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Is this possible to somehow reverse engineer password to .zip file if I have 2 similarly encrypted .zip archives, both need 20-sign password (that has big and small letters and numbers) so Brute Force is impossible to execute when I have speed of 14k passwords per second. I have 20-sign key to one of the archives. Is it possible to understand how they are encrypted and by somehow using reverse engineering decrypt the one that I don't have key to? I'm very new here so I have literally no idea where to start. Cheers.

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As I lack 50 reputation to comment and the question is rather simple, I'll answer directly by assuming some details:

Assuming ZIP-encryption utilizes a proper encryption algorithm, no. Even if two files are similarily (whatever that means. I just assume similar passwords) encrypted, you won't be successful.

Reverse engineering is the wrong term here. You can actually look up how encryption is done by reading the code of an open-source ZIP software or looking up the algorithm in general.

Following the Kerckhoff principle, the security of the algorithm is not determined by the secrecy of the algorithm itself, but the keys being used. No key, no decryption.

In addition, the keys being actually used are probably very different from each other, even if the password are similar, as functions like PBKDF or various cryptographic Hashfunctions are used to derive encryption keys from a password.

Hash("Hey, welcome to Stack Exchange!") is very different from Hash("Hey, welcome to Stack Exchange.").

  • No, passwords are very different. Are there different encryption types encrypting zip archive? – Przemek Dąbrowski Feb 5 '18 at 14:48
  • It depends. If I remember correctly, AES-CBC is used, most of the time. However, this won't help you. Your ultimate goal of decrypting cipher text (your encrypted ZIP) without knowing the key is, according to current knowledge in the field, computanionally infeasable. Your only chance is to guess passwords, as you know one of them and the other password might be similar. But as I said, blindly guessing passwords (Brute Force Attack) won't yield a result before the death of your grand-grand-grand-grand-....-grand children. Sadly, you can't negotiate with mathematics. – GxTruth Feb 5 '18 at 14:53
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    Are there different encryption types encrypting zip archive? Yes. – user155462 Feb 5 '18 at 14:53
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No.

Assuming eg. AES, you have two misconceptions:

  • Knowing one key will not help you to understand how the encryption works, because it is already fully known. Nothing secret about it, except the keys (in an ideal case).
  • Understanding how the encryption works is not enough to break it.

If they unknown password is similar to the known one, this might help. But just that they are passwords for the same algorithm will not.

  • Ok, thank you very much. I guess that's all I wanted to ask. – Przemek Dąbrowski Feb 5 '18 at 14:52

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