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British attacks on the Enigma(s) were similar in concept to the original Polish methods, but based on different specifics. First, the German Army had changed their practices in 1939 (more rotors, different 'message setting', etc), so the Polish techniques no longer worked directly. Second, the German Navy had always used more secure practices, and no one had broken any of their traffic.
One new attack relied on the fact that the reflector (a patented feature of the Enigma machines) guaranteed that no letter could be enciphered as itself, so an A could not be sent as an A. Another technique counted on various common German phrases, like "Heil Hitler" or "please respond", which were found to likely be in this or that plaintext; successful guesses as to the plaintext were known at Bletchley as cribs. With a probable plaintext fragment and the knowledge that no letter could be enciphered as itself, it wasn't uncommon that a corresponding ciphertext fragment could be identified. This often provided a large hint as to the message settings, much in the same way the message setting codes had done for the Poles before the War started.
German operators themselves also gave the cryptanalysts immense help on a number of occasions. In one instance an operator was asked to send a test message, so he simply hit the T key repeatedly and sent it. A British analyst received a long message without a single T in it from the interceptor stations, and immediately realised what had happened. In other cases, Enigma operators would constantly use the same settings for their message codes, often their own initials or those of their girlfriends (one apparently had the initials "C.I.L.", so Bletchley Park named such hints 'cillies'). Analysts were set to finding these messages in the sea of intercepted traffic every day, which winnowed out enough possibilities to allow Bletchley to use the original Polish techniques (more or less) to find the initial settings for the day from those that remained. Other German operators used "form letters" for daily reports, notably weather reports, so the same crib could be used every day.
Had the Germans ever replaced every rotor at the same time, it is possible that the British would not have been able to break back into the system. And had German operational practice been better, things would have been much more difficult. However, both because of the expense and because of the difficulty of getting all those new rotors to all the necessary ships and units, it was never done. Instead the Germans simply added new rotors to the mix every so often, allowing the settings of the newest ones to be deciphered after a short period.
Usable Ultra information came too late to be of great help during the Battle of Britain.
The Allies were seriously concerned to conceal from the Axis command that they had made any breaks into any of the Enigma traffic. This was taken to the extreme that, for instance, though they had intercepted and knew the whereabouts of U-boats lying in wait in mid-Atlantic, they were not generally hunted unless a 'cover story' could be arranged -- often a search plane was 'fortunate enough' to sight the U-boat, thus explaining an attack. Ultra information was used to attack and sink many of the Afrika Korps supply ships travelling to North Africa, but as in the North Atlantic, every time it was used, some 'innocent' explanation had to be provided; scout planes would often be sent on otherwise unnecessary missions to ensure they were seen by the German military. The British were, it is said, more careful about this than the Americans, and the difference in caution on this point was a source of friction between them.
The distribution of Ultra information to the Allied commanders and units in the field involved considerable risk of discovery by the Germans, and great care was taken to control both the information and knowledge of how it was obtained. Liaison officers were appointed for each field command to manage and control dissemination.
In the summer of 1940, British cryptanalysts, who were successfully breaking the German Air Force Enigma cypher variants, were able to give Churchill information about the issuance of maps of England and Ireland to the Sealion invasion forces.
From the beginning, the Naval version of Enigma used a greater variety of rotors than did the Army or Air Force versions, as well as various operational methods that made it much more secure than other Enigma variants. There was no hint at all to the initial settings for the machines, and there was little probable plaintext to use either. Different, and far more difficult methods had to be used to break into Naval Enigma traffic, and with the U-boats running freely in the Atlantic after the Fall of France, a more direct approach recommended itself.
On 7 May 1941 the Royal Navy deliberately captured a German weather ship, together with cipher equipment and codes, and 2 days later U-110 was captured, together with an Enigma machine, code book, operation manual and other information enabling the submarine message traffic to be broken until the end of June. And they did it again shortly afterwards.
In addition to U-110, Naval Enigma machines or settings books were captured from a total of 7 U-boats and 8 German surface ships, including U-boats U-505 ( 1944) and U-559 ( 1942), as well as from a number of German weather-reporting boats, from some converted trawlers, a small vessel (the Krebs) captured during a raid in the Lofoten Islands off Norway, and so on. Several other more imaginative techniques were dreamed up, including Ian Fleming's James Bondian suggestion to "crash" captured German bombers into the sea near German shipping, hoping to be "rescued" by the crew, which would then be taken captive by the Commandos hiding in the plane and the crypto material captured intact.
In other cases the Allies forced the Germans to provide them with a crib. To do this they would drop mines (or take some other action), and then listen for messages thus provoked. In the case of mining this or that channel, they expected the word "Minen" would be in some of them. This technique was called gardening at Bletchley.
Even these brief periods were enough to have dramatic effects on the progress of the War. Charting the amount of traffic decoded against the British shipping losses for that month shows a strong pattern of increased loss when Naval Enigma was blacked out, and vice versa. But, by 1943, so much traffic had been decrypted that the code breakers had an excellent understanding of the messages coming from various locations and times. For instance, a brief message sent from the west at 6am was likely to be sent by a weather reporting boat in the Atlantic, and that meant the message would almost certainly contain these cribs, and similarly for other traffic. From this point on, Naval Enigma messages were being read constantly, even after changes to the ground settings.
However, the new tricks only reduced the number of possible settings for a message. The number remaining was still huge, and due to the new rotors the Germans had added from time to time, that number was much larger than the Poles had been left with. In order to solve this problem the Allies, especially the US, "went industrial", and produced much larger versions of the Polish bomba that could test thousands of possible key settings very rapidly indeed.
Some Germans had suspicions that all was not right with Enigma. Karl Dönitz received reports of "impossible" encounters between U-boats and enemy vessels which made him suspect some compromise of his communications. In one instance, three U-boats met at a tiny island in the Caribbean, and a British destroyer promptly showed up. They all escaped and reported what had happened. Dönitz immediately asked for a review of Enigma's security. The analysis suggested that the signals problem, if there was one, wasn't due to the Enigma itself. Dönitz had the settings book changed anyway, blacking out Bletchley Park for a period. However the evidence was never enough to truly convince him that Naval Enigma was being read by the Allies. The more so, since his counterintelligence B-Dienst group, who had partially broken Royal Navy traffic (including its convoy codes during the early part of the War), supplied enough information to support the idea that the Allies were unable to read Naval Enigma. Coincidentally, German success in this respect almost exactly matched in time an Allied blackout from Naval Enigma.
In 1941 British intelligence learned that the German Navy was about to introduce M4, a new version of Enigma with 4 wheels rather than 3. Fortunately, for the Allies, in December, a U boat mistakenly transmitted a message using the four rotor machine before it was due to be implemented. Realising the error, they re-transmitted the same message using the 3 rotor Enigma, giving the British sufficient clues to break the new machine very shortly after it became operational on February 1 1942. The U-boat network which used the four rotor machine was known as Triton, codenamed Shark by the Allies. Its traffic was routinely readable.
It is commonly claimed that the breaks into Naval enigma resulted in the war being a year shorter, but given its effects on the Battle of the Atlantic (1940) alone, that might be an underestimate.
A break of some messages (not in German Enigma, however) led to the defeat of the Italian Navy at Capa Matapan, and was preceded by another 'fortunate' search plane sighting. British Admiral Cunningham also did some fancy footwork at a hotel in Egypt to prevent Axis agents from taking note of his movements and deducing that some major operation was planned. Ultra information was of considerable assistance to the British at El Alamein in Western Egypt in the long running battle with the Afrika Korps under Rommel. Intelligence from signals between Adolf Hitler and General Günther von Kluge was of considerable help during the campaign in France just after the Allied landings on D-Day, particularly in regard to estimations of when German reserves would be committed to battle.
By 1945 almost all German Enigma traffic (Wehrmacht, Navy, Luftwaffe, Abwehr, SD, etc.) could be decoded within a day or two, yet the Germans remained confident of its security. Had they been better informed, they simply could have, and surely would have, changed systems, forcing Allied code-breakers to start over. The Germans considered Enigma traffic so secure that they openly discussed their plans and movements, handing the Allies a huge amount of very useful information. However, Ultra information was also misused or ignored at times. For instance, Rommel's intentions just prior to the Battle of the Kasserine Pass in North Africa had been suggested by Ultra, but this was not taken into account by the Americans. Likewise, Ultra traffic suggested an attack in the Ardennes in 1944, but the Battle of the Bulge was a surprise to the Allies because the information was disregarded.
After the War, the American TICOM project teams found and detained a considerable number of German crypto personnel. Among the things they learned was that German cryptographers, at least, understood very well that Enigma messages might be read; they knew Enigma was not unbreakable. They just found it impossible to imagine anyone going to the immense effort required. (See Bamford's Body of Secrets in regard to the TICOM missions immediately after the War.)
A most intriguing alleged, and still open, use of Ultra information may have been in the Lucy spy ring. This was an extremely well informed, and rapidly responsive, ring which was able to get information 'directly from the German General Staff Headquarters' -- often on specific request. The allegation is that it was, in major part, a way for the British to get Ultra information to the Soviets in a way which appeared to have come from highly placed espionage, and not from cryptanalysis of German radio traffic. The Lucy ring was operated by, apparently, one man, Rudolf Roessler, and was initially treated with considerable suspicion by the Soviets when it began to operate. The information it provided was accurate and timely, and Soviet agents in Switzerland (including Alexander Rado , the director) eventually took it quite seriously.
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