Making and breaking codes has long been of the practical applications of pure mathematics.

The first codes often were used for military , so that even if a courier captured, his information would be to the enemy. The ancient Spartans used a long strip of skin with writing on it. when this skin was wrapped around a stick of the right size could the message be read.

Later Julius Caesar protected his communications in Gaul writing his messages in Greek, which the Gauls could not understand - a trick was used again by Englishmen on India's north west frontier, where Greek was also .

The most basic codes are called substitution codes, where one letter of the alphabet is replaced by symbol. Although more sophisticated codes have been , today's commonest code is the PIN, or personal identity number. Today PINs are used by all of us in life, to our bank accounts, mobile phones and to on to internet accounts.