Capturing the Enigma machine; why it was less important than you think

Many a journalist, writing about code-breaking in WW2, praises the bravery, enterprise and doggedness through which the Royal Navy captured an Enigma machine. The popular story is that Britain’s codebreakers were toiling unsuccessfully in the dark, unable to make progress, until this capture.

One can understand how these stories emerge, and take hold. An Enigma machine is a real, tangible, chunk of equipment, something which could be physically captured and carried off, a very obvious prize.

As ever, the truth is less romantic. The Enigma machine was developed in Germany in the mid-1920s, and was for some years marketed publicly. Its function, stated briefly, was to provide secrecy for long-distance communications, whether by wireless or cable (or even on paper).

As the market was international, its makers wanted to safeguard their intellectual property rights in countries other than Germany. Quite naturally, successive innovations embodied in this piece of office equipment were patented in the UK, particularly in 1927 and 1929.

Enigma Machine patent - schematic
A patent drawing showing a section through the commercial Enigma machine (note – no steckerboard)

With the widespread publicity given to the machine, it is not surprising that, round the world, military, diplomatic and commercial bodies took some interest in its potential, although not many went as far as to buy one. The UK Foreign Office is reported to have bought one in 1928.

The Polish Cypher Bureau also bought one in the 1920s, so were aware of the mechanics of the basic machine by 1928, at a time when they realised that the German military had started using a sophisticated device to encipher a growing number of their radio communications.

Of course, the military version of Enigma was not identical to the machines which had been sold on the open market, but the brilliant work carried out by Rejewski, Zygalski, and Rozycki enabled the Polish Cypher Bureau to identify the differences, particularly deducing the wiring pattern inside each of the three different rotors then in use.

An Enigma machine patent document from 1927; well before the start of WW2

As any rotor could potentially be wired in any one of 400 million million million million ways, identifying the exact wiring pattern of the three rotors was an amazing achievement. In 1938, two more rotors were added to the original three, so that every machine used by the German Army and Air Force then had a suite of five, from which three were selected each day as part of the new day’s set-up. The Poles managed to work out the wiring of these extra two, as well.

In the summer of 1939, sensing that invasion was imminent, the Poles decided to pass on to their allies – Britain and France – everything they had so far discovered about Enigma and how its ciphers might be broken. They even gave a working Enigma replica to each, as well. Had they not shared their knowledge, the British work at Bletchley Park would have been, at best, delayed.

Sales literature for the Enigma machine (approx 1925)

Every Enigma machine in military service was reconfigured every single day at midnight. As there are literally millions of ways a machine could be set up, merely having access to a machine does not allow the deciphering of an Enigma-encrypted message.

To achieve that requires knowledge of the set-up, as well as a machine itself. The German cipher clerks had copies of carefully guarded books of settings (loosely referred to as ‘code books’) which gave them specific daily set-up  instructions. The Bletchley Park codebreakers had no such aids.

So, by the outbreak of war, the British had two patents, an original Enigma machine (commercial version), and a replica military version, and precise knowledge of the wiring patterns in all five rotors used by the German Army and Air Force. The German Navy used the same machine, but they had another three rotors of their own, making up a suite of eight altogether. It took about a year for Britain to discover (by capture) how these extra three were wired.

The big unknown, then, is how the machines are set up for use. The picture is also complicated by the fact that different regional commands of the German forces used different set-ups, and all of these were changed every day. Unless the Bletchley Park codebreakers could work out the ever-changing set-up details, deciphering the intercepted messages would be impossible.

Thus for much of the war, the most useful targets for capture were not the machines themselves but the instructions that went with them including, but not limited to, the code books. Two examples show this admirably.

In May 1941, the German weather-ship München was deliberately hunted down in the North Atlantic by a Royal Navy task force of 3 cruisers and 4 destroyers. When the München was fired at on 9 May, her crew threw the Enigma machine and current code books overboard. (Destroying or throwing overboard their Enigma machines was standard procedure for the German armed forces, but not all were fully lost – and many Enigma machines have since been found in fields and the sea floor).

What they, and the captain, failed to think about was that other documents including code books for future months were in the captain’s quarters. These were discovered by the Royal Naval boarding party, removed, and ended up in Bletchley Park on 10 May 1941, by which time the weather-ship had been sunk.

Weather ship Lauenburg before it was sunk 28 June 1941 (seen from HMS Tartar)
Enigma documents were captured from weather ship Lauenburg before it was sunk 28 June 1941 (seen from HMS Tartar)

In June 1941, another German weather-ship, the Lauenburg was hunted down by a task force of a cruiser and three destroyers. Again, when the Royal Navy ships were sighted, on 28 June, overboard went the Enigma and the papers from the signalman’s desk. Again, none of the Lauenburg’s crew thought about the papers stored elsewhere. The search party’s haul included the code books for July, and these reached Bletchley on 2 July.

In both these cases, the Navy had no expectation of capturing a machine – documentation was the prize. Of course, if an intact machine could be captured, it might reveal the settings for the day of capture, but a ‘code book’ might provide settings information for every day for a whole month.

Dr Mark Baldwin – aka Dr Enigma – is an international expert and speaker on the Enigma Machine. Book an talk, event or Enigma machine presentation (either in person or remote) here.

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