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German Cryptanalytic Attacks on the British World War II "TYPEX" Machine

24 August 2018

This morning, I saw a link to a fascinating document. Briefly, it's a declassified TICOM document on some German cryptanalytic efforts during World War II. There are a number of interesting things about it, starting with the question of why it took until 2018 to declassify this sort of information. But maybe there's a answer lurking here…

(Aside: I'm on the road and don't have my library handy; I may update this post when I get home.)

TYPEX—originally Type 10, or Type X—was the British high-level cipher device. It was based on the commercial Enigma as modified by the British. The famous German military Enigma was also derived from the commercial model. Although the two parties strengthened it in different ways, there were some fundamental properties—and fundamental weaknesses—that both inherited from the original design. And the Germans had made significant progress against TYPEX—but they couldn't take it to the next level.

The German Amy Cryptanalytic Agency, OKH/In 7/VI, did a lot of statistical work on TYPEX. They eventually figured out more or less everything about how it worked, learning only later that the German army had captured three TYPEX units at Dunkirk. All that they were missing were the rotors, and in particular how they were wired and where the "notch" was on each. (The notch controlled when the rotor would kick over to the next position.) And if they'd had the rotor details and a short "crib" (known plaintext)?

The approximate number of tests required would be about 6 × 143 = 16,464. This was not by any means a large number and could certainly be tackled by hand. No fully mechanised method was suggested, but a semi-mechanised scheme using a converted Enigma and a lampboard was suggested. There can be no doubt that it would have worked if the conditions (a) and (b) had ever been fulfilled. Moreover, the step from a semi-mechanised approach to a fully automatic method would not have been a difficult one.
In other words, the Germans never cracked TYPEX because they didn't know anything about the rotors and never managed to "pinch" any. But the British did have the wiring of the Enigma rotors. How?

It turns out that the British never did figure that one out. It was the work of a brilliant Polish mathematician, Marian Rejewski; the Poles eventually gave their results to the French and the British, since they realized that even perfect knowledge of German plans wouldn't help if their army was too weak to exploit the knowledge.

Rejewski was, according to David Kahn, the first person to use mathematics other than statistics and probability in cryptanalysis. In particular, he used group theory and permutation theory to figure out the rotor wiring. This was coupled with a German mistake in how they encrypted the indicators, the starting positions of the rotors. (Space prohibits a full discussion of what that means. I recommend Kahn's Seizing the Enigma and Budiansky's Battle of Wits for more details.)

But what if the Germans had solved TYPEX? What would that have meant? Potentially, it would have been a very big deal.

The first point is that since TYPEX and the German military Enigma had certain similarities, the ability to crack TYPEX (which is generally considered stronger than Enigma) might have alerted the Germans that the British could do the same to them—which was, of course, the case. If that wasn't enough, the British often used TYPEX to communicate ULTRA—the intelligence derived from cryptanalysis of Engima and some other systems—to field units. (Aside: the British used one-time pads to send ULTRA to army-level commands but used TYPEX for lower-level units.) In other words, had the German army gained the ability to read TYPEX, it might have been extremely serious. And although their early work was on 1940 and earlier TYPEX, "Had they succeeded in reading early traffic it seems reasonable to conjecture that they might have maintained continuity beyond the change on 1/7/40 when the 'red' drums were introduced." It's certainly the case that the British exploited continuity with Enigma; most historians agree that if the Germans had used Enigma at the start of the war as well as they used it at the end, it's doubtful that the British could have cracked it.

There are a couple of other interesting points in the TICOM report. For one thing, at least early in the war British cipher clerks were making the same sorts of mistakes as the German clerks did: "operators were careless about moving the wheels between the end of one message and the start of the next". The British called their insight about similar laziness by the Germans the "Herivel tip". And the British didn't even encipher their indicators; they sent them in the clear. (To be sure, the bad way the Germans encrypted their indicators was what led to the rotor wiring being recovered, thus showing that not even trying can be better than doing something badly!)

So where are we? The Germans knew how TYPEX worked and had devised an attack that was feasible if they had the rotor wiring. But they never captured any rotors and they lacked someone with the brilliance of Marian Rejewski, so they couldn't make any progress. We're also left with a puzzle: why was this so sensitive that it wasn't declassified until more than 70 years the war? Might the information have been useful to someone else, someone who did know the rotor wiring?

It wouldn't have been the U.S. The U.S. and the British cooperated very closely on ULTRA, though the two parties didn't share everything: "None of our allies was permitted even to see the [SIGABA] machine, let alone have it." Besides, TICOM was a joint project; the US had the same information on TYPEX's weaknesses. However, might the Soviets have benefited? They had plenty of well-placed agents in the U.K. Might they have had the rotor wirings? I don't know—but I wonder if something other than sheer inertia kept that report secret for so many years.

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