Terrasque

Proprietary, of course…

Sadly. At least there seem to be an open source implementation of it, don’t know if it’s reverse engineered or if there’s design docs available somewhere: https://github.com/Martichou/rquickshare

Android users use quick share these days

I just send back and forth plain gibberish. Good luck breaking that!

https://en.m.wikipedia.org/wiki/Solitaire_(cipher)

Or a deck of cards

Sell them to zoomers as 3d save button coasters. $19.95 each