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Date: Sat, 4 Sep 2021 17:27:20 +0200
From: magnum <john.magnum@...hmail.com>
To: john-users@...ts.openwall.com
Subject: Re: brute forcing AES key
On 2021-09-04 17:19, magnum wrote:
> On 2021-09-03 17:41, p+password@...atpro.net wrote:
>> I have a string, result of encryption by an App.
>> The code for encryption of this App is visible here:
>> <https://github.com/Anubis901/SafeCrypto/blob/main/lib/encryption.dart>
>> It looks to me that the dev of this App is missing something
>> important: he did not use a derivation fonction to create a secret
>> key, he uses the user provided password directly as an encryption key.
>> So I guess it would be feasible to use a password cracker software
>> like JtR to brute force the password / key and decrypt the string I have.
>>
>> Is that possible with JtR, if at all?
>
> A format for attacking this should be, potentially, very fast. I didn't
> look a lot into that link you gave but the tricky bit is likely to know
> when we made a correct guess. It goes like this: Try a candidate,
> decrypt some ciphertext, verify against some known plaintext (or
> plaintext-ish). If it's correct, we guessed the correct password.
>
> So, if we can identify some plaintext (such as a file magic) or
> plaintext-ish (such as json or xml and/or 100% UTF-8 or ASCII) we can
> hopefully write a format for this.
Oh and another possibility which is used in many formats is *if* that
software use a padding scheme for the very last block that we can use
either solely (if it turns out a correct hit *always* have at least,
say, 6 bytes of padding) or at least as an early reject.
magnum
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