void silc_hash_update(SilcHash hash, const unsigned char *data,
                          SilcUInt32 data_len);

    This function may be called to add data to be used in the digest
    computation.  This can be called multiple times to add data from
    many sources before actually computing the digest.  Once you've
    added all the data you need you can call the silc_hash_final to
    actually produce the message digest value.

    unsigned char digest[20];

    silc_hash_init(hash);
    silc_hash_update(hash, data, data_len);
    silc_hash_update(hash, more_data, more_data_len);
    silc_hash_final(hash, digest);