Visual cryptography for physical key fobs

Visual cryptography is one of those unusual cases that sounds like a good idea, but turns out to be fraught with pitfalls. The idea is quite simple – an image to be encrypted is sampled and a series of sub-pixel patterns are produced, which are distributed to several separate images. When individual images are printed on transparent film and all the films in the set are lined up, an image appears out of randomness. Without at least a minimum number of such images, the original image cannot be resolved. Well, sort of. [anfractuosity] wanted to play with concept of visual cryptography in a slightly different mediumthat of a set of metal plates, in the form of key rings.

Two “share pairs” of images are required as a minimum to form an image when combined

The metal blanks were laser cut, the image being formed by light transmitted through coincident holes in the two pairs of plates, when properly aligned. What is, you hear yourself asking, the problem with this cryptography technique? Well, one problem is fake messages. It is possible for a malicious party, given either key in a pair, to construct a matching key that composes an entirely different message and then replace it with the second key, tricking both parties into origin. Obviously, this would require both parties to be physically compromised, but neither would necessarily notice the substitution, if neither party knew the originally encrypted message. For those wishing to dig a little deeper, check out this classic paper by Naor and Shamir [pdf] of the Wiezmann Institute. Still, despite the issues, for a visual hack, it’s still a pretty fun technique!

Want to learn a bit more about crypto techniques you can do at home? Here is our guide. The encryption is too hard to crack, but you need a way to spy? Just take out a faulty system and you’re good to go.

Briana R. Cross