Internet-Draft M. Brown February 2006 RedPhone Security Expires: August 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. Extension types are carried in the client and server hello messages to confirm that both parties support the authorization messages. The syntax and semantics of the authorization messages are described in detail. Brown & Housley [Page 1] Internet-Draft February 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 when the original design criteria for TLS were determined. 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 allow TLS clients to provide to the TLS server authorization information, and allow TLS server to provide to the TLS client authorization information about the TLS server. The authorization extensions 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 messages associated with the authorization extensions will only be exchanged 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 request the use of 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]. Brown & Housley [Page 2] Internet-Draft February 2006 1.2. Overview Figure 1 illustrates the placement of the authorization messages in the full TLS handshake. In order to avoid unnecessary disclosure of privilege information, the authorization messages appear after the Finished message. This placement ensures that they are encrypted and integrity protected. Client Server ClientHello --------> ServerHello Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone Certificate* ClientKeyExchange CertificateVerify* [ChangeCipherSpec] Finished --------> ClientAuthorizationData --------> [ChangeCipherSpec] <-------- Finished <-------- ServerAuthorizationData 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 that the ClientAuthorizationData message and ServerAuthorizationData message are supported. The ServerHello message also includes an indication that the ClientAuthorizationData message and ServerAuthorizationData message are supported. Both the client and the server MUST indicate support for the authorization messages, otherwise they MUST NOT be included in the handshake. Brown & Housley [Page 3] Internet-Draft February 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: 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_request and server_authz_request. 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_request(TBD), server_authz_request(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 February 2006 2.1. The client_authz_request Extension Type Clients MUST include the client_authz_request 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. The format is indicated with the AuthzDataFormat type defined in Section 2.3. Servers that receive an extended client hello message containing the client_authz_request extension MUST respond with the same client_authz_request extension in the extended server hello message if the server is willing to receive authorization data in the indicated format. The client_authz_request extension MUST be omitted from the extended server hello message if the server is not willing to receive authorization data in the indicated format. 2.2. The server_authz_request Extension Type Clients MUST include the server_authz_request 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 is desired. The format is indicated with the AuthzDataFormat type defined in Section 2.3. Servers that receive an extended client hello message containing the server_authz_request extension MUST respond with the same server_authz_request extension in the extended server hello message if the server is willing to provide authorization data in the requested format. The server_authz_request extension MUST be omitted from the extended server hello message if the server is not able to provide authorization data in the requested format. 2.3. AuthzDataFormat Type The AuthzDataFormat type is used in both the client_authz_request and the server_authz_request 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; 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]. Brown & Housley [Page 5] Internet-Draft February 2006 When the saml_assertion value is present, the authorization data is an assertion composed using the Security Assertion Markup Language (SAML) [SAML]. 3. Handshake Messages This document specifies the use of two new handshake messages: the ClientAuthorizationData message and ServerAuthorizationData message. These messages are described in Section 3.1 and Section 3.2, respectively. The updated handshake message structure becomes: enum { hello_request(0), client_hello(1), server_hello(2), certificate(11), server_key_exchange (12), certificate_request(13), server_hello_done(14), certificate_verify(15), client_key_exchange(16), finished(20), certificate_url(21), certificate_status(22), client_authz_data(TBD), server_authz_data(TBD), (255) } HandshakeType; struct { HandshakeType msg_type; /* handshake type */ uint24 length; /* octets in message */ select (HandshakeType) { case hello_request: HelloRequest; case client_hello: ClientHello; case server_hello: ServerHello; case certificate: Certificate; case server_key_exchange: ServerKeyExchange; case certificate_request: CertificateRequest; case server_hello_done: ServerHelloDone; case certificate_verify: CertificateVerify; case client_key_exchange: ClientKeyExchange; case finished: Finished; case certificate_url: CertificateURL; case certificate_status: CertificateStatus; case client_authz_data: ClientAuthorizationData; case server_authz_data: ServerAuthorizationData; } body; } Handshake; Brown & Housley [Page 6] Internet-Draft February 2006 3.1. ClientAuthorizationData Message The ClientAuthorizationData message contains authorization data associated with the TLS client. The format of the authentication data depends on the format negotiated in the client_authz_request hello message extensions. struct { client_authz_data AuthorizationData; } ClientAuthorizationData; The AuthorizationData structure is described in Section 3.3. 3.2. ServerAuthorizationData Message The ServerAuthorizationData message contains authorization data associated with the TLS server. The format of the authorization data depends on the format negotiated in the server_authz_request hello message extensions. struct { server_authz_data AuthorizationData; } ServerAuthorizationData; The AuthorizationData structure is described in Section 3.3. 3.3. AuthorizationData Type The AuthorizationData structure is defined as follows. For readability, the definition of AuthzDataFormat is repeated from section 2.3. All of the entries in the authz_data_list MUST contain the same authz_format value, and this value MUST match the one advertised by the client in the extended hello message extension. struct{ AuthorizationDataEntry authz_data_list<1..2^16-1>; } AuthorizationData; struct { AuthzDataFormat authz_format; select (authz_format) { case x509_attr_cert: X509AttrCert; case saml_assertion: SAMLAssertion; } authz_data_entry; } AuthorizationDataEntry; Brown & Housley [Page 7] Internet-Draft February 2006 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. 4. IANA Considerations IANA needs to assign two TLS Extension Types: client_authz_request, and server_authz_request. IANA needs to assign two TLS Handshake Message Types: client_authz_data, and server_authz_data. 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. Brown & Housley [Page 8] Internet-Draft February 2006 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. 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. [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. Brown & Housley [Page 9] Internet-Draft February 2006 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 Full Copyright Statement Copyright (C) The Internet Society (2006). 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