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If the encrypted file is from one of the known formats, such as doc, jpg, mp3, etc., is it possible to reverse engineer it and decrypt it?

I "roughly" know the mechanic behind the AES encryption: Matrix operation, Row shifting, Column shuffling, all that stuff.

That's what makes AES "unbreakable", but that all seems built on the assumption of "you don't know what it was", what if you "know" what it was? at least what format it was? Is it possible to "break" it?

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No. You're describing a known-plaintext attack, which AES is resistant to, just like every other cipher worth using today is.

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Matrix operation, Row shifting, Column shuffling, ... That's what makes AES "unbreakable", but that all seems built on the assumption of "you don't know what it was"

Yes, those steps make AES unbreakable in the sense that that they prevent it from being vulnerable to attacks like linear cryptanalysis, differential cryptanalysis and more. They don't do this through "you don't know what it was" though, the algorithm is already publicly known; they do it by removing any exploitable relationship between the ciphertext and plaintext.

But you're also forgetting about the AddRoundKey step. That part "mixes" the secret key with the ciphertext, so that unless you know the secret key, you can't reverse the encryption even if you know everything else.

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