"Gentlemen do not read each other's mail."
— Secretary of State Henry L. Stimson upon closing down The Black Chamber, the U.S. government's peacetime cryptography section.
When fighting a war, cold or hot, you try to anticipate what the enemy is going to do. What, however, if you already knew what they were doing? What if you were able to decipher their messages and know their plans in advance?
This of course leads to problems — what if they know you know
? They'll change their codes. Therefore, you must be very careful- you can't, for example, just attack a ship because you know it's going somewhere- the other side might work out that the only way you would know is if you are...
Of course, stuff is still subject to interpretation.
Cipher machine theft is also a common plot in espionage, provided you are able to cover up said theft.
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- Y: The Last Man. In Paris, Alter is seen going through the mail to find the last "male".
- V hacks Fate, the government's central supercomputer (and, incidentally, the love of the villain's life). He uses it to access both their surveillance systems and the freaking postal system in order to bring down the corrupt regime. Unlike the film, the postal system stuff was just to flip the bird at the establishment by showing them he was not only in their systems, but able to control them.
- In the 1983 film Enigma, Martin Sheen plays a CIA agent who must steal a Soviet scrambler from East Berlin. The CIA already have one- this is just a Batman Gambit to convince the Soviets they don't.
- The main reason for wanting to attack the titular submarine in U571 was to capture the submarine's enigma machine.
- One plot point in Tora! Tora! Tora! is that the US Military can decrypt the Japanese diplomatic codes. However, it is time-consuming, and very very secret. So secret, that at one point the President of the United States is removed from the list of people authorized to read the decoded messages because someone in his office improperly disposed of a decoded message. This of course only serves to add to the information delay that contributes to the Americans' failure to prepare for the impending attack.
- ULTRA- namely the Allied decryption of the German Enigma and Lorenz ciphers for much of the war. Intensive efforts to provide "cover stories" were created- if you couldn't make it look like it had been found another way, you couldn't act on it.
- There is some suggestion that the British lost Crete to keep ULTRA secret. London knew things about the German invasion plans, and they told the general commanding the defence of Crete what they knew. But they didn't dare tell him how they knew. So he didn't place full reliance on the intelligence, didn't commit fully to defending the airfields, and lost.
- The bombing of Coventry during the Blitz is also believed to be a cover-up; ordering the city be evacuated would have revealed that the codes had been broken.
- The Germans managed to crack the British merchant navy's codes in World War II. The Kriegsmarine used this informtaion cautiously, leading Allied intelligence to think German sonar was far more advanced than it really was right up to the end of the war.
- The Americans also cracked the Japanese ciphers in World War II.
- Where British, Germans and Americans cracked the enemy ciphers in World War II, the Italians choose an easier way: in September 1941 (when the US were still neutrals), Italian spies broke in the US embassy in Rome and stole the ciphers. From then on, the Italians were able to read the US diplomatic communications without any problem, something that, thanks to the American consul at Alexandria having access to the British war plans from December 1941 and reporting them to Washington, allowed the Axis forces in Africa to know what the British were doing. While the British caught on it fairly quickly, the Americans didn't believe it until June 1942, when the Germans spoke about it on the radio, allowing ULTRA to prove it and force the US to change codes (and recall that talkative consul).
- So the West couldn't do this, the Soviets used something called "one-time pads", where the coding method for each message was different. However, when one set got accidentally used twice, the West were able to figure out the messages and from 1946 break a good many of them. It was key to identifying a good number of Soviet moles, including the Rosenbergs and Julius Fuchs. Some people weren't prosecuted as VENONA (as it was called) was too valuable to be used in court- it was not declassified until 1995.
- The one-time pad is, as long as the key is not compromised or re-used, completely uncrackable. A communications device was invented by the Norwegians after WWII based on this principle, but since each message requires pre-delivery of a key as long as the message itself, they deemed it completely useless. The machine was later picked up by US Intelligence, who used it as the basis of the Hot Line, since books with thousands of key codes could easily be exchanged by the embassies.
- The Americans used a low-tech solution in World War II by using Navajo code-talkers, nicknamed wind-talkers, not to be confused with the fictional account in the movie Windtalkers. They didn't just speak in the Navajo language (which the Japanese were completely clueless to figure out anyway), but also spoke in code in the language. It was basically double encryption. The British apparently made similar use of Welsh.
- Older Than They Think: in the previous war, Italian communications were handled in Sardinian Language (Sardu): regarded by many scholars to be the living language closest to Classical Latin, it is not an Italian dialect nor by any means related to Italian. In Italy, it is notoriously known for its difficulty to be actually understood by anyone not speaking it (that is, Italians and many Sardinians themselves, being and endangered language), unless spoken very slowly. And to be sure it couldn't be understood, the code-talkers weren't Italians speaking the literary version of Sardinian (that may have been understood), but actual Sardinians speaking their native dialects (which, even though they are all pretty much mutually intelligible, vary greatly the pronunciation of words).
- During the English Civil War, John Thurloe became the Spymaster for the Commonwealth, and later the Protectorate. He also became Postmaster-General, which probably made his job quite a bit easier (It is worth mentioning that he did such a good job with governance that he was not even imprisoned after the Restoration in exchange for assisting with central government.
- During the Irish War of Independence Michael Collins spy network was so sophisticated that top British Generals and Civil Servants mail arrived marked censored by the IRA.
- The Zimmermann Telegram was a diplomatic note that was sent by the German Empire to Mexico, looking at seeing whether Mexico would be interested in declaring war on the United States should the increasingly pro-Triple-Entente US enter World War I. The Germans promised financial and military aid to help Mexico reclaim the "lost territories" of Arizona, New Mexico, and Texas. This telegram was intercepted by the British as it was passing through the US (direct communication lines from Germany to the Western Hemisphere were cut by the British, so the only way the Germans could communicate with their embassies in the Americas was via Canada and the US) and turned over to the Americans (Britain had to find other pieces of evidence so as to avoid admitting it was reading the US's diplomatic mail), thus further enraging American public opinion and leading to the US's entry into WW1 on the side of the Entente. (Mexico, by the way, declined the offer.)