Network Working Group M. Salter Internet-Draft National Security Agency Intended status: Informational E. Rescorla Expires: March 20, 2009 Network Resonance R. Housley Vigil Security September 16, 2008 Suite B Cipher Suites for TLS draft-rescorla-tls-suiteb-05.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 March 20, 2009. Abstract The United States Government has published guidelines for "NSA Suite B Cryptography", which defines cryptographic algorithm policy for national security applications. This document defines a profile of TLS which is conformant with Suite B. Salter, et al. Expires March 20, 2009 [Page 1] Internet-Draft Suite B for TLS September 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used In This Document . . . . . . . . . . . . . . 3 3. Suite B Requirements . . . . . . . . . . . . . . . . . . . . . 3 4. Suite B Compliance Requirements . . . . . . . . . . . . . . . 4 4.1. Security Levels . . . . . . . . . . . . . . . . . . . . . 6 4.2. Acceptable Curves . . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 10 Salter, et al. Expires March 20, 2009 [Page 2] Internet-Draft Suite B for TLS September 2008 1. Introduction National Security Agency posted a Fact Sheet on Suite B Cryptography, and at the time of this writing, it states: 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 a profile of TLS 1.2 [RFC5246] and of the cipher suites defined in [RFC5289], but does not itself define any new cipher suites. 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. Suite B Requirements The Fact Sheet on Suite B Cryptography requires that key establishment and 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, Suite B includes: Salter, et al. Expires March 20, 2009 [Page 3] Internet-Draft Suite B for TLS September 2008 Encryption: Advanced Encryption Standard (AES) - FIPS 197 (with keys sizes of 128 and 256 bits) Digital Signature: Elliptic Curve Digital Signature Algorithm (ECDSA) - FIPS 186-2 (using the curves with 256 and 384-bit prime moduli) Key Exchange: Elliptic Curve Diffie-Hellman (ECDH) - 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) 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. Note: Some people refer to the two security levels based on the AES key size that is employed. At the 128-bit security level, an AES key length of 128 bits is used. However, at the 192-bit security level, an AES key length of 256 bits is used. 4. Suite B Compliance Requirements TLS version 1.1 and earlier does not support Galois Counter Mode (GCM) cipher suites [RFC5289]. However, TLS version 1.2 and later does support GCM. For Suite B TLS compliance, GCM cipher suites are REQUIRED to be used whenever possible. Also, for Suite B TLS compliance, Cipher Block Chaining (CBC) cipher suites are employed when GCM cipher suites cannot be employed. For a client to be considered Suite B compliant, the following cipher suite rules apply: o A Suite B compliant TLS version 1.1 or earlier client MUST offer one cipher suite for each supported security level. For the 128 bit security level, TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA MUST be offered in the ClientHello message. For the 192 bit security level, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA MUST be offered in the ClientHello message. One of these cipher suites MUST be the first (most preferred) cipher suite in the ClientHello message. o A Suite B compliant TLS version 1.2 or later client MUST offer at least two cipher suites for each supported security level. For the 128 bit security level, Salter, et al. Expires March 20, 2009 [Page 4] Internet-Draft Suite B for TLS September 2008 TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 MUST be offered in this order in the ClientHello message. For the 192 bit security level, TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 and TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 MUST be offered in this order in the ClientHello message. o A Suite B compliant TLS version 1.2 or later client that offers backward compatibility with TLS version 1.1 or earlier servers MAY offer an additional cipher suite for each supported security level. If these cipher suites are offered, they MUST appear after the ones discussed above. For the 128 bit security level, TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA MAY be offered in the ClientHello message. For the 192 bit security level, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA MAY be offered in the ClientHello message. o A Suite B compliant TLS (of any version) client that offers interoperability with non-Suite B compliant servers MAY offer additional cipher suites. If any additional cipher suites are offered, they MUST appear after the ones discussed above in the ClientHello message. For a server that is configured to support the 128-bit security level to be considered Suite B compliant, the following cipher suite rules apply: o A Suite B compliant TLS version 1.1 or earlier server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it is offered. If the server is not offered this cipher suite and interoperability with clients that are not compliant with Suite B is desired, then the server MAY accept another offered cipher suite that is considered acceptable by the server administrator. o A Suite B compliant TLS version 1.2 or later server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite if it is offered. If this cipher suite is not offered, then the server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 cipher suite if it is offered. If neither of these cipher suites is offered, then the server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it is offered. If the server is not offered any of those three cipher suites and interoperability with clients that are not compliant with Suite B is desired, then the server MAY accept another offered cipher suite that is considered acceptable by the server administrator. For a server that is configured to support the 192-bit security level to be considered Suite B compliant, the following cipher suite rules apply: Salter, et al. Expires March 20, 2009 [Page 5] Internet-Draft Suite B for TLS September 2008 o A Suite B compliant TLS version 1.1 or earlier server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it is offered. If the server is not offered this cipher suite and interoperability with clients that are not compliant with Suite B is desired, then the server MAY accept another offered cipher suite that is considered acceptable by the server administrator. o A Suite B compliant TLS version 1.2 or later server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite if it is offered. If this cipher suite is offered, then the server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 cipher suite if it is offered. If neither of these cipher suites is offered, then the server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it is offered. If the server is not offered any of those three cipher suites and interoperability with clients that are not compliant with Suite B is desired, then the server MAY accept another offered cipher suite that is considered acceptable by the server administrator. Note that these rules explicitly permit the use of CBC cipher suites in TLS version 1.2 connections in order to permit operation between Suite B compliant and non-Suite B compliant implementations. For instance, a Suite B compliant TLS version 1.2 client might offer TLS version 1.2 with both GCM and CBC cipher suites when communicating with a non-Suite B TLS version 1.2 server which then selected the CBC cipher suites. This connection would nevertheless meet the requirements of this specification. However, any two Suite B compliant implementations will negotiate a GCM cipher suite when doing TLS version 1.2. 4.1. Security Levels As described in Section 1, Suite B specifies two security levels: 128 bit and 192 bit. The following table lists the cipher suites for each security level. Withing each security level, the cipher suites are listed in their preferred order for selection by a TLS version 1.2 implementation. +-----------------------------------------+----------------+ | Cipher Suite | Security Level | +-----------------------------------------+----------------+ | TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | 128 | | TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 | 128 | | TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA | 128 | | TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 | 192 | | TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 | 192 | | TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA | 192 | +-----------------------------------------+----------------+ Salter, et al. Expires March 20, 2009 [Page 6] Internet-Draft Suite B for TLS September 2008 4.2. 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 [DSS] as P-256 and P-384, respectively. For cipher suites at the 128-bit security level, secp256r1 MUST be used. For cipher suites at the 192-bit security level, secp384r1 MUST be used. RFC 4492 requires that uncompressed(0) form be supported. ansiX962_compressed_prime(1) point formats MAY also be supported. Clients desiring to negotiate only a Suite B-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 Suite B-compliant and non-Suite B-compliant connections MAY omit the extension or send the extension but offer other curves as well as the appropriate Suite B ones. Servers desiring to negotiate a Suite B-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 Suite B-compliant or non-Suite B-compliant modes to interoperate with a server which will only do Suite B-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 MUST check the chosen curve to make sure it is acceptable. Server and client certificates used to establish a Suite B-compliant connection MUST be signed with ECDSA. For certificates used at the 128-bit security level, the subject public key MUST use the P-256 curve, and the digital signature MUST be calculated using the P-256 curve and the SHA-256 hash algorithm. For certificates used at the 192-bit security level, the subject public key MUST use the P-384 curve, and the digital signature MUST be calculated using the P-384 curve and the SHA-384 hash algorithm. 5. Security Considerations Most of the security considerations for this document are described in TLS 1.2 [RFC5246], Elliptic Curve Cryptography (ECC) Cipher Suites for TLS [RFC4492], AES-GCM Cipher Suites for TLS [RFC5288], and TLS ECC Cipher Suites with SHA-256/384 and AES-GCM [RFC5289]. Readers should consult those documents. In order to meet the goal of a consistent security level for the Salter, et al. Expires March 20, 2009 [Page 7] Internet-Draft Suite B for TLS September 2008 entire cipher suite, in Suite B mode TLS implementations MUST ONLY use the curves defined in Section 4.2. Otherwise, it is possible to have a set of symmetric algorithms with much weaker or stronger security properties than the asymmetric (ECC) algorithms. 6. IANA Considerations This document defines no actions for IANA. 7. Acknowledgements This work was supported by the US Department of Defense. 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA- 256/384 and AES Galois Counter Mode (GCM)", RFC 5289, August 2008. [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. [DSS] National Institute of Standards and Technology, "Digital Signature Standard", FIPS 186-2, January 2000. 8.2. Informative References [RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, Salter, et al. Expires March 20, 2009 [Page 8] Internet-Draft Suite B for TLS September 2008 August 2008. Authors' Addresses Margaret Salter National Security Agency 9800 Savage Rd. Fort Meade 20755-6709 USA Email: msalter@restarea.ncsc.mil Eric Rescorla Network Resonance 2064 Edgewood Drive Palo Alto 94303 USA Email: ekr@rtfm.com Russ Housley Vigil Security 918 Spring Knoll Drive Herndon 21070 USA Email: housley@vigilsec.com Salter, et al. Expires March 20, 2009 [Page 9] Internet-Draft Suite B for TLS September 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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 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, THE IETF TRUST 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. 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