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Security Area TLS Working Group example This memo describes the use of the Advanced Encryption Standard (AES) in the Counter and CBC-MAC Mode (CCM) of operation within Transport Layer Security (TLS) to provide confidentiality and data-origin authentication. The AES-CCM algorithm is amenable to compact implementations, making it suitable for constrained environments, while at the same time providing a high level of security. The cipher suites defined in this document use Elliptic Curve Cryptography (ECC) and are advantageous in networks with limited bandwidth.

This document describes the use of Advanced Encryption Standard (AES) in Counter with CBC-MAC Mode (CCM) in several TLS cipher suites. AES-CCM provides both authentication and confidentiality (encryption and decryption) and uses as its only primitive the AES encrypt block cipher operation. This makes it amenable to compact implementations, which are advantageous in constrained environments. Of course, adoption outside of constrained environments is necessary to enable interoperability, such as that between web clients and embedded servers, or between embedded clients and web servers. The use of AES-CCM has been specified for the IPsec Encapsulating Security Payload (ESP) and 802.15.4 wireless networks . Authenticated encryption, in addition to providing confidentiality for the plaintext that is encrypted, provides a way to check its integrity and authenticity. Authenticated Encryption with Associated Data, or AEAD , adds the ability to check the integrity and authenticity of some associated data that is not encrypted. This memo utilizes the AEAD facility within TLS 1.2 and the AES-CCM-based AEAD algorithms defined in and . All of these algorithms use AES-CCM; some have shorter authentication tags and are therefore more suitable for use across networks in which bandwidth is constrained and message sizes may be small. The cipher suites defined in this document use Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) as their key establishment mechanism; these cipher suites can be used with DTLS .

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 cipher suites defined in this document are based on the AES-CCM Authenticated Encryption with Associated Data (AEAD) algorithms AEAD_AES_128_CCM and AEAD_AES_256_CCM described in . The following cipher suites are defined: CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM = {0xC0,0xAC} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CCM = {0xC0,0xAD} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 = {0xC0,0xAE} CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 = {0xC0,0xAF} These cipher suites make use of the AEAD capability in TLS 1.2 . Note that each of these AEAD algorithms uses AES-CCM. Cipher suites ending with "8" use eight-octet authentication tags; the other cipher suites have 16-octet authentication tags. The HMAC truncation option described in Section 7 of (which negotiates the "truncated_hmac" TLS extension) does not have an effect on the cipher suites defined in this note, because they do not use HMAC to protect TLS records. The "nonce" input to the AEAD algorithm is as defined in . In DTLS, the 64-bit seq_num field is the 16-bit DTLS epoch field concatenated with the 48-bit sequence_number field. The epoch and sequence_number appear in the DTLS record layer. This construction allows the internal counter to be 32 bits long, which is a convenient size for use with CCM. These cipher suites make use of the default TLS 1.2 Pseudorandom Function (PRF), which uses HMAC with the SHA-256 hash function. The ECDHE_ECDSA key exchange is performed as defined in , with the following additional stipulations: Curves with a cofactor equal to one SHOULD be used; this simplifies their use. The uncompressed point format MUST be supported. Other point formats MAY be used. The client SHOULD offer the elliptic_curves extension, and the server SHOULD expect to receive it. The client MAY offer the ec_point_formats extension, but the server need not expect to receive it. Fundamental ECC algorithms MAY be used as an implementation method. If the server uses a certificate, then the requirements in RFC 4492 apply: "The server's certificate MUST contain an ECDSA-capable public key and be signed with ECDSA." Guidance on acceptable choices of hashes and curves that can be used with each cipher suite is detailed in . The Signature Algorithms extension (Section 7.4.1.4.1 of ) SHOULD be used to indicate support of those signature and hash algorithms. If a client certificate is used, the same criteria SHOULD apply to it. Implementations of these cipher suites will interoperate with but can be more compact than a full implementation of that RFC.

The following AEAD algorithms are used: AEAD_AES_128_CCM is used in the TLS_ECDHE_ECDSA_WITH_AES_128_CCM cipher suite, AEAD_AES_256_CCM is used in the TLS_ECDHE_ECDSA_WITH_AES_256_CCM cipher suite, AEAD_AES_128_CCM_8 is used in the TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 cipher suite, and AEAD_AES_256_CCM_8 is used in the TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 cipher suite.

Implementations SHOULD select elliptic curves and hash functions so that AES-128 is used with a curve and a hash function supporting a 128-bit security level, and AES-256 is used with a curve and a hash function supporting a 192-bit or 256-bit security level. More detailed guidance on cryptographic parameter selection is given in (see especially Tables 2 and 3). describes suitable curves and hash functions that are widely available.

These cipher suites make use of the authenticated encryption with additional data defined in TLS 1.2 . 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 that select an earlier version of TLS MUST NOT select one of these cipher suites. Earlier versions do not have support for AEAD; for instance, the TLSCiphertext structure does not have the "aead" option in TLS 1.1. Because TLS has no way for the client to indicate that it supports TLS 1.2 but not earlier versions, 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.

IANA has assigned the values for the cipher suites defined in from the "TLS Cipher Suite Registry". The DTLS-OK column has been marked as "Y" for each of these algorithms.

The perfect forward secrecy properties of ephemeral Diffie-Hellman cipher suites are discussed in the security analysis of . This analysis applies to the ECDHE cipher suites.

AES-CCM security requires that the counter never be reused. The IV construction in is designed to prevent counter reuse.

A cipher suite can be implemented in such a way that the secret keys and private keys are stored inside a Hardware Security Module (HSM), and the cryptographic operations involving those keys are performed by the HSM on data provided by an application interacting with the HSM through an interface such as that defined by the Cryptographic Token Interface Standard . When an AEAD cipher suite, such as those in this note, are implemented in this way, special handling of the nonce is required. This is because the "salt" part of the nonce is set to the client_write_IV or server_write_IV, which is a function of the TLS master secret. Another potential issue with the Cryptographic Token Interface Standard is that the use of the DecryptUpdate function is not possible with the CCM decrypt operation or the decrypt operation of any other authenticated encryption method. This is because the DecryptUpdate requires that post-decryption plaintext be returned before the authentication check is completed.

This document borrows heavily from . Thanks are due to Robert Cragie for his great help in making this work complete, correct, and useful, and to Peter Dettman for his review. Thanks also to Mike StJohns for pointing out the HSM issues. This document is motivated by the considerations raised in the Zigbee Smart Energy 2.0 working group.

&rfc6090; &rfc5639; &rfc6347; &rfc6655; &rfc4492; &rfc2119; &rfc6066; &rfc5288; &rfc5116; &rfc5246; National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology &rfc4309; IEEE RSA Laboratories

This memo does not mandate any particular elliptic curves or cryptographic algorithms, for the sake of flexibility. However, since the main motivation for the AES-CCM-ECC cipher suites is their suitability for constrained environments, it is valuable to identify a particular suitable set of curves and algorithms. This appendix identifies a set of elliptic curves and cryptographic algorithms that meet the requirements of this note and that are widely supported and believed to be secure. The curves and hash algorithms recommended for each cipher suite are: An implementation that includes either TLS_ECDHE_ECDSA_WITH_AES_128_CCM or TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 SHOULD support the secp256r1 curve and the SHA-256 hash function. An implementation that includes either TLS_ECDHE_ECDSA_WITH_AES_256_CCM or TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8 SHOULD support the secp384r1 curve and the SHA-384 hash function, and MAY support the secp521r1 curve and the SHA-512 hash function. More information about the secp256r1, secp384r1, and secp521r1 curves is available in Appendix A of . It is not necessary to implement the above curves and hash functions in order to conform to this specification. Other elliptic curves, such as the Brainpool curves , for example, meet the criteria laid out in this memo.