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I have access to a pair of plaintexts and their corresponding encrypted texts. Would I be able to decrypt a third message encrypted provided they use the same key? From my little understanding of cryptography I cannot get the key but I would be able to get something out of the reusing of the key (by XORing the message and its cipher?).

Thank you very much and I'm sorry if the question is not well-asked, I'm new.

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    $\begingroup$ Being able to decrypt another message only using a plaintext-ciphertext-pair would imply you are able to recover the key and thus would allow you to perform a known-plaintext-attack against AES, which would imply that AES is fundamentally flawed. So no, it shouldn't be possible (or at least very hard) to decrypt another message. $\endgroup$ – VincBreaker Feb 10 '18 at 18:21
  • $\begingroup$ I think this would be better written as an answer rather than a comment. $\endgroup$ – Jackoson Feb 10 '18 at 19:05
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For some encryption schemes xoring the plaintext and ciphertext would give you the key, eg the one time pad. (So we can use the key once) AES on the other hand is very complicated and so takes thousands of simultaneous equations to solve it. Therefore you'd need many, many, many ciphertext-plaintext pairs to extract the key.

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