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Substitution Cipher

Find the frequency distribution for ordinary English, and use it to help you crack the code.

Secret Transmissions

How can Agent X transmit data on a faulty line and be sure that her message will get through?

Secondary Cipher Challenge Part 1

Here is the start of a six-part challenge. Can you get to the end and crack the final message?

The Best Card Trick?

Stage: 3 and 4 Challenge Level:

In the video below, you can see Alison and Charlie performing a card trick for Yanqing:

Can you figure out how Alison and Charlie's code works?

The video below shows two more examples of the trick; do these examples confirm your initial ideas about the code?

This crib sheet fell out of Charlie's pocket after he had performed the trick.
Can you use it to make sense of the code?

Find someone to work with, and together practise the trick until you can impress someone with your mathemagical skills!

Alternatively, if you are working on your own, here are sets of five cards that might be handed to you:

For each set, work out which four cards you would show, and in which order, so that a partner could work out the fifth card.

Notes and Background

This trick first appears in Wallace Lee's book "Math Miracles" in which he credits its invention to William Fitch Cheney, Jr., a.k.a. "Fitch."

You can read an article by Michael Kleber, the first part of which describes the trick, here.

Modulus arithmetic. STEM - General. Working systematically. Combinatorics. Logo. Programming. Practical Activity. Codes and cryptography. Video. Algorithms.