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What are the major code breaks in the history of cryptography? Which code breaks changed the face of a battle or another major event? Which one had nationwide consequences?

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  • $\begingroup$ Purple first comes to mind. See also this on the Hagelin C(X)-52, which can be regarded as a break from an operational standpoint. $\endgroup$ – fgrieu Feb 20 '17 at 7:49
  • $\begingroup$ The Zimmermann Telegram $\endgroup$ – r3mainer Feb 20 '17 at 10:54
  • $\begingroup$ @ Raza: Such broad and list-alike questions aren't very welcome at our site. What research have you done? See, sharing research efforts helps everyone! Tell us what research you did, what you found, and why it didn’t meet your needs. That shows users you took time trying to help yourself, it saves us from reiterating obvious answers, and (most important) it helps you to get more relevant on-point answers. At worst it will help you frame “a better question”; at best it might even answer it. $\endgroup$ – e-sushi Feb 20 '17 at 16:55
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    $\begingroup$ Right now, I have to agree with @e-sushi that this question is likely too broad or at least unclear what you're asking. Please specify exactly what you want to know, how much you want to know about it and about which time-frames we are talking about (pre-industrialization? Pre-Computer? Modern-Age?). $\endgroup$ – SEJPM Feb 20 '17 at 19:55
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    $\begingroup$ There is a similar Q – Examples of modern, widely used ciphers that suddenly fell? – on the site already, but seems less focused on history-changing results. $\endgroup$ – Ella Rose Feb 20 '17 at 20:20
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Besides the ones mentioned in the already existing Q&A "Examples of modern, widely used ciphers that suddenly fell?", the following major code breaks have been notable in history:

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    $\begingroup$ I've created a community wiki answer in case we want to keep this question alive. Note that there is already a list on wikipedia for modern ciphers (and above that for classical and WWII ciphers). $\endgroup$ – Maarten Bodewes Feb 20 '17 at 22:35
  • $\begingroup$ I've integrated the list from the question into the answer. $\endgroup$ – Maarten Bodewes Mar 17 '17 at 0:35
  • $\begingroup$ thanks for the effort, but now everyone thinks, question has been answered, so no one bothers to add more $\endgroup$ – abraza Mar 17 '17 at 7:04
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    $\begingroup$ Because it is not really an interesting question. :/ $\endgroup$ – Biv Mar 17 '17 at 17:40
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The case of the Queen Mary of Scots is interesting. She paid with her head the cost of using a weak cipher (breakable with frequency analysis). But that happened many centuries ago so ciphers using complicated substitution were the best option at the time.

For more details:

BBC - History - Elizabeth's Spy Network

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