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It depends greatly on what form and mode of encryption you use. For any stream cipher or a block cipher used in a psuedo-stream mode such as CTR or GCM then reusing a key/IV pair even once is absolutely fatal. Never, ever use a static key with them, and if you do, never, ever reuse an IV. They are strictly for one time use keys negotiated via a key exchange ...


In short, yes, key re-use will eventually lead to a growing vulnerability to a persistent and dedicated attacker over a very large data set. Depending on what encryption method you are using, the details get a very complicated very quickly. I believe in a standard AES CBC implementation with a random IV a key change is recommended after 264 bytes of data. ...

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