@Override
  public byte[] computeMac(byte[] data) throws GeneralSecurityException {
    if (plaintextLegacySuffix.length > 0) {
      return Bytes.concat(
          outputPrefix, wrappedPrf.compute(Bytes.concat(data, plaintextLegacySuffix), tagSize));
    }
    return Bytes.concat(outputPrefix, wrappedPrf.compute(data, tagSize));
  }
Creates an object implementing the {@link Mac} interface using an Hmac underneath.
java_src/src/main/java/com/google/crypto/tink/subtle/PrfMac.java