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I've read several claims that Churchill ordered the destruction of the Bombe code-breaking machines, but I don't really understand the motivation for it. Can you explain it to me, or give a reference?

If you, as a nation, have the capability to decode communications, then why give that up? This is independent of whether or not the Enigma machines were destroyed -- that ability should, if nothing else, discourage others from even trying, or at least give a head start in case someone did try to use encrypted communications.

It seems to me this signals a desire to be less threatening, but I don't feel this "fits" into the ... vibe(?) of the times -- and in any case is based solely on my own exceedingly amateur speculation.

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    I have this image in my head of Churchill personally destroying each Bombe by hitting it with his cane. :) – Schwern Apr 11 '17 at 1:00
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    Speculation: Rapid developments in general computers just after WWII rendered the specialized code breaking machines antiquated. England's code breaking capabilities was more likely to be known by others if they where kept at a storage somewhere where the wrong people could stumble upon them. And they occupied a lot of space. – Kjetil S. Apr 13 '17 at 9:07
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    Hint: By the end of the war Churchill was no longer Prime Minister. – Santiago Apr 13 '17 at 18:52
  • And why did we prosecute Turing after the war? Not needed anymore I guess! – Jim W says reinstate Monica Apr 15 '17 at 3:34
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    @KjetilS. - because the bombes were dedicated hardware, they ran much faster than a general purpose computer. I heard (sorry, no reference) that it was only in the 1990s that a general purpose computer could break Enigma as fast as a bombe. – paj28 Apr 17 '17 at 8:52
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If you, as a nation, have the capability to decode communications, then why give that up?

This is a fallacious assumption, and probably the source of your confusion. Destroying the bombes did not amount to the British "giving up" their code breaking capabilities, as the bombes were essentially mechanical ASICs, designed specifically to break the encryption provided by the Nazi Enigma systems. With the defeat of the Nazis, there was simply no need to break Enigma encoded messages on an industrial scale, as there was no one broadcasting such signals in range of British listening stations. So, for almost all practical purposes, these devices became useless with the fall of the Nazi regime. However, the minds behind those machines were still available to the British government, and if needed, they could be called on to build new bombes, or similar machines to break different cipher systems.

The danger of keeping these devices around is that they revealed just how good the British had become at code-breaking, which was a secret that was very closely held. The bombe has even been referred to as the most important machine in the history of Britain, which may be an overstatement, but it certainly had a huge impact on the course of the war. Had the Germans known, they would have undoubtedly deployed countermeasures that the British may not have been able to penetrate, and the massive advantage the British enjoyed would have been lost. This thought was certainly on Churchill's mind as he pondered the fate of these devices - the defeat of the Nazis did not mean that all of Britain's enemies and adversaries were vanquished, and revealing their incredible code-breaking ability to adversaries (such as the USSR) would have been a huge strategic blunder. It was safer, and more prudent, to destroy the now-obsolete machines that were evidence of their code-breaking proficiency, and enlist the mathematical geniuses behind the bombes to build new machines to break the new or different codes that would be used by other countries going forward.

that ability should, if nothing else, discourage others from even trying, or at least give a head start in case someone did try to use encrypted communications.

Well, two things here. It's actually much more advantageous to have someone communicate in a code they think is unbreakable, but that you can listen in on, than it is to try to discourage encryption. If your adversary thinks you can break their code, they'll use a different code, and/or change the medium to one that you can't intercept (directed radio waves, in-person couriers, etc.), but if they're confident you can't listen in, they'll communicate freely and allow you listen in by breaking their "unbreakable" code.

Secondly, the foundations of cryptanalysis were already established by the minds at Bletchley Park, and the bombes, specifically, wouldn't really help much in advancing code breaking on whatever the next cipher was going to be. The real magic behind the machines was the mathematics and the cryptanalytical theory/techniques that underpinned the machines. This is what was valuable for future use. And again, the best way to keep this capability a secret was to destroy the physical evidence (the machines themselves) and keep the minds behind them working in secret.

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    "they revealed just how good the British had become at code-breaking" -- I think this is what I tried to say with my "appear less threatening". Thank you for your post! – KlaymenDK Apr 10 '17 at 19:59
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    An aside only: Competing for " ...the most important machine in the history of Britain." would be the Rolls Royce Merlin. 1 each in a Spitfire, Hurricane, Fulmar, Mustang, 2 each in a Mosquito, Beaufighter, 4 in a Lancaster. Wow! That's just the most obvious applications. Wikipedia lists 40+ aircraft that used it (some were less successful as aircraft than others). Which "achine" takes the prize is, and ever shall be, moot - and odds are there are a number that stand at number 1 on the podium. – Russell McMahon Apr 11 '17 at 7:37
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    Of course, it turned out that the USSR already knew exactly how good we were at code-breaking. – David Richerby Apr 11 '17 at 14:24
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    @DavidRicherby Yeah. As they say, there's no intelligence like human intelligence. But it took them a while to figure it out, and probably cost the Soviets more to do so than it would have if the Brits had kept all their bombes around to gather dust. – HopelessN00b Apr 11 '17 at 14:30
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    @DavidRicherby The Bletchley Park Cryptography Museum has on display a Russian Enigma Based encryption machine that was used up to the point that the secret was exposed in 1973. In my view the secret was worth keeping. – Brian Tompsett - 汤莱恩 Apr 11 '17 at 21:04
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Most people who worked on Enigma code-breaking didn't know the real name of the system, or the scale of the effort being made. That contributed to the secret of the breaking of Enigma being kept until the 1970s.

So "appearing less threatening" is a non-issue. The few nations who know, already know, and have better cryptography. Nobody else knows. The information really was tightly held. A USN submarine officer, John P. Cromwell, deliberately went down with a sinking submarine to avoid capture by the Japanese and possibly being forced to reveal the secret.

The Allies sold captured Enigma machines to third world countries after WWII, because they were widely believed to be secure. This is a normal level of dirty tricks for this field. The amount of material from the new Enigma users was going to be much less than from Germany during the war. Most of the bombes were not going to be needed.

If you destroy most of them, or even the whole lot if there's a better replacement, then if the secret leaks, you have a credible claim that you can't read that code any more. You also avoid the costs of guarding and maintaining large, complex and very secret machines.

  • "because they were widely believed to be secure" -- but, if you throw away your (mass-volume) decoder, then doesn't the Enigma become effectively unbroken again? I mean, the Bombes are what made decryption not just possible but feasible. (Yes, I know that nowadays it's trivial to do in a browser extension. But then, the Enigmas sold today are sold for different purposes.) – KlaymenDK Apr 10 '17 at 20:20
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    @KlaymenDK Even if the machines themselves were destroyed the information on how to build them was likely preserved. – kasperd Apr 10 '17 at 21:17
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    @KlaymenDK: The capability wasn't thrown away. There was simply an attempt to appear is if it had been discarded, and save money at the same time. – John Dallman Apr 11 '17 at 9:13
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    Right up into the 60s people all over the world were still using Enigma machines, not knowing that the Brits were reading their signals. Destroying the hardware let them think the Brits had given up bothering as the war ended. – RedSonja Apr 12 '17 at 7:29
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    @John Surely there was no significant "attempt to appear" anything. In the 1940's to the mid 1970's very few people knew the Bombes had existed at all. There was no need to persuade people that something they didn't know existed no longer existed. Once the volume of vital messages dropped to near zero, there was simply no need for the majority of the Bombes and destroying them was the one way to ensure that no one else could discover them. The Bombes at Bletchley were destroyed but there were Bombes elsewhere sufficient for expected needs & the capability to build better machines when needed. – RedGrittyBrick Apr 12 '17 at 14:16
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If you, as a nation, have the capability to decode communications, then why give that up? This is independent of whether or not the Enigma machines were destroyed -- that ability should, if nothing else, discourage others from even trying, or at least give a head start in case someone did try to use encrypted communications.

The knowledge that your encryption is broken only leads to stronger encryption.

Enigma was broken because it was flawed, both in design, but also in operational use. But those flaws were not fundamental and could be corrected. If people knew about those flaws, and that they could be turned into a practical attack, they'd fix them. If they think it's secure, they'll stick with the status quo: it's cheaper.

The flaws in Enigma were not hard to fix, but you can't fix what you don't know is broken. B-Dienst, the German intelligence service, was both overconfident about Enigma's security, and greatly underestimated their enemy's codebreaking ability. Primarily they judged others based on their own attempts, and their attempts were lax because they considered Enigma unbreakable.

They took solace in the large number of permutations. They did not conceive their enemy might exploit flaws to reduce the possible permutations, nor develop machines to run through them. Developing a special machine to crack codes was a brand new idea, previously it had been done by hand and cleverness, and the existence of a Bombe would send cryptographers scrambling to reassess their algorithms.

In fact, Enimga was fixed during the war... by the Allies. The British Typex is basically an improved Enigma. The US had the ECM Mark II / SIGABA. They were modified to be compatible as the Combined Cipher Machine. They're all rotor cipher machines contemporary with Enigma. While they had flaws, they were not as flawed as Enigma.


We see this played out today: even though better algorithms are available, older cryptographic algorithms are kept in service even as theoretical attacks are developed. It takes a practical attack before it's abandoned in droves. For example, SHA-1 has been crumbling for years, but it's only in the last few years that it's finally being phased out.

Similarly, people are becoming more aware of the greatly increased parallel processing power available. For example, a dozen GPUs can give an attacker the sort of parallel processing power for a few thousand dollars that previously would have cost millions. What would previously have been a purely theoretical attack might now be practical. Security folks use that knowledge to reassess which practices are safe and which are not.

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    Nobody has mentioned the psychological effect on the Germans have being able to read the British Merchant Marine Code for virtually the entire war, having broken it in the mid-1930's. This was a key factor in the German belief the Brits were weak in code breaking, if they could not even fix, or even recognize the need to fix, their own codes. – Pieter Geerkens Apr 11 '17 at 1:08
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    On the flip side, Hitler (despite his blatant madness) had figured out that something was up by Fall 1944, and insisted that all communications reparing for "Operation Watch on the Rhine", aka Battle of the Bulge, completely eschew the Enigma code machines. – Pieter Geerkens Apr 11 '17 at 1:11
  • *preparing, you mean :) – KlaymenDK Apr 11 '17 at 7:32
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As per a few comments here: They weren't all destroyed. Reading a few books on the times there's strong hints that at least some capability was retained - both Bombes and more advanced equipment.

The Brits captured many Enigma machines and also the more advanced Lorenz (if memory serves) which they could also break. Both were either used by/sold to allies including the Russians and of course monitored by the Brits because we're excellent at being devious.

Again, dragging info from comments, very few people knew anything about any of this stuff up to the 1980's so there was no theatre in destroying it. I suspect they destroyed stuff at Bletchley and a few other places purely as they were only temporary during WW2 (or had way more machines than they needed), and retained or re-built the same or better at more permanent / suitable locations staffed by security service regulars rather than the assembled wartime staff who were dissipating back to regular life.

It's true Churchill basically banned further development at home after the war for security purposes, and handed a lot of research & secrets to the USA by way of offsetting the vast wartime debt. IIRC R.V.Jones says something along the lines of "He (Churchill) gave it all away far too cheaply" and felt he'd put the UK at a major disadvantage. (Most Secret War)

Edit to add:

The true details of all this have been very slow to come out. As mentioned above, Churchill was paranoid about the whole thing and pushed hard to bury it after the war, even to the extreme detriment of further R&D.

Given that Enigma, Bombe, Colossus etc. were only revealed 30+ years later, and some of the finer details from Bletchley have only come out in the last decade or so, anything that had any utility to the security services post-war (EG the Lorenz stuff) could still be under wraps. It's not hard to imagine some derivation of that kit being used well into the cold war (BT still had plenty of Telex lines running well into the 1990's). Apply a 30+ year embargo on that and we may not know the full story for another decade or two.

Additionally, it's easy to imagine that all of the stuff worked on by civilians during the war was deliberately destroyed so that they all went home believing it was all dead & buried, rather than spirited away for further use. By that point, the Americans had copies of the kit and we were all sharing mountains of intelligence etc.

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    Correction: A Lorenz machine wasn't captured during the war. Bill Tutte worked out how they were constructed (and hence how to break the code) without ever having seen one. 'Over the following two months up to January 1942, Tutte and colleagues worked out the complete logical structure of the cipher machine. This remarkable piece of reverse engineering was later described as "one of the greatest intellectual feats of World War II"' See en.wikipedia.org/wiki/Lorenz_cipher#Code_breaking – DavidPostill Apr 13 '17 at 23:58
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    "The amazing thing about Lorenz is that the code breakers in Bletchley Park never saw an actual Lorenz machine until right at the end of the war but they had been breaking the Lorenz cipher for two and a half years." codesandciphers.org.uk/lorenz/fish.htm – DavidPostill Apr 13 '17 at 23:59
  • I stand corrected, thanks guys. I'm sure I've read the history of the Lorenz but it obviously slipped my mind. – John U Apr 18 '17 at 11:07
  • Hehe. I only knew because last week I read the book "Lorenz: Breaking Hitler's Top Secret Code at Bletchley Park" by Jerry Roberts :) – DavidPostill Apr 18 '17 at 11:09
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The destruction of the Colossus code breaking computers was a more significant thing, I gather the reasoning was the if the machines were retained other powers would pursue such technology. Britain had their own cipher machine typeX, there was some fear that if the cipher breaking machines became general knowledge then other powers would develop it and be able to read Britain's coded cipher machine communications. If the code break machines were retained they would be much more likely to be discovered, where if they were broken up, and their activities during the war would be harder to discover than if there was a continuing active group working with the machines.

Britain by this decision and suppression everything as an official secret and the breakup of the boffins responsible, threw away much of the British work in early computing, putting British computing back at least a decade when it could have been very much a world leader.

Colossus

TypeX machine

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    That last paragraph is speculative at best: many of the people involved in the computing machines at Bletchley Park and elsewhere went on to develop computers immediately after the war; see, for example, here. – Steve Melnikoff Apr 11 '17 at 8:18
  • yeah. it is speculative used the would COULD so it was consciously so. perhaps not a straight factual answer – pugsville Apr 11 '17 at 11:47
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    But British computing was a world-leader in the immediate aftermath of WWII (see, e.g., EDSAC and the Manchester Mark 1). – David Richerby Apr 11 '17 at 14:30
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Something else to think about. As of the end of the war, Britain was in horrific economic shape, a situation which lasted for a few decades (not just because of the war, also because of policies around currency). The cost to break a thing is much lower than the cost to perpetually store and secure a thing.

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    While it seems a possibility, is there any evidence that the potential maintenance costs were factored into the decision? – Steve Bird Apr 12 '17 at 21:25
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    No evidence that I have, pure conjecture. – user24541 Apr 12 '17 at 21:27
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Remember that rapid developments in general computers just after WWII rendered the specialized code breaking machines antiquated within a few years anyways. To keep them around stored somewhere, where someone could stumble upon them, would only unnecessary increase the chance that England's code breaking capabilities became know to other powers.

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