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rfc:rfc5878

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

Brown & Housley Experimental [Page 2] 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.

Brown & Housley Experimental [Page 3] RFC 5878 TLS Authorization Extensions May 2010

  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:

Brown & Housley Experimental [Page 4] RFC 5878 TLS Authorization Extensions May 2010

    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.

Brown & Housley Experimental [Page 5] RFC 5878 TLS Authorization Extensions May 2010

 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

Brown & Housley Experimental [Page 6] RFC 5878 TLS Authorization Extensions May 2010

 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.

Brown & Housley Experimental [Page 7] RFC 5878 TLS Authorization Extensions May 2010

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.

Brown & Housley Experimental [Page 8] 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;

Brown & Housley Experimental [Page 9] RFC 5878 TLS Authorization Extensions May 2010

    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,

Brown & Housley Experimental [Page 10] RFC 5878 TLS Authorization Extensions May 2010

 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.

Brown & Housley Experimental [Page 11] RFC 5878 TLS Authorization Extensions May 2010

 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]:

Brown & Housley Experimental [Page 12] RFC 5878 TLS Authorization Extensions May 2010

    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.

Brown & Housley Experimental [Page 13] RFC 5878 TLS Authorization Extensions May 2010

 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

Brown & Housley Experimental [Page 14] RFC 5878 TLS Authorization Extensions May 2010

       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.

Brown & Housley Experimental [Page 15] RFC 5878 TLS Authorization Extensions May 2010

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.

Brown & Housley Experimental [Page 16] 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.

Brown & Housley Experimental [Page 17] RFC 5878 TLS Authorization Extensions May 2010

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.

Brown & Housley Experimental [Page 18] 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

Brown & Housley Experimental [Page 19]

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