Suite B Cipher Suites for TLS

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 a profile of of TLS 1.2 and of the cipher suites defined in , but does not itself define any new cipher suites. This profile requires TLS 1.2.

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 .

The "Suite B Fact Sheet" requires that key establishment and authentication algorithms be based on Elliptic Curve Cryptography, that the encryption algorithm be AES , and that the function used for key derivation and data integrity be SHA . 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.

Galois Counter Mode (GCM) cipher suites are preferred for Suite B implementations. However, TLS 1.1 did not support GCM. Therefore, when TLS 1.1 implementations are used in Suite B mode, they support Cipher Block Chaining (CBC) mode. Therefore, in order for a connection to be Suite B compliant, the following rules apply: TLS 1.1 or earlier connections MUST use either TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 or TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384. A compliant TLS 1.1 or earlier client MUST offer at least one of these cipher suites. A compliant TLS 1.1 or earlier server MUST select one of them if offered. TLS 1.2 or greater connections connections MUST use one of TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384, TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 or TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384. A compliant TLS 1.2 or greater client MUST offer one of the two GCM cipher suites. If backward compatibility with TLS 1.1 or earlier is desired, it MAY offer the CBC cipher suites as well. A compliant TLS 1.2 server MUST select one of these cipher suites if offered and MUST select a GCM cipher suite in TLS 1.2 mode if one is offered. Note that these rules explicitly permit the use of CBC cipher suites in TLS 1.2 connections in order to permit operation between Suite B and non Suite B implementations. For instance, a Suite B compliant TLS 1.2 client might offer TLS 1.2 with both CBC and GCM cipher suites when communicating with a non-Suite B TLS 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 implementations will negotiate a GCM cipher suite when doing TLS 1.2. In compliance with the guidance in the Suite B Fact Sheet every TLS 1.1 or earlier implementation of Suite B SHOULD implement TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256. Every TLS 1.2 or greater implementation of Suite B SHOULD implement TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256

As described in , 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_SHA256128 TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256128 TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384192 TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384192

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 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 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.

Most of the security considerations for this document are described in TLS 1.2 , RFC 4492 , , and . Readers should consult those documents. In order to meet the goal of a consistent security level for the entire cipher suite, in Suite B mode TLS implementations MUST ONLY use the curves defined in . Otherwise, it is possible to have a set of symmetric algorithms with much weaker or stronger security properties than the asymmetric (ECC) algorithms.

This document defines no actions for IANA.

This work was supported by the US Department of Defense.