In previous posts we’ve looked at encryption and decryption, and talked about how messages are obscured, but at a very basic level, have you heard of ciphers and codes? Have you ever wondered about the difference between the two?

On a recent visit to Bletchley Park I came across a notice with the image shown above, so I thought I’d share it with you.

A code is a word, phrase or numbers which is converted into different words, phrases or numbers.  All the recipient needs is some kind of guide to explain what the codes mean – in the case of the image, “You Attack at Dawn” was encoded as “Buy Some Milk”, so the recipient would need to know what “Buy Some Milk” really meant.

As well as the example given in the image, there are others seen in movies, TV shos and elsewhere.  For example, the Wikipedia entry for Star Trek II: The Wrath of Khan explains the use of a code by saying “Though Khan believes his foe stranded on Regula I, Kirk and Spock use a coded message to arrange a rendezvous.” This was, as Kirk and Spock said, “by the book” where days became hours and hours became minutes.

A cipher transforms individual letters, or small groups of letters, into some other combination, perhaps even using symbols. To read the message, the enciphered (encrypted) text needs to be deciphered (decrypted) somehow.  This is what codebreakers do – they try to decipher the original message.