Internet-Draft M. Brown March 2006 RedPhone Security Expires: September 2006 R. Housley Vigil Security Transport Layer Security (TLS) Authorization Extensions Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2006). All Rights Reserved. Abstract This document specifies authorization extensions to the Transport Layer Security (TLS) Handshake Protocol. Extensions carried in the client and server hello messages to confirm that both parties support the desired authorization data types. Then, if supported by both the client and the server, authorization information is exchanged in the supplemental data handshake message. Brown & Housley [Page 1] Internet-Draft March 2006 1. Introduction Transport Layer Security (TLS) protocol [TLS1.0][TLS1.1] is being used in an increasing variety of operational environments, including ones that were not envisioned at the time of the original design for TLS. The extensions introduced in this document are designed to enable TLS to operate in environments where authorization information needs to be exchanged between the client and the server before any protected data is exchanged. This document describes authorization extensions for the TLS Handshake Protocol in both TLS 1.0 and TLS 1.1. These extensions observe the conventions defined for TLS Extensions [TLSEXT] that make use of the general extension mechanisms for the client hello message and the server hello message. The extensions described in this document confirm that both the client and the server support the desired authorization data types. Then, if supported, authorization information is exchanged in the supplemental data handshake message [TLSSUPP]. The authorization extensions may be used in conjunction with TLS 1.0 and TLS 1.1. The extensions are designed to be backwards compatible, meaning that the Handshake Protocol Supplemental Data messages will only contain authorization information of a particular type if the client indicates support for them in the client hello message and the server indicates support for them in the server hello message. Clients typically know the context of the TLS session that is being setup, thus the client can use the authorization extensions when they are needed. Servers must accept extended client hello messages, even if the server does not "understand" the all of the listed extensions. However, the server will not indicate support for these "not understood" extensions. Then, clients may reject communications with servers that do not support the authorization extensions. 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. Brown & Housley [Page 2] Internet-Draft March 2006 Client Server ClientHello (w/ extensions) --------> ServerHello(w/ extensions) SupplementalData* Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone SupplementalData* Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> [ChangeCipherSpec] <-------- Finished Application Data <-------> Application Data * Indicates optional or situation-dependent messages that are not always sent. [] Indicates that ChangeCipherSpec is an independent TLS Protocol content type; it is not actually a TLS handshake message. Figure 1. Authorization data exchange in full TLS handshake The ClientHello message includes an indication of the client authorization data formats that are supported and an indication of the server authorization data formats that are supported. The ServerHello message contains similar indications, but any authentication 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. Brown & Housley [Page 3] Internet-Draft March 2006 2. Authorization Extension Types The general extension mechanisms enable clients and servers to negotiate whether to use specific extensions, and how to use specific extensions. As specified in [TLSEXT], the extension format used in the extended client hello message and extended server hello message is repeated here for convenience: struct { ExtensionType extension_type; opaque extension_data<0..2^16-1>; } Extension; The extension_type identifies a particular extension type, and the extension_data contains information specific to the particular extension type. As specified in [TLSEXT], for all extension types, the extension type MUST NOT appear in the extended server hello message unless the same extension type appeared in the corresponding client hello message. Clients MUST abort the handshake if they receive an extension type in the extended server hello message that they did not request in the associated extended client hello message. When multiple extensions of different types are present in the extended client hello message or the extended server hello message, the extensions can appear in any order, but there MUST NOT be more than one extension of the same type. This document specifies the use of two new extension types: client_authz and server_authz. These extension types are described in Section 2.1 and Section 2.2, respectively. This specification adds two new types to ExtensionType: enum { client_authz(TBD), server_authz(TBD), (65535) } ExtensionType; The authorization extensions are relevant when a session is initiated and any subsequent session resumption. However, a client that requests resumption of a session does not know whether the server 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. Brown & Housley [Page 4] Internet-Draft March 2006 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 as described in Section 2.3. Servers that receive an extended client hello message containing the client_authz extension MUST respond with the same client_authz extension in the extended server hello message if the server is willing to receive authorization data in the indicated format. Any unacceptable formats must be removed from the list provided by the client. The client_authz extension MUST be omitted from the extended server hello message if the server is not willing to receive authorization data in any of the indicated formats. 2.2. The server_authz Extension Type Clients MUST include the server_authz extension type in the extended client hello message to indicate their desire to receive authorization data from the server. The extension_data field indicates the format of the authorization data that will be sent in the supplemental data handshake message. The syntax of the server_authz extension_data field as described in Section 2.3. Servers that receive an extended client hello message containing the server_authz extension MUST respond with the same server_authz extension in the extended server hello message if the server is willing to provide authorization data in the requested format. Any unacceptable formats must be removed from the list provided by the client. The server_authz extension MUST be omitted from the extended server hello message if the server is not able to provide authorization data in any of the indicated formats. 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 AuthzDataFormat type definition is: enum{ x509_attr_cert(0), saml_assertion(1), (255) } AuthzDataFormat; Brown & Housley [Page 5] Internet-Draft March 2006 When the x509_attr_cert value is present, the authorization data is an X.509 Attribute Certificate that conforms to the profile in RFC 3281 [ATTRCERT]. When the saml_assertion value is present, the authorization data is an assertion composed using the Security Assertion Markup Language (SAML) [SAML]. 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. The format of the authentication data depends on the format negotiated in the client_authz hello message extension. The AuthorizationData structure is described in Section 3.3. 3.2. Server Authorization Data The SupplementalData message sent from the server to the client contains authorization data associated with the TLS server. The format of the authorization data depends on the format negotiated in the server_authz hello message extensions. The AuthorizationData structure is described in Section 3.3. Brown & Housley [Page 6] Internet-Draft March 2006 3.3. AuthorizationData Type The AuthorizationData structure carried authorization information for either the client or the server. The AuthzDataFormat specified in Section 2.3 for use in the hello extensions is also used in this structure. All of the entries in the authz_data_list MUST employ authorization data formats that were negotiated in the relevant hello message extension. struct{ AuthorizationDataEntry authz_data_list<1..2^16-1>; } AuthorizationData; struct { AuthzDataFormat authz_format; select (AuthzDataFormat) { case x509_attr_cert: X509AttrCert; case saml_assertion: SAMLAssertion; } } AuthorizationDataEntry; enum{ x509_attr_cert(0), saml_assertion(1), (255) } AuthzDataFormat; opaque X509AttrCert<1..2^16-1>; opaque SAMLAssertion<1..2^16-1>; 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); the main difference being that the AC contains no public key. An AC may contain attributes that specify group membership, role, security clearance, or other authorization information associated with the AC holder. When SAMLAssertion is used, the field contains XML constructs with a nested structure defined in [SAML]. SAML is an XML-based framework for exchanging security information. This security information is expressed in the form of assertions about subjects, where a subject is either human or computer with an identity. In this context, the assertions are most likely to convey authorization decisions about whether subjects are allowed to access certain resources. Assertions are issued by SAML authorities, namely, authentication authorities, attribute authorities, and policy decision points. Brown & Housley [Page 7] Internet-Draft March 2006 4. IANA Considerations IANA assigned two TLS Extension Types: client_authz(TBD) and server_authz(TBD). IANA assigned one TLS Supplemental Data Formats: authz_data(TBD). IANA needs to establish a registry for TLS Authorization Data Formats. The first two entries in the registry are x509_attr_cert(0) and saml_assertion(1). TLS Authorization Data Format identifiers with values in the inclusive range 0-63 (decimal) are assigned via RFC 2434 [IANA] Standards Action. Values from the inclusive range 64-223 (decimal) are assigned via RFC 2434 Specification Required. Values from the inclusive range 224-255 (decimal) are reserved for RFC 2434 Private Use. 5. Security Considerations A TLS server can support more than one application, and each application may include several features, each of which requires separate authorization checks. This is the reason that more than one piece of authorization information can be provided. A TLS server that requires different authorization information for different applications or different application features may find that a client has provided sufficient authorization information to grant access to a subset of these offerings. In this situation the TLS Handshake protocol will complete successfully; however, the server must ensure that the client will only be able to use the appropriate applications and application features. That is, the TLS server must deny access to the applications and application features for which authorization has not been confirmed. In many cases, the authorization information is itself sensitive. The double handshake technique can be used to provide protection for the authorization information. Figure 2 illustrates the double handshake, where the initial handshake does not include any authorization extensions, but it does result in protected communications. Then, a second handshake that includes the authorization information is performed using the protected communications. In Figure 2, the number on the right side indicates the amount of protection for the TLS message on that line. A zero (0) indicates that there is no communication protection; a one (1) indicates that protection is provided by the first TLS session; and a two (2) indicates that protection is provided by both TLS sessions. Brown & Housley [Page 8] Internet-Draft March 2006 Client Server ClientHello (no extensions) --------> |0 ServerHello (no extensions) |0 Certificate* |0 ServerKeyExchange* |0 CertificateRequest* |0 <-------- ServerHelloDone |0 Certificate* |0 ClientKeyExchange |0 CertificateVerify* |0 [ChangeCipherSpec] |0 Finished --------> |1 [ChangeCipherSpec] |0 <-------- Finished |1 ClientHello (w/ extensions) --------> |1 ServerHello (w/ extensions) |1 SupplementalData (w/ authz data)* |1 Certificate* |1 ServerKeyExchange* |1 CertificateRequest* |1 <-------- ServerHelloDone |1 SupplementalData (w/ authz data)* |1 Certificate* |1 ClientKeyExchange |1 CertificateVerify* |1 [ChangeCipherSpec] |1 Finished --------> |2 [ChangeCipherSpec] |1 <-------- Finished |2 Application Data <-------> Application Data |2 Figure 2. Double Handshake to Protect Authorization Data Brown & Housley [Page 9] Internet-Draft March 2006 6. Normative References [ATTRCERT] Farrell, S., and R. Housley, "An Internet Attribute Certificate Profile for Authorization", RFC 3281, April 2002. [IANA] Narten, T., and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 3434, October 1998. [TLS1.0] Dierks, T., and C. Allen, "The TLS Protocol, Version 1.0", RFC 2246, January 1999. [TLS1.1] Dierks, T., and E. Rescorla, "The Transport Layer Security (TLS) Protocol, Version 1.1", RFC 4346, February 2006. [TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 3546, June 2003. [TLSSUPP] Santesson, S., " TLS Handshake Message for Supplemental Data", work in progress: draft-santesson-tls-supp, March 2006. [SAML] Organization for the Advancement of Structured Information Standards, "Security Assertion Markup Language (SAML), version 1.1", September 2003. [Version 2.0 is out for public comment; it will replace this reference if approved.] [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Author's Address Mark Brown RedPhone Security 2019 Palace Avenue Saint Paul, MN 55105 USA mark redphonesecurity com Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA housley vigilsec com Brown & Housley [Page 10] Internet-Draft March 2006 Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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