Comments on: Reversing CRC

By: Luke

Luke — Wed, 16 Nov 2011 23:02:07 +0000

The link of the Python script is down. I found an old version here: http://web.archive.org/web/20060211000520/http://w-nz.com/~darkshines/projects/crc-rev-v0.01.zip

By: Dalton

Dalton — Sun, 28 Aug 2011 20:40:37 +0000

Bas, I’m developing a tool for my workplace (We provide IT support) to ideally strip the password of .pst files. I am aware of Nirsoft’s utility PstPassword, however we constantly fight with false positives with Symantec (ugh!) and so we wanted something we could control the coding for. I can not find anything on this other than it is a modified CRC32 algorithm. Can you point me in the right direction on how to pull the CRC32 hash from the .pst and reserse it to get alternative passwords?

We can talk more over e-mail if you want to send me a message there.

Thanks!

By: liqirui

liqirui — Thu, 09 Dec 2010 07:54:04 +0000

Dear Sir:
I am a chinese student,I want to ask you a question that is there a limit of the length of the message, for example,could I calculate the checksum of the message which length is more bigger than 32767 with crc-16?
I am looking forward to hear from you.

By: Kranjcar

Kranjcar — Wed, 13 Oct 2010 01:28:41 +0000

Here�s an example that doesn�t seem to work:
current hash:A6A43EDA
wanted hash:A0A70404

Is this possible? What would the hash patch be in ASCII?

By: Christian

Christian — Tue, 12 Oct 2010 19:44:37 +0000

I’m trying to run your script

CRC that i want: 0xA0A70404

CRC on my file: 0x2985ef5d

Running code:
crc32 = Crc32Provider()
crc32._hash = crc32.xorOut ^ NumberFromHexadecimal(’2985ef5d’)
print “Required patch to make the new hash:”
p = crc32.patch(NumberFromHexadecimal(‘a0a70404′))
print p
print “Applying patch, resulting hash:”
crc32.update(p)
print NumberToHexadecimal(crc32.hash)
print “It works ^_^”

Tells me to add these 4 bytes:
a4,’p',’^',’+’

(0xA4,0×70,0x5E,0x2B)

I try to append this bytes on the file, but the new cksum is not 0xa0a70404, but 0x5EC210BA…

Do you see anything wrong that i do? Do you get the same bytes if you try?

Thanks in advance!

By: Bas Westerbaan

Bas Westerbaan — Tue, 12 Oct 2010 16:26:45 +0000

That shouldn’t be correct. Could you provide an example?

Bas

By: Christian

Christian — Tue, 12 Oct 2010 14:52:04 +0000

It doesn’t seem to work for crc32 used in cksum (UNIX).. is that correct?

By: Olly

Olly — Tue, 24 Aug 2010 11:36:08 +0000

I can directly manipulate the crc of file?

example:

original.txt … crc = CC89F9B7
patch.txt … crc = 1FC2FCD4

…. run your script…done..

patch.txt = crc from original.txt

By: Tushar

Tushar — Mon, 23 Aug 2010 16:46:02 +0000

I am unable to properly generate the check-sum of all the string characters in a loop. There are more than 20 characters in a string. Do to incorrect steps I took, therefore no character is being reversed. You haven’t given the steps for how I should update check-sum for all the string characters block by block, or character by character, in a loop. Please clarify this problem.

By: Tushar

Tushar — Mon, 23 Aug 2010 14:33:17 +0000

Dear Bas,

Well, I can’t understand a lot in your reversing technique. If I have got 16 Characters in a full string, how can I get all these 16 characters back from the CRC32 Checksum? I tried to code but I was lost and totally confused. If you can tell in simple words how to do this. Thanks.

Tushar