# Codes and Ciphers - February 2012, Stage 4&5

We're celebrating the centenary of Turing's birth with lots of engaging tasks on codes and ciphers, devised with the help of James Grime of the Enigma project. We hope they stimulate interest in this intriguing field.

## Problems

### Substitution Cipher

##### Stage: 3 and 4 Challenge Level:

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

### The Best Card Trick?

##### Stage: 3 and 4 Challenge Level:

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?

### Transposition Cipher

##### Stage: 3 and 4 Challenge Level:

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

### Secondary Cipher Challenge Part 1

##### Stage: 3 and 4 Challenge Level:

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

### Substitution Transposed

##### Stage: 3 and 4 Challenge Level:

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

### Secret Transmissions

##### Stage: 4 Challenge Level:

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

### More Secret Transmissions

##### Stage: 5 Challenge Level:

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?

### Probably a Code?

##### Stage: 5 Challenge Level:

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

### Stage 5 Cipher Challenge

##### Stage: 5 Challenge Level:

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