->''"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 -- [[IKnowYouKnowIKnow 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.



[[folder:Comic Book]]
* ''ComicBook/YTheLastMan''. In Paris, Alter is seen going through the mail to find the last "[[IncrediblyLamePun male]]".
* In ''ComicBook/VForVendetta'', V hacks Fate, the government's central supercomputer (and, incidentally, [[CargoShip 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 ''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 BatmanGambit to convince the Soviets they don't.
* The main reason for wanting to attack the titular submarine in ''Film/{{U571}}'' was to capture the submarine's enigma machine.
* One plot point in ''Film/ToraToraTora'' 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 [[PoorCommunicationKills information delay]] that contributes to the Americans' failure to prepare for the impending attack.

* In ''Literature/FromRussiaWithLove'' (originally a novel), Literature/JamesBond must collect a Soviet defector from Turkey, who is bringing a cipher machine with her. He has to start a fire in the embassy to cover up the theft.
* In the Robert Harris novel ''Enigma'', Bletchley Park has been locked out of "Shark", the German U-boat code. Part of the novel involves them getting back in, the other part making sure that the whole thing isn't blown to the Germans.
** In the later part of the story, the heroes use an Enigma machine to decrypt a message that was stolen- [[spoiler: it is a report on the Germans finding the site of the Katyn Massacre, where 20,000 Polish soldiers were murdered in 1940 by the Soviets]].
* In ''Literature/TheThirdWorldWar'', the US had cracked the Atrophos cipher being used by Cuba. In the early stages of the war, the Cubans send a long message stating that they cannot attack the US directly, but they might be able to do some small sabotage operations. While the Soviet cipher clerk is manually decrypting the message and wishing that the long-winded platitudes that begin it are done with, the Americans have broken the whole message. The Soviets interpret the message correctly (You cowards! You're wimping out like Italy in 1939), the Americans don't (They're going to attack us!) and launch air strikes on Cuba. It takes the rest of Latin America to stop a full-scale invasion.
* In ''TheThrawnTrilogy'', eventually the New Republic did figure out that Thrawn was aware of all the plans they made in the Imperial Palace on Coruscant. They spent a lot of effort going through the palace trying to find the listening devices, to no avail. Turns out Delta Source was right in front of them the whole time.
* In DanBrown's ''Literature/DigitalFortress'', the NSA has a supercomputer capable cracking the digital encyrption keys used in e-mails. The titular program is touted as being unbreakable, prompting the NSA to [[spoiler:try and steal the program and secretly install a backdoor into the program, giving them unlimited access to enemy e-mail communications. However, things don't go as planned.]]
* Literature/{{Discworld}}'s Vetinari not only does this regularly in everyday life, but he expects others to do the same to him, and prepares accordingly. After all,
-->What would be the point of ciphering messages that very clever enemies couldn't break? You'd end up not knowing what they thought you thought they were thinking…
* Most of Literature/{{Cryptonomicon}} (the 1940s bits, at least) is based around this. As with the real life section below, the Allies go to incredible amounts of effort to engineer plausible explanations for them having the information gained from intercepting enemy signals.
* Stephen Maturin of the ''Literature/AubreyMaturin'' series does this frequently. He even carefully edits the messages before passing them along to spread misinformation in enemy lines. While his friend Jack Aubrey found the operation very dishonorable, Stephen once mentioned that he has no qualms about violating an entire mail coach if it wins them the war.
* ''Literature/JackRyan'':
** In ''Literature/TheSumOfAllFears'', a significant subplot involves whether or not NSA and State Department encoding methods have been cracked by the Soviets. In fact, they have, and at the end, when the US doesn't believe they've been cracked, it makes things in the climax significantly worse.
** Creator/TomClancy also likes looking into the specifics of certain cryptographic methods. ''Literature/ClearAndPresentDanger'' has a relatively long-winded passage on one-time pads (see below), and even ''Literature/TheHuntForRedOctober'' goes into some length about it.
* Something [[TheWheelOfTime Egwene and the other rebel Aes Sedai]] try to do while Dreaming with regards to Elaida's mail.
* In Creator/VernorVinge's novella "Literature/TrueNames", Erythrina knows the True Name of one of The Mailman's associates, which gives her enough information to tap and decrypt communications between the two.
* The ConnieWillis novel ''To Say Nothing of the Dog'' revolves around the attempt by two British timetravellers to prevent the possible exposure of the ULTRA program (see Real Life below) due to [[GodwinsLawOfTimeTravel accidental alteration of the events of the bombing of Coventry by timetravellers]].
* The W.E.B. Griffin ''Corps'' novels spend a lot of time discussing MAGIC (The codename for intercepted and decrypted Japanese communications), the importance of the data brought in by it, and the efforts necessary to ensure that (As one non-MAGIC communication obliquely put it) the rabbit stayed in the hat.

[[folder:Live Action TV]]
* A sketch on ''Series/HorribleHistories'' has Sir Francis Walsingham advertising his new postal service where your mail will be picked up, sorted, read by a spy...

[[folder:Real Life]]
* 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. This has been denied by people in a position to know - specifically, [[https://en.wikipedia.org/wiki/Reginald_Victor_Jones R.V. Jones]], who maintained that while messages were intercepted and decrypted indicating that Coventry was a target, the decryption came too late to do anything about it.
* The Germans managed to crack the British merchant navy's codes in WorldWarTwo. 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 WorldWarTwo. In particular, the cracking of the Imperial Japanese Navy's JN-25b code by early 1942 allowed the US to get early warning that the IJN was planning a big operation around some place they only called "AF". On a hunch, the US [[BluffTheEavesdropper instructed the US Navy base at Midway Island to fake a water supply problem and broadcast it uncoded over radio]]; within 24 hours the Japanese had parroted the information via JN-25b saying that AF was short on water, confirming that Midway was the target and allowing the US Navy to plan a trap around them.
* Where British, Germans and Americans cracked the enemy ciphers in WorldWarII, 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 [[AwesomeButImpractical completely useless]]. The machine was later picked up by US Intelligence, who used it as the basis of the HotLine, 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 ''Film/{{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.
** OlderThanTheyThink: in the [[WorldWarI previous war]], Italian communications were handled in [[http://en.wikipedia.org/wiki/Sardinian_language 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 [[RightfulKingReturns 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 [[http://en.wikipedia.org/wiki/Zimmermann_Telegram 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 [[MexicoCalledTheyWantTexasBack 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.)