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PLATFORM04

Caesar shift

CIPHER
WHEN
FROM
STATUS

The simplest cipher anyone remembers: write every letter a fixed number of places further along the alphabet. With a shift of three, A becomes D, B becomes E, and the word ATTACK becomes DWWDFN. To read it, walk back the same number of steps.

The machine

A cipher disk

Two rings with the same alphabet, pinned through the middle. Turn the inner ring and read across: outer ring is plain, inner ring is cipher. Romans did it in their heads; the disk is a Renaissance invention, but it shows the idea at a glance. Drag the inner wheel, use the arrow keys or the buttons, and type a message of your own.

Break it yourself

A note in the left-luggage office

Someone shifted every letter by the same secret amount. There are only 25 possible shifts, so the classic attack is simply to try them all. Turn the selector knob: each click re-flaps the whole board, and the meter tells you how much the result looks like English.

ARRIVED · CIPHER CRACKED

The story

The Roman historian Suetonius, writing about a century and a half after Caesar's death, says that when Julius Caesar had anything confidential to say he wrote it in cipher, "changing the order of the letters of the alphabet so that not a word could be made out". To read it, Suetonius explains, you substitute the fourth letter for the first, D for A, and so on. He adds that Augustus used a shift of one, and wrote AA for X rather than wrapping round.

That is nearly everything we know. No enciphered letter of Caesar's survives, and we do not know how often he used the trick or who it was meant to stop. Against enemies who could barely read Latin, let alone think of shifting it, a shift was probably enough.

Why it keeps turning up

Because it is so easy to do by hand, the shift outlived Rome by two thousand years. Children's decoder rings use it. So did Bernardo Provenzano, a Sicilian Mafia boss, whose notes in a number-shift variant helped the police catch him in 2006. On the early internet, ROT13 (a shift of 13, half the alphabet) was used to hide joke punchlines and spoilers: apply it twice and you are back where you started.

The disk on this platform is later than Caesar. Leon Battista Alberti described a cipher disk with a fixed outer ring and a movable inner one in Rome around 1467, and he used it for something much cleverer: he turned the inner disk in the middle of a message, which is the seed of the polyalphabetic ciphers further down the line.

How it was broken

A Caesar shift has only 25 useful keys, so it falls to plain patience: try every shift and stop when you can read the result. Cryptographers call this a brute-force attack, and here it takes about a minute with a pencil.

There is also a quicker way, and it is the subject of the next platform. A shift moves every letter, but it does not change how often each one appears. The commonest letter in an English message is usually E; find the commonest letter in the cipher and you have the shift in one go. The meter beside the knob is doing exactly that: it compares the letter counts on the board with the counts of ordinary English.