Internet Engineering Task Force A. Popov, Ed. Internet-Draft M. Nystroem Intended status: Standards Track Microsoft Corp. Expires: April 8, 2016 D. Balfanz A. Langley Google Inc. October 6, 2015 Transport Layer Security (TLS) Extension for Token Binding Protocol Negotiation draft-ietf-tokbind-negotiation-01 Abstract This document specifies a Transport Layer Security (TLS) [RFC5246] extension for the negotiation of Token Binding protocol [I-D.ietf-tokbind-protocol] version and key parameters. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Expires April 8, 2016 [Page 1] Internet-Draft Token Binding Negotiation TLS Extension October 2015 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 2. Token Binding Negotiation Client Hello Extension . . . . . . 2 3. Token Binding Negotiation Server Hello Extension . . . . . . 3 4. Negotiating Token Binding Protocol Version and Key Parameters 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6.1. Downgrade Attacks . . . . . . . . . . . . . . . . . . . . 6 6.2. Triple Handshake Vulnerability in TLS . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 8.2. Informative References . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction In order to use the Token Binding protocol [I-D.ietf-tokbind-protocol], the client and server need to agree on the Token Binding protocol version and the parameters (signature algorithm, length) of the Token Binding key. This document specifies a new TLS extension to accomplish this negotiation without introducing additional network round-trips. 1.1. Requirements Language 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 [RFC2119]. 2. Token Binding Negotiation Client Hello Extension The client uses the "token_binding" TLS extension to indicate the highest supported Token Binding protocol version and key parameters. enum { token_binding(TBD), (65535) } ExtensionType; The "extension_data" field of this extension contains a "TokenBindingParameters" value. Popov, et al. Expires April 8, 2016 [Page 2] Internet-Draft Token Binding Negotiation TLS Extension October 2015 struct { uint8 major; uint8 minor; } ProtocolVersion; enum { rsa2048_pkcs1.5(0), rsa2048_pss(1), ecdsap256(2), (255) } TokenBindingKeyParameters; struct { ProtocolVersion token_binding_version; TokenBindingKeyParameters key_parameters_list<1..2^8-1> } TokenBindingParameters; "token_binding_version" indicates the version of the Token Binding protocol the client wishes to use during this connection. This SHOULD be the latest (highest valued) version supported by the client. [I-D.ietf-tokbind-protocol] describes version {1, 0} of the protocol. Prototype implementations of Token Binding drafts can indicate support of a specific draft version, e.g. {0, 1} or {0, 2}. "key_parameters_list" contains the list of identifiers of the Token Binding key parameters supported by the client, in descending order of preference. 3. Token Binding Negotiation Server Hello Extension The server uses the "token_binding" TLS extension to indicate support for the Token Binding protocol and to select the protocol version and key parameters. The server that supports Token Binding and receives a client hello message containing the "token_binding" extension, will include the "token_binding" extension in the server hello if all of the following conditions are satisfied: 1. The server supports the Token Binding protocol version offered by the client or a lower version. 2. The server finds acceptable Token Binding key parameters on the client's list. 3. The server is also negotiating Extended Master Secret TLS extension [I-D.ietf-tls-session-hash] (see security considerations section below for more details). The server will ignore any key parameters that it does not recognize. The "extension_data" field of the "token_binding" extension is Popov, et al. Expires April 8, 2016 [Page 3] Internet-Draft Token Binding Negotiation TLS Extension October 2015 structured the same as described above for the client "extension_data". "token_binding_version" contains the lower of the Token Binding protocol version offered by the client in the "token_binding" extension and the highest version supported by the server. "key_parameters_list" contains exactly one Token Binding key parameters identifier selected by the server from the client's list. 4. Negotiating Token Binding Protocol Version and Key Parameters It is expected that a server will have a list of Token Binding key parameters identifiers that it supports, in preference order. The server MUST only select an identifier that the client offered. The server SHOULD select the most highly preferred key parameters identifier it supports which is also advertised by the client. In the event that the server supports none of the key parameters that the client advertises, then the server MUST NOT include "token_binding" extension in the server hello. The client receiving the "token_binding" extension MUST terminate the handshake with a fatal "unsupported_extension" alert if any of the following conditions are true: 1. The client did not include the "token_binding" extension in the client hello. 2. "token_binding_version" is higher than the Token Binding protocol version advertised by the client. 3. "key_parameters_list" includes more than one Token Binding key parameters identifier. 4. "key_parameters_list" includes an identifier that was not advertised by the client. 5. Extended Master Secret [I-D.ietf-tls-session-hash] is not negotiated (see security considerations section below for more details). If the "token_binding" extension is included in the server hello and the client supports the Token Binding protocol version selected by the server, it means that the version and key parameters have been negotiated between the client and the server and SHALL be definitive for the TLS connection. In this case, the client MUST use the negotiated key parameters in the "provided_token_binding" as described in [I-D.ietf-tokbind-protocol]. Popov, et al. Expires April 8, 2016 [Page 4] Internet-Draft Token Binding Negotiation TLS Extension October 2015 If the client does not support the Token Binding protocol version selected by the server, then the connection proceeds without Token Binding. Please note that the Token Binding protocol version and key parameters are negotiated for each TLS connection, which means that the client and server include their "token_binding" extensions both in the full TLS handshake that establishes a new TLS session and in the subsequent abbreviated TLS handshakes that resume the TLS session. 5. IANA Considerations This document defines a new TLS extension "token_binding", which needs to be added to the IANA "Transport Layer Security (TLS) Extensions" registry. This document establishes a registry for identifiers of Token Binding key parameters entitled "Token Binding Key Parameters" under the "Token Binding Protocol" heading. Entries in this registry require the following fields: o Value: The octet value that identifies a set of Token Binding key parameters (0-255). o Description: The description of the Token Binding key parameters. o Specification: A reference to a specification that defines the Token Binding key parameters. This registry operates under the "Expert Review" policy as defined in [RFC5226]. The designated expert is advised to encourage the inclusion of a reference to a permanent and readily available specification that enables the creation of interoperable implementations using the identified set of Token Binding key parameters. An initial set of registrations for this registry follows: Value: 0 Description: rsa2048_pkcs1.5 Specification: this document Value: 1 Popov, et al. Expires April 8, 2016 [Page 5] Internet-Draft Token Binding Negotiation TLS Extension October 2015 Description: rsa2048_pss Specification: this document Value: 2 Description: ecdsap256 Specification: this document 6. Security Considerations 6.1. Downgrade Attacks The Token Binding protocol version and key parameters are negotiated via "token_binding" extension within the TLS handshake. TLS prevents active attackers from modifying the messages of the TLS handshake, therefore it is not possible for the attacker to remove or modify the "token_binding" extension. The signature algorithm and key length used in the TokenBinding of type "provided_token_binding" MUST match the parameters negotiated via "token_binding" extension. 6.2. Triple Handshake Vulnerability in TLS The Token Binding protocol relies on the TLS Exporters [RFC5705] to associate a TLS connection with a Token Binding. The triple handshake attack [TRIPLE-HS] is a known TLS protocol vulnerability allowing the attacker to synchronize exported keying material between TLS connections. The attacker can then successfully replay bound tokens. For this reason, the Token Binding protocol MUST NOT be negotiated unless the Extended Master Secret TLS extension [I-D.ietf-tls-session-hash] has also been negotiated. 7. Acknowledgements This document incorporates comments and suggestions offered by Eric Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Bill Cox, Nick Harper and others. 8. References 8.1. Normative References [I-D.ietf-tokbind-protocol] Popov, A., Nystrom, M., Balfanz, D., and A. Langley, "The Token Binding Protocol Version 1.0", draft-ietf-tokbind- protocol-02 (work in progress), September 2015. Popov, et al. Expires April 8, 2016 [Page 6] Internet-Draft Token Binding Negotiation TLS Extension October 2015 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, . [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, . [RFC5705] Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010, . 8.2. Informative References [I-D.ietf-tls-session-hash] Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley, A., and M. Ray, "Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension", draft-ietf- tls-session-hash-06 (work in progress), July 2015. [TRIPLE-HS] Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti, A., and P. Strub, "Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS. IEEE Symposium on Security and Privacy", 2014. Authors' Addresses Andrei Popov (editor) Microsoft Corp. USA Email: andreipo@microsoft.com Magnus Nystroem Microsoft Corp. USA Email: mnystrom@microsoft.com Popov, et al. Expires April 8, 2016 [Page 7] Internet-Draft Token Binding Negotiation TLS Extension October 2015 Dirk Balfanz Google Inc. USA Email: balfanz@google.com Adam Langley Google Inc. USA Email: agl@google.com Popov, et al. Expires April 8, 2016 [Page 8]