Return to Bronze Age, exchange clay tablets with secret messages in cuneiform. I can fashion a wedge-shaped stylus out of any old stick

Edit: better yet, if we don’t fire the tablets, we can destroy them quickly and easily whenever we need to or just recycle them. This was common for a lot of cuneiform tablets in ancient Mesopotamia, and much of what we have found in archaeology is stuff that was unintentionally fired because the building it was in burned down