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. Authorization information is carried in the client and server hello messages. The syntax and semantics of the authorization messages are described in detail. 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 when the original design criteria for TLS were determined. The authorization 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 are intended for use with both TLS 1.0 and TLS 1.1. The extensions are designed to be backwards compatible, meaning that the authorization information carried in the client hello message and the server hello message can be ignored by any implementation that does not support the included authorization information format. Clients typically know the context of the TLS session that is being setup, thus the client can use of the authorization extensions when 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 make use of the authorization information if the authorization extension is not supported or the authorization information is provided in an unsupported format. 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 March 2006 1.2. Overview Figure 1 illustrates the placement of the authorization messages in the full TLS handshake. Client Server ClientHello (with AuthorizationData) --------> ServerHello (with AuthorizationData) Certificate* ServerKeyExchange* CertificateRequest* <-------- ServerHelloDone 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 Protocol message. Figure 1. AuthorizationData carried in ClientHello and ServerHello The ClientHello message includes the AuthorizationData extension, which contains the authorization data for the client, and then the ServerHello message includes the AuthorizationData extension, which contains the authorization data for the server. If the server does not support the AuthorizationData extension, or the server does not support the authorization information format used by the client, then the server MUST NOT include the AuthorizationData extension in the ServerHello message. The Handshake Protocol continues, but without the benefit of authorization information. 2. AuthorizationData Extension The general extension mechanisms enable clients and servers to negotiate the use of specific extensions. As specified in [TLSEXT], the extension format used in the extended client hello message and Brown & Housley [Page 3] Internet-Draft March 2006 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 one new extension type: authz_data. This specification adds one new type to ExtensionType: enum { authz_data(TBD), (65535) } ExtensionType; The authorization extension is relevant when a session is initiated, regardless of the use of a full handshake or use of session resumption. Clients MUST explicitly present AuthorizationData in every client hello message for which authorization information is desired. Upon receipt of a client hello message that requests session resumption but which contains no acceptable AuthorizationData, the TLS server MAY resume the session but it MUST NOT grant authorization to the session being resumed based on any prior session authorization. These requirements allow a series of resumed sessions to have different authorizations from one another. More importantly, the authorization information is always provided by the client in case the server no longer honors the session resumption at the requested authorization level. Repeated inclusion of the authorization information allows the Handshake Protocol to proceed the same way for Brown & Housley [Page 4] Internet-Draft March 2006 both resume and session origination. 2.1. The authz_data Extension Type Clients MUST include the authz_data extension type in the extended client hello message to send authorization data to the server. The extension_data field contains the authorization data. Section 2.2 specifies the authorization data formats that are supported. Servers that receive an extended client hello message containing the authz_data extension MUST respond with the authz_data extension in the extended server hello message if the server is willing to make use of the received authorization data in the provided format. If the server has any authorization information to send to the client, then the server MUST include the information in the authz_data extension type in the extended server hello message. The AuthorizationData structure is described in Section 2.3. 2.2. AuthzDataFormat Type The AuthzDataFormat type is used in the authz_data extension. It indicates the format of the authorization information that will be transferred. The AuthzDataFormat type definition is: enum { x509_attr_cert(0), saml_assertion(1), x509_attr_cert_url(2), saml_assertion_url(3), (255) } AuthzDataFormat; 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) [SAML]. 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. Brown & Housley [Page 5] Internet-Draft March 2006 Additional formats can be registered in the future using the procedures in section 3. 2.3. AuthorizationData Type The AuthorizationData type is carried in the extension_data field for the authz_data extension. When it appears in the extended client hello message, it carries authorization information for the TLS client. When it appears in the extended server hello message, it carries authorization information for the TLS server. 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; case x509_attr_cert_url: URLandHash; case saml_assertion_url: URLandHash; } authz_data_entry; } AuthorizationDataEntry; opaque X509AttrCert<1..2^16-1>; opaque SAMLAssertion<1..2^16-1>; struct { opaque url<1..2^16-1>; HashType hash_type; select (hash_type) { case sha1: SHA1Hash; case sha256: SHA256Hash; } hash; } URLandHash; enum { sha1(0), sha256(1), (255) } HashType; opaque SHA1Hash[20]; opaque SHA1Hash[32]; When X509AttrCert is used, the field contains an ASN.1 DER-encoded X.509 Attribute Certificate (AC) that follows the profile in RFC 3281 Brown & Housley [Page 6] Internet-Draft March 2006 [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. Since X509AttrCert and SAMLAssertion can lead to a significant increase in the size of the hello messages, 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; however, to avoid circular dependencies, supported schemes SHOULD NOT themselves make use of TLS, such as the https scheme. Implementations that support either x509_attr_cert_url or saml_assertion_url MUST support both SHA-1 [SHA1] and SHA-256 [SHA2] as one-way hash functions. Other one-way hash functions may also be supported. Additional one-way hash functions can be registered in the future using the procedures in section 3. 3. IANA Considerations IANA has assigned one TLS Extension Types: authz_data(TBD). IANA has established 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 7] Internet-Draft March 2006 IANA has established a registry for TLS Hash Types. The first two entries in the registry are sha1(0) and sha256(1). TLS Hash Type identifiers with values in the inclusive range 0-158 (decimal) are assigned via RFC 2434 [IANA] Standards Action. Values from the inclusive range 159-223 (decimal) are assigned via RFC 2434 Specification Required. Values from the inclusive range 224-255 (decimal) are reserved for RFC 2434 Private Use. 4. 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 information, 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 |0 (no AuthorizationData) --------> |0 ServerHello |0 (no AuthorizationData) |0 Certificate* |0 ServerKeyExchange* |0 CertificateRequest* |0 <-------- ServerHelloDone |0 Certificate* |0 ClientKeyExchange |0 CertificateVerify* |0 [ChangeCipherSpec] |0 Finished --------> |1 [ChangeCipherSpec] |0 <-------- Finished |1 ClientHello |1 (with AuthorizationData) --------> |1 ServerHello |1 (with AuthorizationData) |1 Certificate* |1 ServerKeyExchange* |1 CertificateRequest* |1 <-------- ServerHelloDone |1 Certificate* |1 ClientKeyExchange |1 CertificateVerify* |1 [ChangeCipherSpec] |1 Finished --------> |2 [ChangeCipherSpec] |1 <-------- Finished |2 Application Data <-------> Application Data |2 Figure 2. Protection of Authorization Data (Two Full Handshakes) Brown & Housley [Page 9] Internet-Draft March 2006 Public key operations can be minimized by making the second handshake a resumption. This is much more efficient in term of computation and message exchanges. Figure 3 illustrates this more efficient double handshake. Client Server ClientHello |0 (no AuthorizationData) --------> |0 ServerHello |0 (no AuthorizationData) |0 Certificate* |0 ServerKeyExchange* |0 CertificateRequest* |0 <-------- ServerHelloDone |0 Certificate* |0 ClientKeyExchange |0 CertificateVerify* |0 [ChangeCipherSpec] |0 Finished --------> |1 [ChangeCipherSpec] |0 <-------- Finished |1 ClientHello |1 (with AuthorizationData) --------> |1 ServerHello |1 (with AuthorizationData) |1 [ChangeCipherSpec] |1 <-------- Finished |2 [ChangeCipherSpec] |1 Finished --------> |2 Application Data <-------> Application Data |2 Figure 3. Protection of Authorization Data (Resumption) Brown & Housley [Page 10] Internet-Draft March 2006 5. 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.] [SHA1] National Institute of Standards and Technology (NIST), FIPS PUB 180-1, Secure Hash Standard, 17 April 1995. [SHA2] National Institute of Standards and Technology (NIST), FIPS PUB 180-2: Secure Hash Standard, 1 August 2002. [STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Brown & Housley [Page 11] Internet-Draft March 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). 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. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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