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Assuming that what I read about the Nazi enigma encryption being broken in ww1 and ww2, where the code breakers hid the fact that they had broken the code, allowing the world to feel that the code was still good.

My question is, how do you know that all the encryption techniques which people use today have not been compromised, where if it has been compromised, is hidden from the world?

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    We cannot know that. Although Bruce Schneier suggested in "Applied Cryptography" to send a newly created, good joke in encrypted form. If you heard the joke elsewhere you'd know somebody had decrypted it. The idea was that even NSA employees could not resist the urge to share a good joke.
    – S.L. Barth
    Apr 21 '15 at 8:07
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There are encryption techniques like the One Time Pad that are provably non-compromisable when employed properly (but the one-time pad as single encryption technology has other problem like the possibility to change the encrypted message unnoticed).

Otherwise, it is often good enough to know that there are no publically known breaks to the encryption scheme; this will reduce the number of possible attackers drastically.

Yet another aspect is that even a perfect unbreakable encryption is useless when the attacker can watch you typing in the clear text (with a camera or a key logger).

So, in security you need a "theat model" and defense techniques against those threats. This is a never ending art.

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You can never know for certain if your cryptosystem is still safe. But there are few classical ways of going about this.

If you are the one whose cryptosystem may be broken:

Use your cryptosystem to send false information and see if the enemy responds to the false information. If they do, they've read your message.
Note that that indicates that your cryptosystem was broken, but the false information may also have been obtained and leaked in another way - for example, an enemy agent within your own organisation.

If you are the one who broke a cryptosystem:

Every time you use the knowledge obtained by decrypting enemy messages, you risk the enemy becoming aware of yours having broken their crypto.
You need to somehow convince your enemy that you obtained the information in another way. Clearly this is not a trivial task.

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