Black Storm Networks NIST This document describes the use of the HMAC algorithm in conjunction with the SHA-256 algorithm as an experimental authentication mecha- nism within the context of the IPsec AH and ESP protocols. This algo- rithm is intended to provide data origin authentication and integrity protection. Given the current lack of practical experience with SHA-256, implementations based on this document will be experimental in nature, and implementation is not required in order to claim com- pliance with the IPsec proposed standards. The version of the HMAC- SHA-256 authenticator described in this document specifies truncation to 128 bits, and is therefore named HMAC-SHA-256-128.