Scientific American presents Math Dude by Quick & Dirty Tips. Scientific American and Quick & Dirty Tips are both Macmillan companies.

Would you believe it’s possible to send someone a secret message secured with absolutely unbreakable encryption using only a bit of simple arithmetic? Well, it is—the solution is surprisingly simple and was used by British, German, and American spy agencies throughout World War II. Curious to know how it works? You’re in luck because today we’re taking our first steps into the world of secret-agent math.

Secret Agent Math, Part 1
Here’s the somewhat ridiculous but hopefully amusing scenario: Imagine you’re a secret agent working for your government’s top-secret espionage agency. You’re sitting alone in a restaurant when the waiter approaches your table and slides a small piece of folded paper in front of you. He indicates that the note was sent by a person seated across the room—you take a look and quickly recognize him to be a trusted colleague. The waiter leaves and you unfold the paper. Given your wealth of experience in these situations, you’re not at all surprised to find a senseless looking series of letters scrawled across the paper: “P–B–A”. Having taken stock of the situation, you realize that this sequence of letters is an encrypted message.