fill_known_data
fill_known_data(md, MD5_DIGEST_LENGTH);
fill_known_data(md, SHA_DIGEST_LENGTH);
fill_known_data(md, SHA256_DIGEST_LENGTH);
fill_known_data(md, SHA384_DIGEST_LENGTH);
fill_known_data(md, SHA512_DIGEST_LENGTH);
fill_known_data(out + plen, SHA_DIGEST_LENGTH);