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I have an encrypted file which I want to decrypt but I dont know the last 5 letters (which are probably literally numbers / letters) of the key / password.
The Initialization vector is known.

Is it possible to bruteforce this within a reasonable amount of time?

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    $\begingroup$ What's wrong with brute force? If what's missing is 5 bytes form the key known to be letters (of unknown case) or digits, there is a mere $62^5<2^{30}$ keys to try. How much time it takes depend heavily on unstated details, including exactly how the AES key is built, the encryption mode, what's known on the plaintext.. and of course on the computing power at hand, quality of implementation.. $\endgroup$ – fgrieu Jun 4 '18 at 15:05
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    $\begingroup$ What do you mean by letters? An AES-128 key consists of 16 uniformly distributed bytes. $\endgroup$ – CodesInChaos Jun 4 '18 at 17:50
  • $\begingroup$ Why was my comment deleted? $\endgroup$ – yuikonnu Jun 4 '18 at 18:11
  • $\begingroup$ @yuikonnu Could our mod, SEJPM, have integrated it in your question maybe? We tend to do that because the question should contain enough information without anybody having to consult to the comment section. $\endgroup$ – Maarten Bodewes Jun 4 '18 at 21:00
  • $\begingroup$ @yuikonnu indeed Maarten is right. I saw your comment as a request to clarify the question. Jack answered with the information. I edited this info into the question and removed the request and the response to tidy things up. $\endgroup$ – SEJPM Jun 5 '18 at 16:06
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fgrieu has it down in the comments:

What's wrong with brute force? If what's missing is 5 bytes from the key known to be letters (of unknown case) or digits, there is a mere $62^5<2^{30}$ keys to try. How much time it takes depend heavily on unstated details, including exactly how the AES key is built, the encryption mode, what's known on the plaintext.. and of course on the computing power at hand, quality of implementation..

with the additional information that usually $2^{40}$ is the standard estimate for anything computable within a reasonable amount of time on a home computer, so even with some minor hinderance by a weak-ish password-based key derivation function this should be no problem.

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