PLATFORM08

Playfair

CIPHER

WHEN

FROM

STATUS

Frequency analysis counts single letters. So encipher two at a time. Playfair takes the message in pairs and turns each pair into another pair with a five-by-five square of letters. There are 600 possible pairs instead of 26 letters, and no single letter always comes out the same.

The machine

A Playfair square

Write the keyword into the square, skipping letters you have already used, then the rest of the alphabet (J shares with I). Split the message into pairs. If the two letters sit in different rows and columns, they mark the corners of a rectangle: take the other two corners. If they share a row, move each one step right; if they share a column, one step down. Change the keyword and watch the tiles rearrange; click any pair to see its rule.

Break it yourself

A coastwatcher's report

A message from an island lookout, enciphered in Playfair. You do not have the key, but the sender left a clue to it. Type keywords: the square rebuilds and the board is deciphered with it at once. When the board reads English, you have it.

ARRIVED · CIPHER CRACKED

The story

The cipher was invented in 1854 by Charles Wheatstone, the physicist who built some of the first practical electric telegraphs. It carries the name of his friend Lyon Playfair, a chemist and later a politician, who championed it in government circles. The story goes that the two of them demonstrated it to ministers, that the Foreign Office thought it too complicated, and that Wheatstone offered to teach it to boys from a nearby school in a quarter of an hour.

Its strength was speed: a trained clerk can encipher by hand with nothing but a keyword in his head, and the square can be rebuilt and thrown away. It became a field cipher for the British Army, used in the Boer War and in the First World War, where it was meant to protect messages only for the few hours they mattered.

PT-109

It was still in use in the Second World War. In the Solomon Islands, Australian and New Zealand coastwatchers hid on Japanese-held islands with radios and reported ships and aircraft. In the early hours of 2 August 1943 the American torpedo boat PT-109, commanded by Lieutenant John F. Kennedy, was rammed and cut in two by a Japanese destroyer in Blackett Strait. According to David Kahn, a Playfair message about the lost boat, keyed on the words ROYAL NEW ZEALAND NAVY, reached the coastwatcher network. Kennedy and the ten other survivors swam to small islands, were found by the Solomon Islander scouts Biuku Gasa and Eroni Kumana, and were rescued a few days later.

How it was broken

Playfair hides single letters but not pairs. Some pairs are far commoner in English than others (TH, HE, IN, ER), and a reversed pair always comes out reversed: if AB gives XY, BA gives YX. With enough cipher, or a guessed word, a codebreaker can rebuild the square piece by piece. During the First World War German cryptanalysts are said to have read British Playfair traffic regularly.

That was acceptable for a cipher meant to last a few hours. Against a determined enemy with time, Playfair is a speed bump, not a wall.