PLATFORM02

Scytale

CIPHER

WHEN

FROM

STATUS

A Spartan general sets out with a wooden staff. At home the magistrates keep its twin, cut to exactly the same thickness. To send him orders they wind a strip of leather round their staff, write along it, and unwind it. On the road it is a belt with a meaningless row of letters. Only a staff of the same thickness lines them up again.

The machine

A staff and a leather strip

Type a message. Choose how thick the staff is: that is how many letters fit once round it. The message is written in rows along the staff; drag the staff up or down (or use the buttons) to turn it and bring each row to the front. Then press Unwind the strip and watch the letters come off in a different order.

Break it yourself

A strip for Lysander

A strip of 55 letters has arrived, and the rack holds staffs from 2 to 12 letters round. Nothing about the letters has changed, only their order. Pick a staff: the board winds the strip on it and shows the rows. When they read as English, you have the right one.

ARRIVED · CIPHER CRACKED

The story

The scytale (in Greek skytalē, a staff or baton) is the oldest device anyone has described for hiding a message by moving its letters around rather than replacing them. Cryptographers call that a transposition cipher.

The fullest description comes from Plutarch, writing in the late 1st or early 2nd century AD, long after the events. In his Life of Lysander he explains that when the ephors, Sparta's magistrates, sent a commander abroad they gave him one of two matching rods and kept the other. A strip of parchment or leather was wound round the rod, the message written along it, and the strip sent off. Plutarch tells it in the story of Lysander, the admiral who won the Peloponnesian War for Sparta in 404 BC and was later called home by just such a message.

Was it really a cipher?

Here historians disagree. The word skytalē turns up in older Greek writers too, from around the 7th century BC onwards, but often it seems to mean simply an official Spartan message, or the staff a messenger carried. Some scholars think the secret-writing version is a later explanation, invented or embroidered by authors who no longer knew how the Spartans did it. Others take it at face value. The rod may have been a way of proving a message genuine rather than hiding what it said: anyone could read a strip with a little patience, but only the true rod made it fit neatly.

Either way, the idea behind it lasted. Rearranging letters by writing them in rows and reading them out in columns is the core of the column transposition, used by armies well into the 20th century.

How it was broken

A scytale has very few keys: the thickness of the staff, which must divide the length of the strip if the sender did not pad it. An enemy who catches a strip can simply try a few sticks of different thickness, or skip the wood and write the letters out in rows of 2, 3, 4 and so on until words appear. That is exactly what the board above lets you do.

Because transposition keeps every letter, a letter count looks just like ordinary English. That is the giveaway: plenty of E, T and A, but no words. The code-breaker knows at once that the letters are right and only their order is wrong.