Network Working Group M. Salter Internet-Draft National Security Agency Expires: June 16, 2007 E. Rescorla Network Resonance December 13, 2006 SuiteB CipherSuites for TLS draft-rescorla-tls-suiteb-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 RFC 4492 describes elliptic curve cipher suites for Transport Layer Security (TLS). However, all those cipher suites use SHA-1 as their MAC algorithm, which makes them unsuitable for some applications. This document describes eight new CipherSuites for TLS/DTLS which specify stronger digest algorithms and therefore are suitable for use in applications which require compliance with the United States Government's guidelines for "NSA Suite B Cryptography" dated July, Salter & Rescorla Expires June 16, 2007 [Page 1] Internet-Draft SuiteB for TLS December 2006 2005. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used In This Document . . . . . . . . . . . . . . 3 3. SuiteB Requirements . . . . . . . . . . . . . . . . . . . . . 3 4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. HMAC-based Cipher Suites . . . . . . . . . . . . . . . . . 4 4.2. Galois Counter Mode-based Cipher Suites . . . . . . . . . 5 5. Suite B Compliance Requirements . . . . . . . . . . . . . . . 6 5.1. Security Levels . . . . . . . . . . . . . . . . . . . . . 6 5.2. Acceptable Curves . . . . . . . . . . . . . . . . . . . . 6 6. TLS Versions . . . . . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7.1. Downgrade Attack . . . . . . . . . . . . . . . . . . . . . 7 7.2. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 8 7.3. Counter Reuse with GCM . . . . . . . . . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 11 Salter & Rescorla Expires June 16, 2007 [Page 2] Internet-Draft SuiteB for TLS December 2006 1. Introduction In July, 2005 the National Security Agency posted "Fact Sheet, NSA Suite B Cryptography" which stated: To complement the existing policy for the use of the Advanced Encryption Standard (AES) to protect national security systems and information as specified in The National Policy on the use of the Advanced Encryption Standard (AES) to Protect National Security Systems and National Security Information (CNSSP-15), the National Security Agency (NSA) announced Suite B Cryptography at the 2005 RSA Conference. In addition to the AES, Suite B includes cryptographic algorithms for hashing, digital signatures, and key exchange. Suite B only specifies the cryptographic algorithms to be used. Many other factors need to be addressed in determining whether a particular device implementing a particular set of cryptographic algorithms should be used to satisfy a particular requirement. Among those factors are "requirements for interoperability both domestically and internationally". This document is intended to address those requirements in the particular case of TLS [RFC4346] and Datagram TLS [RFC4347]. Much of what is needed to define the Suite B CipherSuites is specified in RFC 4492 "ECC for TLS" [RFC4492]. We use the terminology, notation, and details from that document to the extent possible. A key ingredient of SuiteB not found in RFC4492--or the definition of TLS itself prior to 1.2--is the use of SHA256 or SHA384 for key derivation. TLS 1.2 [I-D.ietf-tls-rfc4346-bis] allows for the use of these hash in the pseudo-random function (PRF) used to derive the keys. Unless specifically stated herein, details of the protocol are identical to those given in [I-D.ietf-tls-rfc4346-bis] and [RFC4492] 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. SuiteB Requirements The "Suite B Fact Sheet" requires that key establishment and Salter & Rescorla Expires June 16, 2007 [Page 3] Internet-Draft SuiteB for TLS December 2006 authentication algorithms be based on Elliptic Curve Cryptography, that the encryption algorithm be AES [AES], and that the function used for key derivation and data integrity be SHA [SHS]. It defines two security levels, of 128 and 192 bits. In particular it states: SUITE B includes: Encryption: Advanced Encryption Standard (AES) - FIPS 197 with keys sizes of 128 and 256 bits) Digital Signature: Elliptic Curve Digital Signature Algorithm - FIPS 186-2 (using the curves with 256 and 384-bit prime moduli) Key Exchange: Elliptic Curve Diffie-Hellman or Elliptic Curve MQV Draft NIST Special Publication 800-56 (using the curves with 256 and 384-bit prime moduli) Hashing: Secure Hash Algorithm - FIPS 180-2 (using SHA-256 and SHA-384) All implementations of Suite B must, at a minimum, include AES with 128-bit keys, the 256-bit prime modulus elliptic curve and SHA-256 as a common mode for widespread interoperability. The 128-bit security level corresponds to an elliptic curve size of 256 bits, AES-128, and SHA-256. The 192-bit security level corresponds to an elliptic curve size of 384 bits, AES-256, and SHA- 384. Because both settings require a digest algorithm other than SHA-1, new cipher suites are required and defined in this document. 4. Cipher Suites This document defines 8 new cipher suites to be added to TLS. All use Elliptic Curve Cryptography for key exchange and digital signature, as defined in RFC 4492. 4.1. HMAC-based Cipher Suites The first four cipher suites use AES in CBC mode with an HMAC-based MAC: Salter & Rescorla Expires June 16, 2007 [Page 4] Internet-Draft SuiteB for TLS December 2006 CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX} These four cipher suites are the same as the corresponding cipher suites in RFC 4492 (TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA, and TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA) except that SHA-256 is used for the MAC with AES-128 and SHA-384 is used for the MAC with AES-256. As described in TLS 1.2, the digest used for the MAC MUST also be used in the PRF. 4.2. Galois Counter Mode-based Cipher Suites The second four cipher suites use the new authenticated encryption modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM]: CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX} These cipher suites use the authenticated encryption with additional data algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM described in [I-D.mcgrew-auth-enc]. The "nonce" input to the AEAD algorithm SHALL be 12 bytes long, and constructed as follows: struct { case client: uint32 client_write_IV; // low order 32-bits case server: uint32 server_write_IV; // low order 32-bits uint64 seq_num; } GCMNonce. In DTLS, the 64-bit seq_num is the 16-bit epoch concatenated with the 48-bit seq_num. This construction allows the internal counter to be 32-bits long, which is the most convenient size for use with GCM. Note: the role played by the client_write_IV and server_write_IV is often called "salt" in counter mode specifications [RFC3686]. Because GCM does not use HMAC as a MAC function, the hash function for the TLS PRF must be explicitly specified. Salter & Rescorla Expires June 16, 2007 [Page 5] Internet-Draft SuiteB for TLS December 2006 For TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 it SHALL be SHA-256. For TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 it SHALL be SHA-384. 5. Suite B Compliance Requirements The following requirements apply only to SuiteB compliant implementations. However, ordinary TLS implementations MAY use these cipher suites even if they do not comply with the requirements in this section. To be considered "SuiteB compatible" at least one of the CipherSuites defined in this document MUST be negotiated. In compliance with the guidance in the Suite B Fact Sheet every TLS implementation of SuiteB SHOULD implement TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256. 5.1. Security Levels As described in Section 1, Suite B specifies two security levels, 128 and 192 bit. The following table lists the security levels for each cipher suite: +-----------------------------------------+----------------+ | Cipher Suite | Security Level | +-----------------------------------------+----------------+ | TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 | 128 | | TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 | 192 | | TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 | 128 | | TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 | 192 | | TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | 128 | | TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 | 192 | | TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 | 128 | | TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 | 192 | +-----------------------------------------+----------------+ 5.2. Acceptable Curves RFC 4492 defines a variety of elliptic curves. For cipher suites defined in this specification, only secp256r1 (23) or secp384r1 (24) may be used. (These are the same curves that appear in FIPS 186-2 and ANSI X9.62 as P256 and P384, respectively). For cipher suites at the 128-bit security level, secp256r1 MUST be used. For cipher suites at the 192-bit security level, secp256r MUST be used. Both uncompressed (0) and ansiX962_compressed_prime(1) point formats SHOULD be supported. Salter & Rescorla Expires June 16, 2007 [Page 6] Internet-Draft SuiteB for TLS December 2006 Clients desiring to negotiate only a SuiteB-compliant connection MUST generate a "Supported Elliptic Curves Extension" containing only the allowed curves. These curves MUST match the cipher suite security levels being offered. Clients which are willing to do both SuiteB- compliant and non-SuiteB-compliant connections MAY omit the extension or send the extension but offer other curves as well as the appropriate SuiteB ones. Servers desiring to negotiate a SuiteB-compliant connection SHOULD check for the presence of the extension, but MUST NOT negotiate inappropriate curves even if they are offered by the client. This allows a Client which is willing to do either SuiteB-compliant or non-SuiteB-compliant modes to interoperate with a server which will only do SuiteB-compliant modes. If the client does not advertise an acceptable curve, the server MUST generate a fatal "handshake_failure" alert and terminate the connection. Clients SHOULD check the chosen curve to make sure it is acceptable. 6. TLS Versions Because these cipher suites depend on features available only in TLS 1.2 (PRF flexibility and combined authenticated encryption cipher modes), they MUST NOT be negotiated in older versions of TLS. Clients MUST NOT offer these cipher suites if they do not offer TLS 1.2 or later. Servers which select an earlier version of TLS MUST NOT select one of these cipher suites. Because TLS has no way for the client to indicate that it supports TLS 1.2 but not earlier, a non-compliant server might potentially negotiate TLS 1.1 or earlier and select one of the cipher suites in this document. Clients MUST check the TLS version and generate a fatal "illegal_parameter" alert if they detect an incorrect version. 7. Security Considerations The security considerations in RFC 4346 and RFC 4492 apply to this document as well. The remainder of this section describes security considerations specific to the cipher suites described in this document. 7.1. Downgrade Attack TLS negotiation is only as secure as the weakest cipher suite that is supported. For instance, an implementation which supports both 160- bit and 256-bit elliptic curves can be subject to an active downgrade attack to the 160-bit security level. An attacker who can attack that can then forge the Finished handshake check and successfully Salter & Rescorla Expires June 16, 2007 [Page 7] Internet-Draft SuiteB for TLS December 2006 mount a man-in-the-middle attack. In environments where there is a concern about this form of attack, implementations SHOULD only offer cipher suites which are as strong as their minimum acceptable security level. 7.2. Perfect Forward Secrecy The static ECDH cipher suites specified in this document do not provide perfect forward secrecy (PFS). Thus, compromise of a single static key leads to potential decryption of all traffic protected using that key. Implementors of this specification SHOULD provide at least one ECDHE mode of operation. 7.3. Counter Reuse with GCM AES-GCM is only secure if the counter is never reused. The IV construction algorithm above is designed to ensure that that cannot happen. 8. IANA Considerations IANA has assigned the following values for the SuiteB cipher suites: CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX} CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX} 9. Acknowledgements This work was supported by the US Department of Defense. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3686] Housley, R., "Using Advanced Encryption Standard (AES) Salter & Rescorla Expires June 16, 2007 [Page 8] Internet-Draft SuiteB for TLS December 2006 Counter Mode With IPsec Encapsulating Security Payload (ESP)", RFC 3686, January 2004. [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. [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)", RFC 4492, May 2006. [I-D.mcgrew-auth-enc] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", draft-mcgrew-auth-enc-01 (work in progress), October 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. [I-D.ietf-tls-ctr] Modadugu, N. and E. Rescorla, "AES Counter Mode Cipher Suites for TLS and DTLS", draft-ietf-tls-ctr-01 (work in progress), June 2006. [AES] National Institute of Standards and Technology, "Specification for the Advanced Encryption Standard (AES)", FIPS 197, November 2001. [SHS] National Institute of Standards and Technology, "Secure Hash Standard", FIPS 180-2, August 2002. [GCM] National Institute of Standards and Technology, "Recommendation for Block Cipher Modes of Operation: Galois;/Counter Mode (GCM) for Confidentiality and Authentication", SP 800-38D, April 2006. 10.2. Informative References Salter & Rescorla Expires June 16, 2007 [Page 9] Internet-Draft SuiteB for TLS December 2006 Authors' Addresses Margaret Salter National Security Agency 9800 Savage Rd. Fort Meade 20755-6709 USA Email: msalter@restarea.ncsc.mil Eric Rescorla Network Resonance 2483 E. Bayshore #212 Palo Alto 94303 USA Email: ekr@networkresonance.com Salter & Rescorla Expires June 16, 2007 [Page 10] Internet-Draft SuiteB for TLS 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. 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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. Salter & Rescorla Expires June 16, 2007 [Page 11]