code: public Optional<byte[]> open(byte[] ciphertext) {
      if (ciphertext.length < SecretBox.NONCE_SIZE) {
        return Optional.empty();
      }
      final byte[] nonce = Arrays.copyOfRange(ciphertext, 0, SecretBox.NONCE_SIZE);
      final byte[] x = Arrays.copyOfRange(ciphertext, SecretBox.NONCE_SIZE, ciphertext.length);
      return box.open(nonce, x);
    }
docs: Decrypt the ciphertext with the given key.
  
  @param ciphertext an encrypted message
  @return an {@link Optional} of the original plaintext, or if either the key, nonce, or
  ciphertext was modified, an empty {@link Optional}