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?

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

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

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

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

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

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.

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?