Codes at war: the security check, the BBC and Bletchley

Two messages arrive from the same agent in occupied France. One of them is genuine. Which?

The security check

Agents agreed a deliberate mistake before they left: a misspelled word, a wrong letter in a fixed place. A message without it meant the radio had been captured and somebody else was sending.

In the Netherlands, agents dropped the check and London kept transmitting anyway, assuming carelessness. The Germans ran the captured radios for nearly two years, and the agents parachuted in afterwards were met on the ground.

The cipher was never broken. The people using it were, and then the people receiving it did not listen.

Hidden in the open

Messages personnels

The BBC read strings of nonsense after the news. The giraffe has a long neck. John has a long moustache. Each phrase was agreed with one circuit and meant one thing. Lines of Verlaine announced the Normandy landings, in the open, to everybody in Europe at once.

Poem codes on silk

Agents memorised a poem and built keys from it. Leo Marks wrote original poems so the enemy could not guess Shakespeare. Later came one-time pads printed on silk, which could be burned line by line as they were used.

A language as a key

Navajo code talkers in the Pacific spoke a language almost nobody outside their nation knew, with an agreed vocabulary on top: a submarine was an iron fish. It was never broken.

Silk maps in board games

Escape maps printed on silk, folded into Monopoly sets and posted to prisoner of war camps by a fake charity. Silk because it does not rustle, does not tear, and survives being sewn into a coat.

The machines, and why they fell

Enigma was broken first by Polish mathematicians before the war, then at Bletchley Park by ten thousand people, most of them women, almost none of whom told anybody for thirty years. Lorenz, a heavier machine, was broken by Colossus, the first electronic computer, built to do one job. SIGABA, the American machine, was never broken at all.

And the way in was almost never the mathematics. It was weather reports that began the same way every morning, operators reusing settings, and one design decision that meant a letter could never encipher as itself.

Making the enemy talk

Midway. The Americans suspected the code group AF meant Midway. So they had Midway report in clear that its water plant had failed. Japan duly transmitted that AF was short of water, and a guess became a certainty. That is a known plaintext attack run as theatre.

Gardening. The British dropped mines in a chosen patch of sea so the Germans would broadcast that patch's name, handing Bletchley a word it already knew.

And deception on a scale that is hard to believe. Inflatable tanks and sound trucks. An entire invented army in Kent. A corpse floated ashore with false papers. Every German agent in Britain captured and turned, so that the reports going back were written by the people they were spying on.

Which is the lesson this whole site keeps arriving at: the codes mostly held. It was the humans, the habits and the assumptions that gave way.

Known plaintext Hidden in Plain Sight

Codes at war

The security check

Hidden in the open

The machines, and why they fell

Making the enemy talk

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