### Probably a Code?

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

### Stage 5 Cipher Challenge

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

### More Secret Transmissions

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?

# Semicircle

##### Stage: 5 Challenge Level:

Have you managed to solve the entire Stage 5 Cipher Challenge? Solutions are now closed, but perhaps you want to take up the full challenge.

Successful solvers of this part were:

Mahdokht from Farzanegan of Kermanshah
Patrick from Woodbridge School, England
An Anonymous Solver from Somewhere in the US
Joseph from Hong Kong

The solution is:

If you can read this, well done! This sort of cipher is called a polyalphabetic cipher. For examples like this one with a keyword of length two, it's possible to solve it by hand just trying possibilities. However, with a longer keyword this gets very hard. There are more sophisticated techniques, one of which is called Kasiski examination. This involves looking for repeated letters in the ciphertext. It's likely that the number of letters inbetween these repeats is a multiple of the keyword length. Look at wikipedia for more information! By the way, if you didn't work it out, the previous cipher was a keyword substitution cipher with keyword keyword. So far, we've only considered substitution ciphers, but there are other alternatives. We could for example reorder the ciphertext.

vigenere keyword: pi