Enigma Chapters 1 and 2

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The Enigma Cipher

An electrical/mechanical ciphering machine that basically worked by sending electrical current from switches that represented letters to lights that also represented letters.

A system of rotating disks scrambled the signal between switches and lights, so that the letter that lit up when you pressed a particular switch was different from the letter you pressed. Because the disks rotated after each letter, the same letter was encrypted differently at different times — simple frequency analysis wouldn’t work to break Enigma!

An additional system of pluggable wires further scrambled letters on their way into and out of the disks.

So an Enigma “key” was…

• Which disks were used (it could typically be any 3 of 5 available)
• What order they were used in
• What their starting positions were
• How the plugboard was set up

The Enigma circuitry was arranged so that as long as a message sender and receiver had their machines set to the same key, the receiver could type ciphertext on their switches and see the sender’s plaintext on their lights, i.e., the machine was equally an encrypter and decrypter.

A big problem, and avenue for attack, was key exchange: how to get sender and receiver to agree on the key to use. Solutions evolved during the war, but generally involved initially sending per-message key information using some sort of pre-agreed daily key.

Having codebooks or equivalent information about how the daily keys were generated was therefore valuable for people who wanted to break into Enigma ciphers.

“Cribs” were probably the main clues used to break Enigma: fragments of plaintext known to correspond to a piece of ciphertext, from which one could (ideally) work backwards to figure out what the Enigma disk and plugboard configuration that produced that ciphertext was, from which one could further work backwards to what the initial settings for the whole message were, from which one could maybe work backwards to the daily key.

An Enigma (or other cipher) “network” is a group of users who all use the same codebooks, initial settings, etc.

Chapters 1 and 2

What more do you know about the characters and emerging plot of the novel?

Jericho is trying to break the submarine naval Enigma (shark), at Bletchley Park (Britain’s code-breaking headquarters during World War 2).

Before the story starts, shark had been broken based on codes used by the Germans to send weather reports from ships at sea. Then the Germans changed their weather codes, and a shark “black out” started. As the story opens, 3 big convoys from the US are heading into the Atlantic, Britain needs to break shark again in order to protect those convoys from U-boat attack.

Meanwhile, Jericho has just broken up with his girl friend Claire.

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Getting further into the novel.

Please read chapters 3 and 4 for Friday.

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