Network Working Group E. Rescorla Internet-Draft Network Resonance Expires: June 16, 2007 M. Salter National Security Agency December 13, 2006 Opaque PRF Inputs for TLS draft-rescorla-tls-opaque-prf-input-00.txt 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. This Internet-Draft will expire on June 16, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document describes a mechanism for using opaque PRF inputs with Transport Layer Security (TLS) and Datagram TLS (DTLS). Rescorla & Salter Expires June 16, 2007 [Page 1] Internet-Draft TLS Opaque PRF Inputs December 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used In This Document . . . . . . . . . . . . . . . 3 3. The OpaquePRFInput Extension . . . . . . . . . . . . . . . . . 3 3.1. Negotiating the OpaquePRFInput Extension . . . . . . . . . 4 3.2. PRF Modifications . . . . . . . . . . . . . . . . . . . . . 4 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 4.1. Threats to TLS . . . . . . . . . . . . . . . . . . . . . . 5 4.2. New Security Issues . . . . . . . . . . . . . . . . . . . . 5 4.3. Scope of Randomness . . . . . . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 8 Rescorla & Salter Expires June 16, 2007 [Page 2] Internet-Draft TLS Opaque PRF Inputs December 2006 1. Introduction TLS [RFC4346] and DTLS [RFC4347] use a 32-byte "Random" value consisting of a 32-bit time value time and 28 randomly generated bytes: struct { uint32 gmt_unix_time; opaque random_bytes[28]; } Random; The client and server each contribute a Random value which is then mixed with secret keying material to produce the final per- association keying material. In a number of United States Government applications, it is desirable to have some material with the following properties: 1. It is contributed both by client and server. 2. It is arbitrary-length. 3. It is mixed into the eventual keying material. 4. It is structured and decodable by the receiving party. These requirements are incompatible with the current Random mechanism, which supports a short, fixed-length value. This document describes a mechanism called "Opaque PRF Inputs for TLS" that meets these requirements. 2. Conventions Used In This Document 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]. 3. The OpaquePRFInput Extension The OpaquePRFInput is carried in a new TLS extension called "OpaquePRFInput". struct { opaque opaque_prf_input_value<0..2^16-1>; } OpaquePRFInput; The opaque_prf_input_value is an opaque byte-string which is generated in an implementation-dependent fashion. It MAY be generated by and/or made available to the TLS/DTLS-using application. Rescorla & Salter Expires June 16, 2007 [Page 3] Internet-Draft TLS Opaque PRF Inputs December 2006 3.1. Negotiating the OpaquePRFInput Extension The client requests support for the opaque PRF input feature by sending an "opaque_prf_input" extension in its ClientHello. The "extension_data" field contains an OpaquePRFInput value. When a server which does not recognize the "opaque_prf_input" extension receives one, it will ignore it as required by [RFC4366]. A server which recognizes the extension MAY choose to ignore it, in which case it SHOULD continue with the exchange as if it had not received the extension. If the server wishes to use the opaque PRF input feature, it MUST send its own "opaque_prf_input" extension with an opaque_prf_input_value equal in length to the client's opaque_prf_input_value. Clients SHOULD check the length of the server's opaque_prf_input_value and generate a fatal "illegal_parameter" error if it is present but does does not match the length that was transmitted in the ClientHello. Because RFC 4366 does not permit servers to request extensions which the client did not offer, the client may not offer the "opaque_prf_input" extension even if the server requires it. In this case, the server should generate a fatal "handshake_failure" alert. Because there is no way to mark extensions as critical, the server may ignore the "opaque_prf_input" extension even though the client requires it. If a client requires the opaque PRF input feature but the server does not negotiate it, the client SHOULD generate a fatal "handshake_failure" alert. 3.2. PRF Modifications When the opaque PRF input feature is in use, the opaque PRF input values MUST be mixed into the PRF along with the client and server random values during the PMS->MS conversion. Thus, the PRF becomes: master_secret = PRF(pre_master_secret, "master secret", ClientHello.random + ClientHello.opaque_prf_input_value + ServerHello.random + ServerHello.opaque_prf_input_value)[0..47]; Because new extensions may not be introduced in resumed handshakes, mixing in the opaque PRF inputs during the MS->keying material conversion would simply involve mixing in the same material twice. Therefore, the opaque PRF inputs are only used when the PMS is converted into the MS. Rescorla & Salter Expires June 16, 2007 [Page 4] Internet-Draft TLS Opaque PRF Inputs December 2006 4. Security Considerations 4.1. Threats to TLS When this extension is in use it increases the amount of data that an attacker can inject into the PRF. This potentially would allow an attacker who had partially compromised the PRF greater scope for influencing the output. Hash-based PRFs like the one in TLS are designed to be fairly indifferent to the input size (the input is already greater than the block size of most hash functions), however there is currently no proof that a larger input space would not make attacks easier. Another concern is that bad implementations might generate low entropy opaque PRF input values. TLS is designed to function correctly even when fed low-entropy random values because they are primarily used to generate distinct keying material for each connection. 4.2. New Security Issues As noted in Section 3 it is anticipated that applications may want to have access to the opaque PRF input values and that they may contain data that is meaningful at a higher layer. Because the values are covered by the TLS Finished message, they are integrity-protected by TLS. However, the application must independently provide any confidentiality necessary for those values. 4.3. Scope of Randomness RFC 4366 specifies that when a session is resumed the extensions from the original connection are used: If, on the other hand, the older session is resumed, then the server MUST ignore the extensions and send a server hello containing none of the extension types. In this case, the functionality of these extensions negotiated during the original session initiation is applied to the resumed session. This motivates why the the opaque PRF input does not get mixed into the PRF when generating the keying material from the master secret. Because the same opaque PRF inputs would be used for every connection in a session, they would not provide any differentiation in the keying material between the connections. 5. IANA Considerations Rescorla & Salter Expires June 16, 2007 [Page 5] Internet-Draft TLS Opaque PRF Inputs December 2006 This document defines an extension to TLS, in accordance with [RFC4366]: enum { opaque_prf_input (??) } ExtensionType; [[ NOTE: These values need to be assigned by IANA ]] 6. Acknowledgements This work was supported by the US Department of Defense. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366, April 2006. [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, April 2006. [I-D.ietf-tls-rfc4346-bis] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.2", draft-ietf-tls-rfc4346-bis-02 (work in progress), October 2006. 7.2. Informative References Rescorla & Salter Expires June 16, 2007 [Page 6] Internet-Draft TLS Opaque PRF Inputs December 2006 Authors' Addresses Eric Rescorla Network Resonance 2483 E. Bayshore #212 Palo Alto, CA 94303 USA Email: ekr@networkresonance.com Margaret Salter National Security Agency 9800 Savage Rd. Fort Meade 20755-6709 USA Email: msalter@restarea.ncsc.mil Rescorla & Salter Expires June 16, 2007 [Page 7] Internet-Draft TLS Opaque PRF Inputs December 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Rescorla & Salter Expires June 16, 2007 [Page 8]