H. Krawczyk, M. Bellare, R. Canetti. <a href="http://www.ietf.org/rfc/rfc2104.txt"><cite>HMAC: Keyed-Hashing for Message Authentication</cite></a>. February 1997. IETF RFC 2104.  URL: <a href="http://www.ietf.org/rfc/rfc2104.txt">http://www.ietf.org/rfc/rfc2104.txt</a>