void silc_hmac_set_key(SilcHmac hmac, const unsigned char *key,
                           SilcUInt32 key_len);

    Sets the key to be used in the HMAC operation.  This must be set
    before calling silc_hmac_make or silc_hmac_final functions.  If
    you do not want to set the key you can still produce a MAC by
    calling the silc_hmac_make_with_key where you give the key as
    argument.  Usually application still wants to set the key.