void silc_hmac_make_truncated(SilcHmac hmac,
                                  unsigned char *data,
                                  SilcUInt32 data_len,
                                  SilcUInt32 truncated_len,
                                  unsigned char *return_hash);

    Same as the silc_hmac_make except that the returned MAC is
    truncated to the length indicated by the `truncated_len'.  Some
    special applications may need this function.  The `return_hash'
    must be at least the size of `truncated_len'.

    For security reasons, one should not truncate to less than half
    of the length of the true MAC lenght.  However, since this routine
    may be used to non-critical applications this allows these dangerous
    truncations.