16 questions · Maths & Logic
For as long as people have written, they have wanted to keep some things for certain readers only. The Spartans wound a strip of leather around a staff, the scytale; unwound, the letters made no sense. Julius Caesar is said to have shifted every letter three places along the alphabet, the Caesar cipher. Such methods were broken by frequency analysis, which the Arab scholar al-Kindi described as early as the ninth century: if you know which letters occur most often, you can read the message anyway. That a broken code can cost a life was learned in 1587 by Mary, Queen of Scots. Her enciphered letters to Anthony Babington were intercepted and decoded, and the plans against Elizabeth I took her to the scaffold. Safer seemed the Vigenère cipher with its keyword, long thought unbreakable until it fell in the nineteenth century. In 1854 Charles Wheatstone invented a method using pairs of letters that was named after his friend Lord Playfair. In the twentieth century codes helped decide wars. In 1917 the British deciphered a telegram from the German foreign secretary Arthur Zimmermann offering Mexico an alliance; soon afterwards the United States entered the war. The Enigma was first broken by the Pole Marian Rejewski, and later Alan Turing built his “Bombe”. In the Pacific the Navajo code talkers spoke in their own language, which the enemy never cracked. Today everyone encrypts, mostly without noticing. RSA works with a public key that anyone may know, hash functions produce irreversible fingerprints, and steganography hides messages in harmless pictures. And some puzzles still keep researchers busy: the Voynich manuscript in its unknown script and the Kryptos sculpture outside CIA headquarters.
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Caesar cipher
A becomes D, B becomes E; with only 25 possible shifts it can be broken by trial and error.
💡 Did you know? The historian Suetonius reports that Augustus shifted by just one place.
Scytale
The message was written along the staff; only someone with a staff of the same thickness could read it again.
💡 Did you know? The scytale is an early example of transposition: the letters stay the same, only their order changes.
Vigenère cipher
Each letter is shifted differently according to the letters of the keyword, which defeats simple frequency analysis.
💡 Did you know? Friedrich Kasiski published a way to break it in 1863; Charles Babbage had managed it earlier but published nothing.
Playfair cipher
It was named after Wheatstone’s friend Lord Playfair, who promoted it in government circles.
💡 Did you know? The British still used it as a field cipher in the Boer War and the First World War.
Frequency analysis
You count how often each symbol appears; in English the most common letter is E, followed by T.
💡 Did you know? Edgar Allan Poe had a treasure map deciphered exactly this way in his story “The Gold-Bug”.
Mary Stuart
Francis Walsingham’s spies intercepted the letters, and his decipherer Thomas Phelippes broke the cipher.
💡 Did you know? Phelippes even forged a postscript to learn the names of the fellow conspirators.
Arthur Zimmermann
British codebreakers intercepted and deciphered the message; its publication helped bring the United States into the war in April 1917.
💡 Did you know? Zimmermann himself publicly confirmed that the telegram was genuine instead of dismissing it as a forgery.
Marian Rejewski
He reconstructed the wiring of the rotors using group theory; with Henryk Zygalski and Jerzy Różycki he then read the traffic for years.
💡 Did you know? A few weeks before the war began in 1939 the Poles handed their knowledge and replica machines to the British and French.
Alan Turing
During the war the Bombe tested huge numbers of possible rotor settings, building on the earlier work of the Poles.
💡 Did you know? Its name goes back to the Polish “bomba”, an earlier device by Marian Rejewski.
Navajo
Around 400 Navajo passed on radio messages in their language, with code words for military terms.
💡 Did you know? To them a tank was a “tortoise” and a submarine an “iron fish”.
RSA
The name stands for Ron Rivest, Adi Shamir and Leonard Adleman.
💡 Did you know? The British intelligence mathematician Clifford Cocks had found an equivalent method in 1973 but was not allowed to talk about it.
Public key
It is used to encrypt; only the holder of the matching private key can decrypt.
💡 Did you know? Whitfield Diffie and Martin Hellman published the idea in 1976, solving the old problem of handing over keys.
Hash function
Even a tiny change to the data produces a completely different value, which makes it useful for checking passwords and files.
💡 Did you know? Well run websites do not store your password, only its hash.
Steganography
Unlike cryptography, it hides not the content but the very fact that there is a message.
💡 Did you know? Herodotus tells of a slave whose shaved head was tattooed with a message; he was sent off only once his hair had grown back.
Voynich manuscript
Radiocarbon dating places its vellum in the early fifteenth century; it is now held by Yale University’s Beinecke Library.
💡 Did you know? Besides the text it shows puzzling plants that no botanist has been able to identify with certainty, and bathing women in green pools.
Kryptos
The artist Jim Sanborn installed it at Langley in 1990; three of its four sections were solved in the 1990s.
💡 Did you know? The Greek name means “hidden”.
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