void silc_hmac_update(SilcHmac hmac, const unsigned char *data,
                          SilcUInt32 data_len);

    This function may be called to add data to be used in the MAC
    computation.  This can be called multiple times to add data from
    many sources before actually performing the HMAC.  Once you've
    added all the data you need you can call the silc_hmac_final to
    actually produce the MAC.

    unsigned char mac[20];
    SilcUInt32 mac_len;

    silc_hmac_init(hmac);
    silc_hmac_update(hmac, data, data_len);
    silc_hmac_update(hmac, more_data, more_data_len);
    silc_hmac_final(hmac, mac, &mac_len);