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Internet Engineering Task Force (IETF) M. Brown
Request for Comments: 5878 RedPhone Security
Updates: 5246 R. Housley
Category: Experimental Vigil Security
ISSN: 2070-1721 May 2010
Transport Layer Security (TLS) Authorization Extensions
Abstract
This document specifies authorization extensions to the Transport
Layer Security (TLS) Handshake Protocol. Extensions are 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, such as
attribute certificates (ACs) or Security Assertion Markup Language
(SAML) assertions, is exchanged in the supplemental data handshake
message.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5878.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Brown & Housley Experimental [Page 1]
RFC 5878 TLS Authorization Extensions May 2010
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
The Transport Layer Security (TLS) protocol ([TLS1.0], [TLS1.1],
[TLS1.2]) 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.
The format and content of the authorization information carried in
these extensions are extensible. This document references Security
Assertion Markup Language (SAML) assertion ([SAML1.1], [SAML2.0]) and
X.509 attribute certificate (AC) [ATTRCERT] authorization formats,
but other formats can be used. Future authorization extensions may
include any opaque assertion that is digitally signed by a trusted
issuer. Recognizing the similarity to certification path validation,
this document recommends the use of TLS Alert messages related to
certificate processing to report authorization information processing
failures.
Straightforward binding of identification, authentication, and
authorization information to an encrypted session 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 TLS 1.0, TLS 1.1, and TLS 1.2. These
extensions observe the conventions defined for TLS extensions that
were originally defined in [TLSEXT1] and revised in [TLSEXT2]; TLS
extensions are now part of TLS 1.2 [TLS1.2]. TLS extensions use
general extension mechanisms for the client hello message and the
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RFC 5878 TLS Authorization Extensions May 2010
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,
TLS 1.1, and TLS 1.2. 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
set up; 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" 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.
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.
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
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.
Successful session resumption uses the same authorization information
as the original session.
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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
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 [TLS1.2], 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. 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:
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enum {
client_authz(7), server_authz(8), (65535)
} ExtensionType;
The authorization extensions are relevant when a session is initiated
and on any subsequent session resumption. However, a client that
requests resumption of a session does not know whether the server
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.
When a session is resumed, ClientHello is followed immediately by
ChangeCipherSpec, which does not provide an opportunity for different
authorization information can be exchanged. Successful session
resumption MUST use the same authorization information as the
original session.
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 is described in Section 2.3.
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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.
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 5755 [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 5755 [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.
Implementations that support either x509_attr_cert_url or
saml_assertion_url MUST support URLs that employ the http scheme
[HTTP]. These implementations MUST confirm that the hash value
computed on the fetched authorization matches the one received in the
handshake. Mismatch of the hash values SHOULD be treated as though
the authorization was not provided, which will result in a
bad_certificate_hash_value alert (see Section 4). Implementations
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MUST deny access if the authorization cannot be obtained from the
provided URL, by sending a certificate_unobtainable alert (see
Section 4).
3. Supplemental Data Handshake Message Usage
As shown in Figure 1, supplemental data can be exchanged 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.
enum {
authz_data(16386), (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, and not within the TLS Handshake Protocol.
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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, and the following fictitious example of a single 5-octet
SAML assertion illustrates its use:
17 # Handshake.msg_type == supplemental_data(23)
00 00 11 # Handshake.length = 17
00 00 0e # length of SupplementalData.supp_data = 14
40 02 # SupplementalDataEntry.supp_data_type = 16386
00 0a # SupplementalDataEntry.supp_data_length = 10
00 08 # length of AuthorizationData.authz_data_list = 8
01 # authz_format = saml_assertion(1)
00 05 # length of SAMLAssertion
aa aa aa aa aa # SAML assertion (fictitious: "aa aa aa aa aa")
3.3. AuthorizationData Type
The AuthorizationData structure carries 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.
The HashAlgorithm type is taken from [TLS1.2], which allows
additional one-way hash functions to be registered in the IANA TLS
HashAlgorithm registry in the future.
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RFC 5878 TLS Authorization Extensions May 2010
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>;
HashAlgorithm hash_alg;
select (hash_alg) {
case md5: MD5Hash;
case sha1: SHA1Hash;
case sha224: SHA224Hash;
case sha256: SHA256Hash;
case sha384: SHA384Hash;
case sha512: SHA512Hash;
} hash;
} URLandHash;
enum {
none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5),
sha512(6), (255)
} HashAlgorithm;
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opaque MD5Hash[16];
opaque SHA1Hash[20];
opaque SHA224Hash[28];
opaque SHA256Hash[32];
opaque SHA384Hash[48];
opaque SHA512Hash[64];
3.3.1. X.509 Attribute Certificate
When X509AttrCert is used, the field contains an ASN.1 Distinguished
Encoding Rules (DER)-encoded X.509 attribute certificate (AC) that
follows the profile in RFC 5755 [ATTRCERT]. An AC is a structure
similar to a public key certificate (PKC) [PKIX1]; the main
difference is 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 either the 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 MUST contain well-formed XML
[XML1.0] and MUST use either UTF-8 [UTF-8] or UTF-16 [UTF-16]
character encoding. UTF-8 is the preferred character encoding. The
XML text declaration MUST be followed by an <Assertion> element using
the AssertionType complex type as defined in [SAML1.1] and [SAML2.0].
The XML text MUST also follow the rules of [XML1.0] for including the
Byte Order Mark (BOM) in encoded entities. SAML is an XML-based
framework for exchanging security information. This security
information is expressed in the form of assertions about subjects,
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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
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 6 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.
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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. When dereferencing these URLs,
circular dependencies MUST be avoided. Avoiding TLS when
dereferencing these URLs is one way to avoid circular dependencies.
Therefore, clients using the HTTP scheme MUST NOT use these TLS
extensions if UPGRADE in HTTP [UPGRADE] is used. For other schemes,
similar care must be taken to avoid using these TLS extensions.
Implementations that support either x509_attr_cert_url or
saml_assertion_url MUST support both SHA-1 [SHS] and SHA-256 [SHS] as
one-way hash functions. Other one-way hash functions may also be
supported. Additional one-way hash functions can be added to the
IANA TLS HashAlgorithm registry in the future.
Implementations that support x509_attr_cert_url MUST support
responses that employ the "application/pkix-attr-cert" Multipurpose
Internet Mail Extension (MIME) media type as defined in [ACTYPE].
Implementations that support saml_assertion_url MUST support
responses that employ the "application/samlassertion+xml" MIME type
as defined in Appendix A of [SAMLBIND].
TLS authorizations SHOULD follow the additional guidance provided in
Section 3.3 of [TLSEXT2] regarding client certificate URLs.
4. Alert Messages
This document specifies the reuse of TLS Alert messages related to
public key certificate processing for any errors that arise during
authorization processing, while preserving the AlertLevels as
authoritatively defined in [TLS1.2] or [TLSEXT2]. All alerts used in
authorization processing are fatal.
The following updated definitions for the Alert messages are used to
describe errors that arise while processing authorizations. For ease
of comparison, we reproduce the Alert message definition from
Section 7.2 of [TLS1.2], augmented with two values defined in
[TLSEXT2]:
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enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
user_canceled(90),
no_renegotiation(100),
unsupported_extension(110),
certificate_unobtainable(111),
bad_certificate_hash_value(114),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
TLS processing of alerts includes some ambiguity because the message
does not indicate which certificate in a certification path gave rise
to the error. This problem is made slightly worse in this extended
use of alerts, as the alert could be the result of an error in
processing of either a certificate or an authorization.
Implementations that support these extensions should be aware of this
imprecision.
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The AlertDescription values are used as follows to report errors in
authorizations processing:
bad_certificate
In certificate processing, bad_certificate indicates that a
certificate was corrupt, contained signatures that did not
verify correctly, and so on. Similarly, in authorization
processing, bad_certificate indicates that an authorization was
corrupt, contained signatures that did not verify correctly,
and so on. In authorization processing, bad_certificate can
also indicate that the handshake established that an
AuthzDataFormat was to be provided, but no AuthorizationData of
the expected format was provided in SupplementalData.
unsupported_certificate
In certificate processing, unsupported_certificate indicates
that a certificate was of an unsupported type. Similarly, in
authorization processing, unsupported_certificate indicates
that AuthorizationData uses a version or format unsupported by
the implementation.
certificate_revoked
In certificate processing, certificate_revoked indicates that a
certificate was revoked by its issuer. Similarly, in
authorization processing, certificate_revoked indicates that
authorization was revoked by its issuer, or a certificate that
was needed to validate the signature on the authorization was
revoked by its issuer.
certificate_expired
In certificate processing, certificate_expired indicates that a
certificate has expired or is not currently valid. Similarly,
in authorization processing, certificate_expired indicates that
an authorization has expired or is not currently valid.
certificate_unknown
In certificate processing, certificate_unknown indicates that
some other (unspecified) issue arose while processing the
certificate, rendering it unacceptable. Similarly, in
authorization processing, certificate_unknown indicates that
processing of AuthorizationData failed because of other
(unspecified) issues, including AuthzDataFormat parse errors.
unknown_ca
In certificate processing, unknown_ca indicates that a valid
certification path or partial certification path was received,
but the certificate was not accepted because the certification
authority (CA) certificate could not be located or could not be
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matched with a known, trusted CA. Similarly, in authorization
processing, unknown_ca indicates that the authorization issuer
is not known and trusted.
access_denied
In certificate processing, access_denied indicates that a valid
certificate was received, but when access control was applied,
the sender decided not to proceed with negotiation. Similarly,
in authorization processing, access_denied indicates that the
authorization was not sufficient to grant access.
certificate_unobtainable
The client_certificate_url extension defined in RFC 4366
[TLSEXT2] specifies that download errors lead to a
certificate_unobtainable alert. Similarly, in authorization
processing, certificate_unobtainable indicates that a URL does
not result in an authorization. While certificate processing
does not require this alert to be fatal, this is a fatal alert
in authorization processing.
bad_certificate_hash_value
In certificate processing, bad_certificate_hash_value indicates
that a downloaded certificate does not match the expected hash.
Similarly, in authorization processing,
bad_certificate_hash_value indicates that a downloaded
authorization does not match the expected hash.
5. IANA Considerations
This document defines two TLS extensions: client_authz(7) and
server_authz(8). These extension type values are assigned from the
TLS Extension Type registry defined in [TLSEXT2].
This document defines one TLS supplemental data type:
authz_data(16386). 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 four 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 5226 [IANA] IETF Review. Values from
the inclusive range 64-223 (decimal) are assigned via RFC 5226
Specification Required. Values from the inclusive range 224-255
(decimal) are reserved for RFC 5226 Private Use.
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6. 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 cases where the authorization information itself is 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.
The use of bearer SAML assertions allows an eavesdropper or a man-in-
the-middle to capture the SAML assertion and try to reuse it in
another context. The constraints discussed in Section 3.3.2 might be
effective against an eavesdropper, but they are less likely to be
effective against a man-in-the-middle. Authentication of both
parties in the TLS session, which involves the use of client
authentication, will prevent an undetected man-in-the-middle, and the
use of the double handshake illustrated in Figure 2 will prevent the
disclosure of the bearer SAML assertion to any party other than the
TLS peer.
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RFC 5878 TLS Authorization Extensions May 2010
AuthzDataFormats that point to authorization data, such as
x509_attr_cert_url and saml_assertion_url, rather than simply
including the authorization data in the handshake, may be exploited
by an attacker. Implementations that accept pointers to
authorization data SHOULD adopt a policy of least privilege that
limits the acceptable references that they will attempt to use. For
more information, see Section 6.3 of [TLSEXT2].
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. Acknowledgement
The authors thank Scott Cantor for his assistance with the SAML
assertion portion of the document.
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8. References
8.1. Normative References
[ACTYPE] Housley, R., "The application/pkix-attr-cert Media Type
for Attribute Certificates", RFC 5877, May 2010.
[ATTRCERT] Farrell, S., Housley, R., and S. Turner, "An Internet
Attribute Certificate Profile for Authorization",
RFC 5755, January 2010.
[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[PKIX1] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[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", March 2005.
[SAMLBIND] OASIS Security Services Technical Committee, "Bindings
for the OASIS Security Assertion Markup Language (SAML)
V2.0", March 2005.
[SHS] National Institute of Standards and Technology (NIST),
FIPS PUB 180-3, Secure Hash Standard (SHS), October 2008.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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, April 2006.
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RFC 5878 TLS Authorization Extensions May 2010
[TLS1.2] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[TLSEXT2] Blake-Wilson, S., Nystrom, M., Hopwood, D.,
Mikkelsen, J., and T. Wright, "Transport Layer Security
(TLS) Extensions", RFC 4366, April 2006.
[TLSSUPP] Santesson, S., "TLS Handshake Message for Supplemental
Data", RFC 4680, October 2006.
[UPGRADE] Khare, R. and S. Lawrence, "Upgrading to TLS Within
HTTP/1.1", RFC 2817, May 2000.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, November 2003.
[UTF-16] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
ISO 10646", RFC 2781, February 2000.
[XML1.0] Bray, T., J. Paoli, C. M. Sperberg-McQueen, E. Maler, and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", http://www.w3.org/TR/xml/, November 2008.
8.2. Informative References
[TLSEXT1] Blake-Wilson, S., Nystrom, M., Hopwood, D.,
Mikkelsen, J., and T. Wright, "Transport Layer Security
(TLS) Extensions", RFC 3546, June 2003.
Authors' Addresses
Mark Brown
RedPhone Security
1199 Falls View Court
Mendota Heights, MN 55118
USA
EMail: mark@redphonesecurity.com
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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