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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5878

Internet-Draft                                                  M. Brown
Intended Status: Experimental                          RedPhone Security
Updates: 5246 (once approved)                                 R. Housley
                                                          Vigil Security
Expires: 14 April 2010                                   14 October 2009

        Transport Layer Security (TLS) Authorization Extensions
                 <draft-housley-tls-authz-extns-09.txt>


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   document authors.  All rights reserved.

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Abstract

   This document specifies authorization extensions to the Transport
   Layer Security (TLS) Handshake Protocol.  Extensions carried in the
   client and server hello messages to confirm that both parties support
   the desired authorization data types.  Then, if supported by both the
   client and the server, authorization information, such as Attribute



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   Certificates or SAML Assertions, is exchanged in the supplemental
   data handshake message.

1. Introduction

   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 is extensible.  This document references SAML
   assertion [SAML] and X509 Attribute Certificate [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 both 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
   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,



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   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
   setup, thus the client can use the authorization extensions when they
   are needed.  Servers must accept extended client hello messages, even
   if the server does not "understand" the all of the listed extensions.
   However, the server will not indicate support for these "not
   understood" extensions.  Then, clients may reject communications with
   servers that do not support the authorization extensions.

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



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   types: client_authz and server_authz.  These extension types are
   described in Section 2.1 and Section 2.2, respectively.  This
   specification adds two new types to ExtensionType:

      enum {
        client_authz(TBD), server_authz(TBD), (65535)
      } ExtensionType;

   The authorization extensions are relevant when a session is initiated
   and any subsequent session resumption.  However, a client that
   requests resumption of a session does not know whether the server
   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



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   server_authz extension_data field as described in Section 2.3.

   Servers that receive an extended client hello message containing the
   server_authz extension MUST respond with the same server_authz
   extension in the extended server hello message if the server is
   willing to provide authorization data in the requested format.  Any
   unacceptable formats must be removed from the list provided by the
   client.  The server_authz extension MUST be omitted from the extended
   server hello message if the server is not able to provide
   authorization data in any of the indicated formats.

2.3. AuthzDataFormat Type

   The AuthzDataFormat type is used in both the client_authz and the
   server_authz extensions.  It indicates the format of the
   authorization data that will be transferred.  The AuthzDataFormats
   type definition is:

      enum {
         x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2),
         saml_assertion_url(3), (255)
      } AuthzDataFormat;

      AuthzDataFormats authz_format_list<1..2^8-1>;

   When the x509_attr_cert value is present, the authorization data is
   an X.509 Attribute Certificate (AC) that conforms to the profile in
   RFC 3281 [ATTRCERT].

   When the saml_assertion value is present, the authorization data is
   an assertion composed using the Security Assertion Markup Language
   (SAML) [SAML1.1][SAML2.0].

   When the x509_attr_cert_url value is present, the authorization data
   is an X.509 AC that conforms to the profile in RFC 3281 [ATTRCERT];
   however, the AC is fetched with the supplied URL.  A one-way hash
   value is provided to ensure that the intended AC is obtained.

   When the saml_assertion_url value is present, the authorization data
   is a SAML Assertion; however, the SAML Assertion is fetched with the
   supplied URL.  A one-way hash value is provided to ensure that the
   intended SAML Assertion is obtained.

   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



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   the authorization was not provided, which will result in a
   bad_certificate_hash_value alert (see Section 4).  Implementations
   MUST deny access if the authorization cannot be obtained from the
   provided URL, sending certificate_unobtainable alert (see Section 4).

3. Supplemental Data Handshake Message Usage

   As shown in Figure 1, supplemental data can be exchanges in two
   places in the handshake protocol.  The client_authz extension
   determines what authorization data formats are acceptable for
   transfer from the client to the server, and the server_authz
   extension determines what authorization data formats are acceptable
   for transfer from the server to the client.  In both cases, the
   syntax specified in [TLSSUPP] is used along with the authz_data type
   defined in this document.

      enum {
         authz_data(TBD), (65535)
      } SupplementalDataType;

      struct {
         SupplementalDataType supplemental_data_type;
         select(SupplementalDataType) {
            case authz_data:  AuthorizationData;
         }
      } SupplementalData;

3.1. Client Authorization Data

   The SupplementalData message sent from the client to the server
   contains authorization data associated with the TLS client.
   Following the principle of least privilege, the client ought to send
   the minimal set of authorization information necessary to accomplish
   the task at hand.  That is, only those authorizations that are
   expected to be required by the server in order to gain access to the
   needed server resources ought to be included.  The format of the
   authorization data depends on the format negotiated in the
   client_authz hello message extension.  The AuthorizationData
   structure is described in Section 3.3.

   In some systems, clients present authorization information to the
   server, and then the server provides new authorization information.
   This type of transaction is not supported by SupplementalData
   messages.  In cases where the client intends to request the TLS
   server to perform authorization translation or expansion services,
   such translation services ought to occur within the ApplicationData
   messages, not within the TLS Handshake protocol.




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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 the use:

      17             # Handshake.msg_type == supplemental_data(23)
      00 00 11       # Handshake.length = 17
      00 00 0e       # length of SupplementalData.supp_data = 14
      ?? ??          # SupplementalDataEntry.supp_data_type = TBD
      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")

      {{ RFC Editor: Please replace "TBD" with the value assigned by
         IANA, and replace "?? ??" with the hexadecimal value assigned
         for TBD. }}

3.3. AuthorizationData Type

   The AuthorizationData structure carried authorization information for
   either the client or the server.  The AuthzDataFormat specified in
   Section 2.3 for use in the hello extensions is also used in this
   structure.

   All of the entries in the authz_data_list MUST employ authorization
   data formats that were negotiated in the relevant hello message
   extension.

   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.

      struct{
         AuthorizationDataEntry authz_data_list<1..2^16-1>;
      } AuthorizationData;






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      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;

      opaque MD5Hash[16];

      opaque SHA1Hash[20];

      opaque SHA224Hash[28];

      opaque SHA256Hash[32];

      opaque SHA384Hash[48];

      opaque SHA512Hash[64];



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3.3.1. X.509 Attribute Certificate

   When X509AttrCert is used, the field contains an ASN.1 DER-encoded
   X.509 Attribute Certificate (AC) that follows the profile in RFC 3281
   [ATTRCERT].  An AC is a structure similar to a public key certificate
   (PKC) [PKIX1]; the main difference being that the AC contains no
   public key.  An AC may contain attributes that specify group
   membership, role, security clearance, or other authorization
   information associated with the AC holder.

   When making an authorization decision based on an AC, proper linkage
   between the AC holder and the public key certificate that is
   transferred in the TLS Certificate message is needed.  The AC holder
   field provides this linkage.  The holder field is a SEQUENCE allowing
   three different (optional) syntaxes: baseCertificateID, entityName
   and objectDigestInfo.  In the TLS authorization context, the holder
   field MUST use the either baseCertificateID or entityName.  In the
   baseCertificateID case, the baseCertificateID field MUST match the
   issuer and serialNumber fields in the certificate.  In the entityName
   case, the entityName MUST be the same as the subject field in the
   certificate or one of the subjectAltName extension values in the
   certificate.  Note that [PKIX1] mandates that the subjectAltName
   extension be present if the subject field contains an empty
   distinguished name.

3.3.2. SAML Assertion

   When SAMLAssertion is used, the field 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][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,
   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



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   a strong binding, such as a bearer mechanism.  SAML assertion
   recipients MUST decide which subject confirmation methods are
   acceptable; such decisions MAY be specific to the SAML assertion
   contents and the TLS session context.

   There is no general requirement that the subject of the SAML
   assertion correspond directly to the subject of the certificate.
   They may represent the same or different entities.  When they are
   different, SAML also provides a mechanism by which the certificate
   subject can be identified separately from the subject in the SAML
   assertion subject confirmation method.

   Since the SAML assertion is being provided at a part of the TLS
   Handshake that is unencrypted, an eavesdropper could replay the same
   SAML assertion when they establish their own TLS session.  This is
   especially important when a bearer mechanism is employed, the
   recipient of the SAML assertion assumes that the sender is an
   acceptable attesting entity for the SAML assertion.  Some constraints
   may be included to limit the context where the bearer mechanism will
   be accepted.  For example, the period of time that the SAML assertion
   can be short-lived (often minutes), the source address can be
   constrained, or the destination endpoint can be identified.  Also,
   bearer assertions are often checked against a cache of SAML assertion
   unique identifiers that were recently received in order to detect
   replay.  This is an appropriate countermeasure if the bearer
   assertion is intended to be used just once.  Section 5 provides a way
   to protect authorization information when necessary.

3.3.3. URL and Hash

   Since the X.509 AC and SAML assertion can be large, alternatives
   provide a URL to obtain the ASN.1 DER-encoded X.509 AC or SAML
   Assertion.  To ensure that the intended object is obtained, a one-way
   hash value of the object is also included.  Integrity of this one-way
   hash value is provided by the TLS Finished message.

   Implementations that support either x509_attr_cert_url or
   saml_assertion_url MUST support URLs that employ the http scheme.
   Other schemes may also be supported.  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 used 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



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   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" Multipart
   Internet Mail Extension (MIME) 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 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 [TLSEXT2]:

      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),



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          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 an error in process
   of either a certificate or an authorization.  Implementations that
   support these extensions should be aware of this imprecision.

   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



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         authorization processing, certificate_revoked indicates that
         authorization was revoked by its issuer, or a certificate
         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 other
         (unspecified) issue, 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 CA certificate
         could not be located or could not be 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,



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         bad_certificate_hash_value indicates that a downloaded
         authorization does not match the expected hash.

5. IANA Considerations

   This document defines a two TLS extensions: client_authz(TBD) and
   server_authz(TBD).  These extension type values are assigned from the
   TLS Extension Type registry defined in [TLSEXT2].

   This document defines one TLS supplemental data type:
   authz_data(TBD).  This supplemental data type is assigned from the
   TLS Supplemental Data Type registry defined in [TLSSUPP].

   This document establishes a new registry, to be maintained by IANA,
   for TLS Authorization Data Formats.  The first 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.

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 many cases, the authorization information is itself sensitive.
   The double handshake technique can be used to provide protection for
   the authorization information.  Figure 2 illustrates the double
   handshake, where the initial handshake does not include any
   authorization extensions, but it does result in protected
   communications.  Then, a second handshake that includes the
   authorization information is performed using the protected
   communications.  In Figure 2, the number on the right side indicates



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   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.

   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
   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].




















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    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 Content
                Type for Attribute Certificates", work in progress.

   [ATTRCERT]   Farrell, S., and R. Housley, "An Internet Attribute
                Certificate Profile for Authorization", RFC 3281,
                April 2002.

   [HTTP]       Fielding, R, J. Gettys, J. Mogul, H. Frystyk,
                L. Masinter, P. Leach, 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", RFC 5226,
                May 2008.

   [PKIX1]      Cooper, D., S. Santesson, S. Farrell, S. Boeyen,
                R. Housley, 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.






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   [TLS1.1]     Dierks, T., and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol, Version 1.1", RFC 4346,
                February 2006.

   [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, September 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", RFC 3629, November 2003.

   [UTF-16]     Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                ISO 10646", RFC 2781, February 2000.

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
   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






Brown & Housley                                                [Page 19]


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