IETF RFC 4868 PDF

This specification describes the use of Hashed Message Authentication Mode (HMAC) in conjunction with the SHA-256, SHA-384, and SHA-512 algorithms in IPsec. These algorithms may be used as the basis for data origin authentication and integrity verification mechanisms for the Authentication Header (AH), Encapsulating Security Payload (ESP), Internet Key Exchange Protocol (IKE), and IKEv2 protocols, and also as Pseudo-Random Functions (PRFs) for IKE and IKEv2. Truncated output lengths are specified for the authentication-related variants, with the corresponding algorithms designated as HMAC-SHA-256-128, HMAC-SHA-384-192, and HMAC-SHA-512-256. The PRF variants are not truncated, and are called PRF-HMAC-SHA-256, PRF-HMAC-SHA-384, and PRF-HMAC-SHA-512.