docs: @Override
    public TsiFrameProtector createFrameProtector(int maxFrameSize, ByteBufAllocator alloc) {
      Preconditions.checkState(!isInProgress(), "Handshake is not complete.");
  
      byte[] key = handshaker.getKey();
      Preconditions.checkState(key.length == AltsChannelCrypter.getKeyLength(), "Bad key length.");
  
      return new AltsTsiFrameProtector(maxFrameSize, new AltsChannelCrypter(key, isClient), alloc);
    }