If you’re carrying around an exposed circuit board and a bunch of wires people are going to notice you. But a dry erase marker won’t turn any heads. And this one holds its own little secret. It acts as a master key for hotel room door locks.

This is really more of a repackaging hack. The exploit is already quite well-known. The Onity brand of key card locks most commonly used in hotels have a power jack on the bottom that doubles as a 1-wire communications port. The first published proof of concept used an Arduino board and a simple adapter to unlock any door in under one second. Now that hardware has been reduced in size so that it fits in the hollow shell of a dry erase marker. Even better, the felt tip has been replaced with the appropriately sized barrel jack. Check out the ultra-fast and inconspicuous use of it after the break. We think using this is no more obvious than actually having the key card.