Applied Kid Cryptography

A low-tech solution

The solution described here can be carried out with very simple accessories. Alice and Bob need a large piece of cardboard (at least twice as large as the picture in each dimension) with a small rectangle cut in the middle. In addition, they need paper clips to fix the correct page in the book. To show that she knows where Waldo is, Alice puts the rectangle on top of Waldo while Bob is not looking (to actually execute it Alice should place her finger on Waldo while navigating the cardboard). Bob sees Waldo and at worse learns something about Waldo's immediate surroundings. See Figure for an illustration of Bob's view.

Bob's view

However, since the cardboard is large enough to cover the picture (no matter where Waldo is), he learns nothing else about the location of Waldo. Before Bob is completely satisfied Alice must also demonstrate that she has the correct Waldo picture (and hasn't flipped a page). Therefore she should pull the book beneath the cardboard in front of Bob's eyes. This last step should be done with care, so as not reveal information about the place from which she is pulling the book. (At the very least the hole in the cardboard should be covered while the book is pulled out.) 

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