Internet-Draft M. Brown
March 2006 RedPhone Security
Expires: September 2006 R. Housley
Vigil Security
Transport Layer Security (TLS) Authorization Extensions
<draft-housley-tls-authz-extns-01.txt>
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Copyright (C) The Internet Society (2006). All Rights Reserved.
Abstract
This document specifies authorization extensions to the Transport
Layer Security (TLS) Handshake Protocol. Authorization information
is carried in the client and server hello messages. The syntax and
semantics of the authorization messages are described in detail.
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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 when the original design criteria for
TLS were determined. The authorization 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.
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 allow TLS clients to provide to the TLS server authorization
information, and allow TLS server to provide to the TLS client
authorization information about the TLS server.
The authorization extensions are intended for use with both TLS 1.0
and TLS 1.1. The extensions are designed to be backwards compatible,
meaning that the authorization information carried in the client
hello message and the server hello message can be ignored by any
implementation that does not support the included authorization
information format.
Clients typically know the context of the TLS session that is being
setup, thus the client can use of the authorization extensions when
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 make use of the authorization
information if the authorization extension is not supported or the
authorization information is provided in an unsupported format.
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].
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1.2. Overview
Figure 1 illustrates the placement of the authorization messages in
the full TLS handshake.
Client Server
ClientHello
(with AuthorizationData) -------->
ServerHello
(with AuthorizationData)
Certificate*
ServerKeyExchange*
CertificateRequest*
<-------- ServerHelloDone
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 Protocol message.
Figure 1. AuthorizationData carried in ClientHello and ServerHello
The ClientHello message includes the AuthorizationData extension,
which contains the authorization data for the client, and then the
ServerHello message includes the AuthorizationData extension, which
contains the authorization data for the server. If the server does
not support the AuthorizationData extension, or the server does not
support the authorization information format used by the client, then
the server MUST NOT include the AuthorizationData extension in the
ServerHello message. The Handshake Protocol continues, but without
the benefit of authorization information.
2. AuthorizationData Extension
The general extension mechanisms enable clients and servers to
negotiate the use of specific extensions. As specified in [TLSEXT],
the extension format used in the extended client hello message and
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extended server hello message is:
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 one new extension type:
authz_data.
This specification adds one new type to ExtensionType:
enum {
authz_data(TBD), (65535)
} ExtensionType;
The authorization extension is relevant when a session is initiated,
regardless of the use of a full handshake or use of session
resumption. Clients MUST explicitly present AuthorizationData in
every client hello message for which authorization information is
desired. Upon receipt of a client hello message that requests
session resumption but which contains no acceptable
AuthorizationData, the TLS server MAY resume the session but it MUST
NOT grant authorization to the session being resumed based on any
prior session authorization.
These requirements allow a series of resumed sessions to have
different authorizations from one another. More importantly, the
authorization information is always provided by the client in case
the server no longer honors the session resumption at the requested
authorization level. Repeated inclusion of the authorization
information allows the Handshake Protocol to proceed the same way for
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both resume and session origination.
2.1. The authz_data Extension Type
Clients MUST include the authz_data extension type in the extended
client hello message to send authorization data to the server. The
extension_data field contains the authorization data. Section 2.2
specifies the authorization data formats that are supported.
Servers that receive an extended client hello message containing the
authz_data extension MUST respond with the authz_data extension in
the extended server hello message if the server is willing to make
use of the received authorization data in the provided format. If
the server has any authorization information to send to the client,
then the server MUST include the information in the authz_data
extension type in the extended server hello message.
The AuthorizationData structure is described in Section 2.3.
2.2. AuthzDataFormat Type
The AuthzDataFormat type is used in the authz_data extension. It
indicates the format of the authorization information that will be
transferred. The AuthzDataFormat type definition is:
enum {
x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2),
saml_assertion_url(3), (255)
} AuthzDataFormat;
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) [SAML].
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.
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Additional formats can be registered in the future using the
procedures in section 3.
2.3. AuthorizationData Type
The AuthorizationData type is carried in the extension_data field for
the authz_data extension. When it appears in the extended client
hello message, it carries authorization information for the TLS
client. When it appears in the extended server hello message, it
carries authorization information for the TLS server.
struct {
AuthorizationDataEntry authz_data_list<1..2^16-1>;
} AuthorizationData;
struct {
AuthzDataFormat authz_format;
select (authz_format) {
case x509_attr_cert: X509AttrCert;
case saml_assertion: SAMLAssertion;
case x509_attr_cert_url: URLandHash;
case saml_assertion_url: URLandHash;
} authz_data_entry;
} AuthorizationDataEntry;
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;
enum {
sha1(0), sha256(1), (255)
} HashType;
opaque SHA1Hash[20];
opaque SHA1Hash[32];
When X509AttrCert is used, the field contains an ASN.1 DER-encoded
X.509 Attribute Certificate (AC) that follows the profile in RFC 3281
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[ATTRCERT]. An AC is a structure similar to a public key certificate
(PKC); 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 SAMLAssertion is used, the field contains XML constructs with a
nested structure defined in [SAML]. 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
assertions are most likely to convey authorization decisions about
whether subjects are allowed to access certain resources. Assertions
are issued by SAML authorities, namely, authentication authorities,
attribute authorities, and policy decision points.
Since X509AttrCert and SAMLAssertion can lead to a significant
increase in the size of the hello messages, 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
supported. Additional one-way hash functions can be registered in
the future using the procedures in section 3.
3. IANA Considerations
IANA has assigned one TLS Extension Types: authz_data(TBD).
IANA has established a registry for TLS Authorization Data Formats.
The first two entries in the registry are x509_attr_cert(0) and
saml_assertion(1). 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.
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IANA has established a registry 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.
4. 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.
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 information, 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.
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Client Server
ClientHello |0
(no AuthorizationData) --------> |0
ServerHello |0
(no AuthorizationData) |0
Certificate* |0
ServerKeyExchange* |0
CertificateRequest* |0
<-------- ServerHelloDone |0
Certificate* |0
ClientKeyExchange |0
CertificateVerify* |0
[ChangeCipherSpec] |0
Finished --------> |1
[ChangeCipherSpec] |0
<-------- Finished |1
ClientHello |1
(with AuthorizationData) --------> |1
ServerHello |1
(with AuthorizationData) |1
Certificate* |1
ServerKeyExchange* |1
CertificateRequest* |1
<-------- ServerHelloDone |1
Certificate* |1
ClientKeyExchange |1
CertificateVerify* |1
[ChangeCipherSpec] |1
Finished --------> |2
[ChangeCipherSpec] |1
<-------- Finished |2
Application Data <-------> Application Data |2
Figure 2. Protection of Authorization Data (Two Full Handshakes)
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Public key operations can be minimized by making the second handshake
a resumption. This is much more efficient in term of computation and
message exchanges. Figure 3 illustrates this more efficient double
handshake.
Client Server
ClientHello |0
(no AuthorizationData) --------> |0
ServerHello |0
(no AuthorizationData) |0
Certificate* |0
ServerKeyExchange* |0
CertificateRequest* |0
<-------- ServerHelloDone |0
Certificate* |0
ClientKeyExchange |0
CertificateVerify* |0
[ChangeCipherSpec] |0
Finished --------> |1
[ChangeCipherSpec] |0
<-------- Finished |1
ClientHello |1
(with AuthorizationData) --------> |1
ServerHello |1
(with AuthorizationData) |1
[ChangeCipherSpec] |1
<-------- Finished |2
[ChangeCipherSpec] |1
Finished --------> |2
Application Data <-------> Application Data |2
Figure 3. Protection of Authorization Data (Resumption)
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5. 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.
[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.
[SAML] Organization for the Advancement of Structured Information
Standards, "Security Assertion Markup Language (SAML),
version 1.1", September 2003. [Version 2.0 is out for
public comment; it will replace this reference if approved.]
[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.
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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
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