docs: public byte[] getKey() {
      if (result == null) {
        return null;
      }
      if (result.getKeyData().size() < KEY_LENGTH) {
        throw new IllegalStateException("Could not get enough key data from the handshake.");
      }
      byte[] key = new byte[KEY_LENGTH];
      result.getKeyData().copyTo(key, 0, 0, KEY_LENGTH);
      return key;
    }