void silc_hmac_init(SilcHmac hmac);

    Sometimes calling the silc_hmac_make might not be the most
    optimal case of doing MACs.  If you have a lot of different data
    that you need to put together for computing a MAC you may either
    put them into a buffer and compute the MAC from the buffer by
    calling the silc_hmac_make, or you can use the silc_hmac_init,
    silc_hmac_update and silc_hmac_final to do the MAC.  This function
    prepares the allocated HMAC context for this kind of MAC
    computation.  The caller must have been called the function
    silc_hmac_set_key before calling this function.  To add the
    data to be used in the MAC computation call the silc_hmac_update
    function.