There are **27** NRICH Mathematical resources connected to **Codes and cryptography**, you may find related items under Cross-curricular Connections.

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

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?

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

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

Jenny Murray describes how she developed her interest in making and breaking codes.

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

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

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

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?

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

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

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?

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?

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

I was looking at the number plate of a car parked outside. Using my special code S208VBJ adds to 65. Can you crack my code and use it to find out what both of these number plates add up to?

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?