Matt Blaze1d ago

Physical security and cryptography can learn from each other, part 11367:

Hotels wisely don't put the room number on guest keycards so if someone finds your card, they'd have to exhaustively search the hotel to find the room it opens.

Some hotels now have elevators programmed to only let you call the floor for which your keycard is coded, preventing guests from wandering to other floors.

But it also means the elevator can be used as an efficient oracle to determine the floor of a found key.

Show threadMatt Blaze23h ago
In other words, restricting the elevator in this way is a bad tradeoff. It makes it harder for guests to visit their friends on other floors, but it reduces the complexity for an outsider burglar from O(|rooms|) to O(|floors|) + O(|rooms_per_floor|), a much more feasible search space.
Show threadmfdeakin22h ago
@mattblaze I enjoy the idea, but are you sure they don't print the room number for security reasons? I was under the impression it was because they reprogrammed them when they gave them to you
Show threadCarolyn
@mfdeakin @mattblaze Both could be true?
Mar 27 at 2:58amWeb