code: public byte[] seal(byte[] plaintext) {
      final byte[] nonce = box.nonce(plaintext);
      final byte[] ciphertext = box.seal(nonce, plaintext);
      final byte[] combined = new byte[nonce.length + ciphertext.length];
      System.arraycopy(nonce, 0, combined, 0, nonce.length);
      System.arraycopy(ciphertext, 0, combined, nonce.length, ciphertext.length);
      return combined;
    }
docs: Encrypt the plaintext with the given key.
  
  @param plaintext any arbitrary bytes
  @return the ciphertext