternate confirmation methods that do not provide
a strong binding, such as a bearer mechanism. SAML assertion
recipients MUST decide which subject confirmation methods are
acceptable; such decisions MAY be specific to the SAML assertion
contents and the TLS session context.
There is no general requirement that the subject of the SAML
assertion correspond directly to the subject of the certificate.
They may represent the same or different entities. When they are
different, SAML also provides a mechanism by which the certificate
subject can be identified separately from the subject in the SAML
assertion subject confirmation method.
Since the SAML assertion is being provided at a part of the TLS
Handshake that is unencrypted, an eavesdropper could replay the same
SAML assertion when they establish their own TLS session. This is
especially important when a bearer mechanism is employed, the
recipient of the SAML assertion assumes that the sender is an
acceptable attesting entity for the SAML assertion. Some constraints
may be included to limit the context where the bearer mechanism will
be accepted. For example, the period of time that the SAML assertion
can be short-lived (often minutes), the source address can be
constrained, or the destination endpoint can be identified. Also,
bearer assertions are often checked against a cache of SAML assertion
unique identifiers that were recently received in order to detect
replay. This is an appropriate countermeasure if the bearer
assertion is intended to be used just once. Section 5 provides a way
to protect authorization information when necessary.
3.3.3. URL and Hash
Since the X.509 AC and SAML assertion can be large, alternatives
provide a URL to obtain the ASN.1 DER-encoded X.509 AC or SAML
Assertion. To ensure that the intended object is obtained, a one-way
hash value of the object is also included. Integrity of this one-way
hash value is provided by the TLS Finished message.
Implementations that support either x509_attr_cert_url or
saml_assertion_url MUST support URLs that employ the http scheme.
Other schemes may also be supported; however, to avoid circular
dependencies, supported schemes SHOULD NOT themselves make use of
TLS, such as the https scheme.
Implementations that support either x509_attr_cert_url or
saml_assertion_url MUST support both SHA-1 [SHA1] and SHA-256 [SHA2]
as one-way hash functions. Other one-way hash functions may also be
Brown & Housley [Page 10]
Internet-Draft May 2006
supported. Additional one-way hash functions can be registered in
the future using the procedures in section 3.
4. IANA Considerations
This document defines a two TLS extensions: client_authz(TBD) and
server_authz(TBD). These extension type values are assigned from the
TLS Extension Type registry defined in [TLSEXT].
This document defines one TLS supplemental data type:
authz_data(TBD). This supplemental data type is assigned from the
TLS Supplemental Data Type registry defined in [TLSSUPP].
This document establishes a new registry, to be maintained by IANA,
for TLS Authorization Data Formats. The first five entries in the
registry are x509_attr_cert(0), saml_assertion(1),
x509_attr_cert_url(2), and saml_assertion_url(3). TLS Authorization
Data Format identifiers with 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.
This document establishes a new registry, to be maintained by IANA,
for TLS Hash Types. The first two entries in the registry are
sha1(0) and sha256(1). TLS Hash Type identifiers with values in the
inclusive range 0-158 (decimal) are assigned via RFC 2434 [IANA]
Standards Action. Values from the inclusive range 159-223 (decimal)
are assigned via RFC 2434 Specification Required. Values from the
inclusive range 224-255 (decimal) are reserved for RFC 2434 Private
Use.
5. Security Considerations
A TLS server can support more than one application, and each
application may include several features, each of which requires
separate authorization checks. This is the reason that more than one
piece of authorization information can be provided.
A TLS server that requires different authorization information for
different applications or different application features may find
that a client has provided sufficient authorization information to
grant access to a subset of these offerings. In this situation the
TLS Handshake protocol will complete successfully; however, the
server must ensure that the client will only be able to use the
appropriate applications and application features. That is, the TLS
server must deny access to the applications and application features
for which authorization has not been confirmed.
Brown & Housley [Page 11]
Internet-Draft May 2006
In many cases, the authorization information is itself sensitive.
The double handshake technique can be used to provide protection for
the authorization information. Figure 2 illustrates the double
handshake, where the initial handshake does not include any
authorization extensions, but it does result in protected
communications. Then, a second handshake that includes the
authorization information is performed using the protected
communications. In Figure 2, the number on the right side indicates
the amount of protection for the TLS message on that line. A zero
(0) indicates that there is no communication protection; a one (1)
indicates that protection is provided by the first TLS session; and a
two (2) indicates that protection is provided by both TLS sessions.
The placement of the SupplementalData message in the TLS Handshake
results in the server providing its authorization information before
the client is authenticated. In many situations, servers will not
want to provide authorization information until the client is
authenticated. The double handshake illustrated in Figure 2 provides
a technique to ensure that the parties are mutually authenticated
before either party provides authorization information.
6. Acknowledgement
The authors thank Scott Cantor for his assistance with the SAML
Assertion portion of the document.
Brown & Housley [Page 12]
Internet-Draft May 2006
Client Server
ClientHello (no extensions) --------> |0
ServerHello (no extensions) |0
Certificate* |0
ServerKeyExchange* |0
CertificateRequest* |0
<-------- ServerHelloDone |0
Certificate* |0
ClientKeyExchange |0
CertificateVerify* |0
[ChangeCipherSpec] |0
Finished --------> |1
[ChangeCipherSpec] |0
<-------- Finished |1
ClientHello (w/ extensions) --------> |1
ServerHello (w/ extensions) |1
SupplementalData (w/ authz data)* |1
Certificate* |1
ServerKeyExchange* |1
CertificateRequest* |1
<-------- ServerHelloDone |1
SupplementalData (w/ authz data)* |1
Certificate* |1
ClientKeyExchange |1
CertificateVerify* |1
[ChangeCipherSpec] |1
Finished --------> |2
[ChangeCipherSpec] |1
<-------- Finished |2
Application Data <-------> Application Data |2
Figure 2. Double Handshake to Protect Authorization Data
7. Normative References
[ATTRCERT] Farrell, S., and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281,
April 2002.
[IANA] Narten, T., and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", RFC 3434,
October 1998.
Brown & Housley [Page 13]
Internet-Draft May 2006
[PKIX1] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[TLS1.0] Dierks, T., and C. Allen, "The TLS Protocol, Version 1.0",
RFC 2246, January 1999.
[TLS1.1] Dierks, T., and E. Rescorla, "The Transport Layer Security
(TLS) Protocol, Version 1.1", RFC 4346, February 2006.
[TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS) Extensions",
RFC 3546, June 2003.
[TLSSUPP] Santesson, S., " TLS Handshake Message for Supplemental
Data", work in progress: draft-santesson-tls-supp,
March 2006.
[SAML1.1] OASIS Security Services Technical Committee, "Security
Assertion Markup Language (SAML) Version 1.1
Specification Set", September 2003.
[SAML2.0] OASIS Security Services Technical Committee, "Security
Assertion Markup Language (SAML) Version 2.0
Specification Set", March2005.
[SHA1] National Institute of Standards and Technology (NIST),
FIPS PUB 180-1, Secure Hash Standard, 17 April 1995.
[SHA2] National Institute of Standards and Technology (NIST),
FIPS PUB 180-2: Secure Hash Standard, 1 August 2002.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Brown & Housley [Page 14]
Internet-Draft May 2006
Author's Address
Mark Brown
RedPhone Security
2019 Palace Avenue
Saint Paul, MN 55105
USA
mark <at> redphonesecurity <dot> com
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
housley <at> vigilsec <dot> com
Full 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.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
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.
Brown & Housley [Page 15]
Internet-Draft May 2006
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Brown & Housley [Page 16]
