RSA based AES-GCM Cipher Suites for TLS

This document describes the use of AES in Galois Counter Mode (GCM) (AES-GCM) with RSA based key exchange as a ciphersuite for TLS. This mechanism is not only efficient and secure, hardware implementations can achieve high speeds with low cost and low latency, because the mode can be pipelined. Applications like CAPWAP, which uses DTLS, can benefit from the high-speed implementations when wireless termination points (WTPs) and controllers (ACs) have to meet requirements to support higher throughputs in the future. AES-GCM has been specified as a mode that can be used with IPsec ESP and 802.1AE MAC Security . It also is part of the NSA suite B ciphersuites for TLS ; however in order to meet Suite B requirements these ciphersuites require ECC base key exchange and TLS 1.2. The ciphersuites defined in this document are based on RSA which allows the use of AES-GCM in environments that have not deployed ECC algorithms and do not need to meet NSA Suite B requirements. AES-GCM is an authenticated encryption with associated data (AEAD) cipher, as defined in TLS 1.2. The ciphersuites defined in this draft may be used with Datagram TLS defined in . This memo uses GCM in a way similar to .

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 following ciphersuites use the new authenticated encryption modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) : CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {TBD1,TBD1} CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {TBD2,TBD2} CipherSuite TLS_RSA_DHE_WITH_AES_128_GCM_SHA256 = {TBD3,TBD3} CipherSuite TLS_RSA_DHE_WITH_AES_256_GCM_SHA384 = {TBD4,TBD4} These ciphersuites use the AES-GCM authenticated encryption with associated data (AEAD) algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM described in . Note that this specification uses a 128-bit authentication tag with GCM. The "nonce" SHALL be 12 bytes long and it is "partially implicit" (see section 3.2.1 in ). Part of the nonce is generated as part of the handshake process and is static for the entire session and part is carried in each packet.

Struct{ opaque salt[4]; opaque explicit_nonce_part[8]; } GCMNonce The salt is the "implicit" part of the nonce and is not sent in the packet. It is either the client_write_IV if the client is sending or the server_write_IV if the server is sending. These IVs SHALL be 4 bytes long. The explicit_nonce_part is chosen by the sender and included in the packet. Each value of the explicit_nonce_part MUST be distinct for each distinct invocation of GCM encrypt function for any fixed key. Failure to meet this uniqueness requirement can significantly degrade security. The explicit_nonce_part is carried in the IV field of the GenericAEADCipher structure. Therefore, for all the algorithms defined in this section, SecurityParameters.iv_length=8. In the case of TLS the explicit_nonce_part MAY be the 64-bit sequence number. In the case of Datagram TLS the explicit_nonce_part MAY be formed from the concatenation of the 16-bit epoch with the 48-bit DTLS seq_num. The RSA and RSA-DHE key exchange is performed as defined in . The PRF algorithms SHALL be as follows:For TLS_RSA_WITH_AES_128_GCM_SHA256 and TLS_RSA_DHE_WITH_AES_128_GCM_SHA256 the hash function is SHA256. For TLS_RSA_WITH_AES_256_GCM_SHA384 and TLS_RSA_DHE_WITH_AES_256_GCM_SHA384 the hash function is SHA384.

If multiple cryptographic processors are in use by the sender, then the sender MUST ensure that each value of the explicit_nonce_part that is used by each processor is distinct. In this case each encryption processor SHOULD include in the explicit_nonce_part a a fixed value that is distinct for each processor. The recommended format is

explicit_nonce_part = FixedDistinct || Variable where the FixedDistinct field is distinct for each encryption processor, but is fixed for a given processor, and the Variable field is distinct for each distinct nonce used by a particular encryption processor. When this method is used, the FixedDistinct fields used by the different processors MUST have the same length. In the terms of Figure 2 in , the Salt is the Fixed-Common part of the nonce (it is fixed, and it is common across all encryption processors), the FixedDistinct field exactly corresponds to the Fixed-Distinct field, and the Variable field corresponds to the Counter field, and the explicit part exactly corresponds to the explicit_nonce_part. For clarity, we provide an example for TLS in which there are two distinct encryption processors, each of which uses a one-byte FixedDistinct field:

Salt = eedc68dc FixedDistinct = 01 (for the first encryption processor) FixedDistinct = 02 (for the second encryption processor) The GCMnonces generated by the first encryption processor, and their corresponding explicit_nonce_parts, are:

GCMNonce explicit_nonce_part ------------------------ ---------------------------- eedc68dc0100000000000000 0100000000000000 eedc68dc0100000000000001 0100000000000001 eedc68dc0100000000000002 0100000000000002 ... The GCMnonces generated by the second encryption processor, and their corresponding explicit_nonce_parts, are

GCMNonce explicit_nonce_part ------------------------ ---------------------------- eedc68dc0200000000000000 0200000000000000 eedc68dc0200000000000001 0200000000000001 eedc68dc0200000000000002 0200000000000002 ...

These ciphersuites 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 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.

IANA has assigned the following values for the ciphersuites defined in this draft: CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {TBD1,TBD1} CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {TBD2,TBD2} CipherSuite TLS_RSA_DHE_WITH_AES_128_GCM_SHA256 = {TBD3,TBD3} CipherSuite TLS_RSA_DHE_WITH_AES_256_GCM_SHA384 = {TBD4,TBD4}

The security considerations in apply to this document as well. The remainder of this section describes security considerations specific to the cipher suites described in this document.

The perfect forward secrecy properties of RSA based TLS ciphersuites are discussed in the security analysis of . It should be noted that only the ephemeral Diffie-Hellman based ciphersuites (RSA_DHE) are capable of providing perfect forward secrecy.

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

This draft borrows heavily from .