<!-- ##### SECTION Title ##### -->
HMAC

<!-- ##### SECTION Short_Description ##### -->

cryptographic message authentication.

<!-- ##### SECTION Long_Description ##### -->
<para>

HMAC is a cryptographic mechanism for message authentication described
in RFC2104.  It allows a sender and receiver with a shared
cryptographic key to verify the authenticity of a message.

</para>

<!-- ##### SECTION See_Also ##### -->
<para>

<ulink url="http://www.ietf.org/rfc/rfc2104.txt">RFC2104 - HMAC: Keyed-Hashing for Message Authentication</ulink>

</para>

<!-- ##### FUNCTION hmac_md5 ##### -->
<para>

</para>

@data: 
@data_len: 
@key: 
@key_len: 
@digest: 


