Internet Engineering Task Force M. Badra INTERNET DRAFT O. Cherkaoui UQAM University I. Hajjeh Expires: 8, February 2004 A. Serhrouchni ENST, Paris August, 10 2004 Pre-Shared-Key key Exchange methods for TLS Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 February 8, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document specifies new key exchange methods for Transport Layer Security protocol to support authentication based on pre installed key and to allow anonymous exchanges, identity protection And Perfect Forward Secrecy. Badra, et. al. Expires February 2005 [Page 1] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 1. Introduction Transport Layer Security (TLS) [TLS] is an authentication protocol that establishes a secure channel, as well as mutual authentication, protected cipher suite negotiation and key exchange between two entities. TLS handshake uses certificates and PKI for mutual authentication and key exchange. In many cases, a TLS public-key- based handshake is unnecessary; especially for closed environments or for clients pre-configured. This document specifies how to establish a TLS session using symmetric keys. Although several Internet Draft authors ([TLSPSK], [TLSSK], [TSLEXP], etc) propose the pre shared key mechanism, none of them provides neither anonymous exchanges and identity protection against eavesdropping nor Perfect Forward Secrecy (PFS). On the other hand, some approaches like [ISATLS], propose a radical change to the TLS protocol. Other like [SPTLS], propose Password-based cipher suite for TLS Handshake scheme. This document specifies new key exchange methods for TLS for pre shared key. The advantageous use of the pre shared key regarding the Public Key Infrastructure (PKI) based certificates is that the pre shared key reduces the cryptographic operations, the messages load and the number of round trips. 1.1. Requirements language The key words "MUST", "SHALL", "SHOULD", and "MAY", in this document are to be interpreted as described in RFC-2119. 2. Changes to the TLS Handshake protocol TLS [TLS] defines the client key exchange message that is always sent by the client. With this message [TLS], the premaster secret is set, either though direct transmission of the RSA-encrypted secret, or by the transmission of Diffie-Hellman parameters which will allow each side to agree upon the same premaster secret. The structure of this message depends on which key exchange method has been selected. The actual TLS standard defines two methods using RSA or Diffie_Hellman algorithms. The rest of this document describes the changes to the handshake messages contents when the pre shared key is being used. 2.1. Client Hello In order to negotiate and to signal to the server that the client wishes to use a pre_shared_key key exchange method, the client MAY include an extension of type "psk_key_exchange (9)" in the extended client hello, such is defined in [TLSEXT]. The "extension_data" Badra, et. al. Expires - February 2005 [Page 2] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 field of the psk key exchange extension SHALL contain "PSKKeyExchangeMethod" where: struct { PSKMethod psk_methods_list<0..2^16-1>; } PSKKeyExchangeMethod; struct { MethodType method_type; Select (method_type) { case rsa_psk : RSAPSK case diffie_hellman_psk : DHPSK } method; } PSKMethod; enum { rsa_psk(0), diffie_hellmen_psk(1), (255) } MethodType; Here, "PSKKeyExchangeMethod" provides a list of PSK key exchange methods that the client supports. 2.3. Server Key Exchange The format of ServerKeyExchange is as follow: struct { select (KeyExchangeAlgorithm) { case diffie_hellman: ServerDHParams params; Signature signed_params; case rsa: ServerRSAParams params; Signature signed_params; case rsa_psk: /*NEW/ ServerRSAParamsPSK params; Signature signed_params; /*optional/ case diffie_hellman_psk: /*NEW/ ServerDHParamsPSK params; Signature signed_params;/*optional/ }; } ServerKeyExchange; rsa_psk and diffie_hellman_psk cases are respectively identical to rsa and diffie_hellman cases that are definied in [TLS]. Note that because the pre_shared_key SHOULD protect entities against man-in-the-middle attack (see section 2.4), the server MAY not sign its Diffie_Hellman parameters and thus the signed_params field MAY be omitted. For more information, see security considerations section. Badra, et. al. Expires - February 2005 [Page 3] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 2.2. Client Key Exchange This document adds two new key exchange methods to the enumerated KeyExchangeAlgorithm originally defined in [TLS]. enum { rsa, diffie_hellman, rsa_psk, diffie_hellman_psk } KeyExchangeAlgorithm; Thus, the structure of the client key exchange becomes as follow: struct { select (KeyEchangeAlgorithm){ case rsa: EncryptedPreMasterSecret; case diffie_hellman: ClientDiffieHellmanPublic; case rsa_psk: EncryptPreMasterSecretPSK; /*NEW/ case diffie_hellman_psk: ClientDiffieHellmanPublicPSK; /*NEW/ } exchange_key; } ClientKeyExchange; 2.2.1. rsa_psk encrypted premaster secret message If rsa_psk is being used for key agreement, the client generates a 30-byte random value, concatenates it with the pre shared key identity, encrypts the result (premaster secret) using the server public key and sends it in an encrypted premaster secret message. Structure of the premaster secret: struct { ProtocolVersion client_version; opaque random[30]; opaque psk_identity<1..2^16-1>; opaque pad[16-psk_identity.length]; } PreMasterSecret; struct { public-key-encrypted PreMasterSecret pre_master_secret; } EncryptedPreMasterSecretPSK; For interoperation issues, this document uses the same definition used in [TLSSRP]. Thus, the psk_identity SHALL be UTF-8 encoded Unicode, where the psk_identity is the pre shared key identifier. If the psk_identity is less than 16 bytes in length, the premaster secret will be padded to obtain 46 bytes. For example, if the psk_identity length is 13 bytes, then the last three bytes of the premaster secret will be 0x03 0x03 0x03. This mechanism will allow the server to extract the psk_identity from the premaster secret. Badra, et. al. Expires - February 2005 [Page 4] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 2.2.2. diffie_hellman_psk encrypted premaster secret message Because the client does not use any certificate, its value Yc needs to be sent. As a result, the case implicit MAY be omitted. struct { select (PublicValueEncoding) { case implicit: struct { }; case explicit: opaque dh_Yc<1..2^16-1>; } dh_public; opaque psk_identity<1..2^16-1>; } ClientDiffieHellmanPublicPSK; dh_Yc The client's Diffie-Hellman public value (Yc). psk_identity The pre shared key identifier. The psk_identity helps the client to indicate which key it wants to use and the server to retrieve the corresponding pre shared key value, if exists. When using a Diffie-Hellman based key exchange method, the psk_identity is sent in the clear. 2.4. Computing the master secret This document uses the same mechanism defined in [TLS] for keys computation and calculation, except the master secret key. It generates the master secret by applying the PRF on the premaster secret XOR pre_shared_key value instead of the premaster secret: master_secret = PRF(pre_master_secret XOR pre_shared_key, "master_secret", ClientHello.random + ServerHello.random)[0..47]; As a result, if the server uses a static private key and if this key is compromised, the intruder must have the pre_shared_key to decrypt old sessions. On the other hand, if either the client or the server calculates an incorrect premaster_secret XOR pre_shared_key value, the finished messages will fail to decrypt properly and the other party will return a bad_record_mac alert. This MAY happen when the server does not send its certificate and that a man-in-the-middle intercepts the session exchanges and sends its public key instead of the server public key. Badra, et. al. Expires - February 2005 [Page 5] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 2.5. Error Alerts Three new TLS error alerts are defined by this document (This section is inspired by [TLSSRP]): a) "unknown_psk_key_exchange" (integer) - this alert MAY be sent by a server that does not support any PSK key exchange methods sent by the client. This alert is always a warning. Upon receiving this alert, the client MAY send a new hello message on the same connection using another TLS authentication methods. b) "unknown_psk_identity" (integer) - this alert MAY be sent by a server that receives an unknown ticket identity. This alert is always fatal. c) "missing_psk_identity" (integer) - this alert MAY be sent by a server that would like to select an offered PSK key exchange method, if the MethodType extension is absent from the client's hello message. This alert is always a warning. Upon receiving this alert, the client MAY send a new hello message on the same connection, this time including the MethodType extension. 2.6. Handshake In order to indicate the support of the shared key type, the client adds the extension "psk_key_exchange (9)" to its extended hello message. When the server receives an extended client hello message, it replies by its hello that contains the following attributes: Protocol Version, Random, Session ID, Cipher Suite, and Compression Method. If the server is able to agree on a key exchange method using the pre shared key, it will send its server key exchange message that contains the selected method. In this case, the Certificate message MAY be omitted from the response. If the server does not support any PSK key exchange methods sent by the client, the server MAY abort the handshake with a unknown_key_exchange alert. Now the server will send the server hello done message, indicating that the hello-message phase of the handshake is completed. The client send its client key exchange message. The content of this message depends on the method selected between the client hello and the server key exchange messages. Badra, et. al. Expires - February 2005 [Page 6] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 The handshake exchange is given in the following diagram: ClientHello --------> (MethodType) ServerHello Certificate* ServerKeyExchange <-------- ServerHelloDone ClientKeyExchange ChangeCipherSpec Finished --------> ChangeCipherSpec <-------- Finished * Indicates an optional message which is not always sent. 3. Security considerations The server MUST stock the shared key in a secure and protected manner in order to prevent attackers from retrieving its value. During the handshake phase, the server MAY send its certificate. The certificate's use protects entities against man-in-the-middle attack. If the server certificate is omitted, the client and the server authenticate each other via the finished messages. In fact, the finished value is computed using the master_secret calculated during the establishment session and the pre shared key. Thus, if the client is intercepted by a bogus server, this later will be detectable by the client during the finished phase. As a result, no third party can calculate the same finished value without having the correct pre_shared_key. Instead, the third party MAY discover the pre shared key identity sent in the client key exchange message. When using a Diffie-Hellman based PSK key exchange method, the client sends its psk_identity in the clear. In order to avoid this issue, the client could first open a conventional anonymous and then renegotiate a PSK key exchange method with the handshake protected by the first connection. Another solution MAY be done using the pseudonym management. 3.1. Key management with non-human support In the case where the client does not enter his credentials manually during the session establishment and that he does not need to remember them, then he can stock them on a secure token (e.g. smartcard) or in a local file. In this case, the server and the client MAY update the pre shared key value after each session has been formed. In this case, the both MAY add a seed to their credentials entries. By this method, the client's support and the Badra, et. al. Expires - February 2005 [Page 7] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 server calculate the seed and update the pre shared key as following (in the session i): seed(0) is a random on 16 bytes. seed(i) = P_MD5(seed(i-1) XOR psk_identity, "seed" + ClientHello.random + ServerHello.random)[0..16]; psk(i) = PRF(psk(i-1) XOR premaster secret(i), "pre shared key", ServerHello.random + ClientHello.random)[0..48]; With this mechanism, the psk_identity remains unchanged. However, when the client connect to the server, it sends the seed (seed(i-1) for session i) instead of the psk_identity. The rest of the protocol is unchangeable. This SHALL ensure, among other, PFS and anonymity. 4. IANA Considerations To be specified. 5. Acknowledgment This document has been inspired by [TLS], [TLSSRP] and [TLSPSK]. Thus, it reused extracts of these documents. 6. References 6.1. Normative References [TLSEXT] Blake-Wilson, S., Nystrom, M., Hopwwod, D., Mikkelsen, J. and Wright, T., "Transport Layer Security (TLS) Extensions", RFC 3546, June 2003. [TLS] Dierks, T., and Allen, C., "The TLS Protocol Version 1.0", RFC 2246, November 1998. [ISATLS] Hajjeh, I., and Serhrouchni, A., "ISAKMP Handshake for SSL/TLS", IEEE GLOBECOM'03, Vol. 3, San Francisco, USA, 1-5 December 2003, Pages: 1481-1485. [SPTLS] Steiner, Michael, et. al., "Secure Password-Based Cipher Suite for TLS", ACM Transaction on Information and System Security, Vol. 4, No. 2, May 2001, Pages 134-157. 6.2. Informative References [TLSSRP] Taylor, D., Wu, T., Mavroyanopoulos, N., and Perrin, T., "Using SRP for TLS Authentication", draft-ietf-tls-srp-07.txt, Internet Draft (work in progress), June 2004. Badra, et. al. Expires - February 2005 [Page 8] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 [TLSPSK] Eronen, P., and Tschofenig, H., "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", draft-eronen-tls-psk-00.txt, Internet Draft (work in progress), August 2004. [TLSSK] Gutmann, P.,"Use of Shared Keys in the TLS Protocol", draft-ietf-tls-sharedkeys-02.txt, Internet Draft (expired), October 2003. [TSLEXP] Badra, M., Serhrouchni, A., and Urien, P., "TLS Express", draft-badra-tls-express-00.txt, Internet Draft (work in progress), June 2004. 6. Author's Addresses Mohamad Badra ENST Telecom 46 rue Barrault 75634 Paris Phone: NA France Email: Mohamad.Badra@enst.fr Omar Cherkaoui UQAM University Montreal (Quebec) Phone: NA Canada Email: cherkaoui.omar@uqam.ca Ibrahim Hajjeh ENST Telecom 46 rue Barrault 75634 Paris Phone: NA France Email: Ibrahim.Hajjeh@enst.fr Ahmed Serhrouchni ENST Telecom 46 rue Barrault 75634 Paris Phone: NA France Email: Ahmed.Serhrouchni@enst.fr 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 IETF's procedures with respect to rights in IETF 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 Badra, et. al. Expires - February 2005 [Page 9] INTERNET-DRAFT PSK key Exchange methods for TLS August 2004 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 (2004). 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. Badra, et. al. Expires - February 2005 [Page 10]