**The Problem **

A friend of mine gave me a riddle this morning regarding “Where’s Waldo?”. The riddle is as follows:

You and a friend play “Where’s Waldo?”. You solve the puzzle before your friend, and you want to prove to your friend you solved the puzzle, without giving him any hints. How do you do this?

Obviously, this is of course very reminiscent of zero knowledge proofs. A good zero knowledge proof will allow your friend to be convinced before he found Waldo himself, and even if he never finds him at all. A somewhat less interesting solution is a solution that will allow your friend to be convinced if and when he finds Waldo himself and can then verify your proof.

There are obviously many “intuitive” solutions, and I will not write them here… I will write here the second solution I thought of, but I consider it to be the more interesting one. However, this solution doesn’t solve the original problem – it only allows your friend to verify your solution once he solved it himself.

**The Solution**

Take a sheet of paper of identical dimensions to the picture, and mark a spot on it in the position where Waldo would have been on that sheet of paper. Fold that sheet of paper to some kind of origami animal, and give it to your friend. Once he solves the puzzle, he can open the folding, and see for himself that the point was marked correctly.

This is obviously not a very good solution. It is just a glorified note – you write down your solution on a note, and ask your friend not to peek until he solves it himself. So I came up with an improvement:

Agree beforehand with your friend on some (large) origami folding (such as a beetle). He shouldn’t know the instructions to fold it.Take a sheet of paper, and mark Waldo’s position on it (with a dot). Hold the paper above the fold, and mark on the fold (with another dot) the projection of the original dot on the folding. Now unfold the origami – you have a crease pattern. Give the crease pattern to your friend. When he solves the puzzle, refold the origami, and prove to your friend that the projection of the dot on the fold coincides with the dot on the picture. As an added bonus – your friend just learned how to fold another origami beast!

Of course, this solution isn’t water-tight. It also relies on crease patterns being hard to solve. It is mostly security by obscurity – but this time, ‘security by obscurity’ becomes ‘security by origami’. I just found that fascinating – that origami may be considered a function that is ‘hard to reverse engineer’ (even if it is not so) – such as a hash function. Origami does behave a little bit like a hash…

Tell me your original solutions to the problem.