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Date: Tue, 7 Mar 2017 23:33:52 +0200
From: Юлиан Машаев <mashaev93@...il.com>
To: john-users@...ts.openwall.com
Subject: Re: Extracting CRC of files from RAR archive

The point is - I recover the whole file from to get the remaining 8-10
bytes by  brute-force generating last lines of file and computing theirs'
CRC - if it matches - I'm done. I'm not brute-forcing the password, since
it is 64 characters long, lowercase, upercase, digits, special symbols, and
no way it can be guessed in rational time.

As I understand c43d7b00400700 is "Terminator" block, and it is constant,
that's why it is possible to speed up brute-force of password by testing
encryption on small block, not on whole file. But it seems that with -hp
there is no way to extract CRC of non encrypted files(unrar uses them to
test if files were correctly deciphered with given password), because file
headers are encrypted just as usual data. Still there is a hope, because it
is just a single file, even with known filename and size, but without
CRC(of original file) it is not possible to perform successful attack.

2017-03-05 16:10 GMT+02:00 magnum <john.magnum@...hmail.com>:

> On 2017-03-04 17:13, Юлиан Машаев wrote:
>
>> I want to obtain few bytes from password protected RAR file, knowing all
>> other bytes. I tested "brute-force" attack and I obatined 10 bytes(just
>> enough for me). Now the problem is that sometimes this RAR file can be
>> encrypted both data and file list. E.g. on file without full encryption
>> rar2john returns:
>> 2017_04_roul.rar:$rar3$**1**de8ad915db6e3d57**69ad6816*(this is
>> CRC32)*19424*125278*...
>> but when filelist is encrypted:
>> 2017_04_roul_n.rar:$rar3$**0*
>> *61f78a76817adc3d*5023d65d291834942e414015cca48084
>>
>> No CRC is returned :(
>> Is it possible somehow to obtain it?(there is just one text file, and I
>> know most of the data except few bytes)
>>
>> As far as I remember, CRC is also encrypted in second case, and at opening
>> - both CRC and data are decrypted, then CRC of decrypted data is compared
>> with decrypted CRC. Seems safe enough, but still, I would like to find
>> some
>> methods to extract checksum of the file.
>>
>
> I'm not sure what you are trying to accomplish. For a "rar -hp" archive,
> we decrypt a certain single block of the data and compare it with a static
> known plaintext of (hex) 'c43d7b00400700'. If it matches, we're done. How
> would knowing the CRC of some file in that blob of data make it any better?
> It would still be a known plaintext but a whole lot harder to find (if at
> all possible).
>
> magnum
>
>

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