This big box adds something to any number that goes into it. If you know the numbers that come out, what addition might be going on in the box?

Shapes are added to other shapes. Can you see what is happening? What is the rule?

Here's a very elementary code that requires young children to read a table, and look for similarities and differences.

A case is found with a combination lock. There is one clue about the number needed to open the case. Can you find the number and open the case?

Letters have different values in Scrabble - how are they decided upon? And would the values be the same for other languages?

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

Time for a little mathemagic! Choose any five cards from a pack and show four of them to your partner. How can they work out the fifth?

Can you work out what size grid you need to read our secret message?

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

Substitution and Transposition all in one! How fiendish can these codes get?

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

In 'Secret Transmissions', Agent X could send four-digit codes error free. Can you devise an error-correcting system for codes with more than four digits?

Is the regularity shown in this encoded message noise or structure?

Can you crack these very difficult challenge ciphers? How might you systematise the cracking of unknown ciphers?

There were lots of good ideas sent in for this challenge. Everyone had good reasons for their choice as to who won each race.

We had the most blogs yet for this task, many from Brynmill School in Swansea. These and others can be viewed at our Infinities blog. Randley School in England also sent in a number of observations from their children, Olivia, Anna, Jamie and Harvi.

This was a very popular problem and we received many useful suggestions on how to tackle it. Thank you all.

Reading through and understanding the solution to this very difficult challenge will be of value to the mathematical enthusiast!

This article describes the underlying mathematical ideas and skills involved in the important mathematical application of coding.

The Enigma Project's James Grime has created a video code challenge. Watch it here!

Simon Singh describes PKC, its origins, and why the science of code making and breaking is such a secret occupation.

Dr James Grime takes an Enigma machine in to schools. Here he describes how the code-breaking work of Turing and his contemporaries helped to win the war.

Our toolkit removes the drudgery of codebreaking while leaving you to do the hard thinking!