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Date: Fri, 19 Oct 2012 22:53:06 +0200
From: magnum <>
Subject: Re: cracking 40-bit RC4 keys of Office and PDF files

On 18 Oct, 2012, at 6:52 , Dhiru Kholia <> wrote:
> On Thu, Oct 18, 2012 at 4:08 AM, magnum <> wrote:
>> On 14 Oct, 2012, at 10:54 , Dhiru Kholia <> wrote:
>>> This program guarantees decryption of office documents which are using RC4 40-bit encryption.
>> So what would you do with a cracked key? Is there any free program that can use this RC4 key to unlock a document?
> Sadly, I couldn't find any program which can do this. I wrote the
> program for fun. Good luck with the OpenCL implementation.

For a while I thought we could nail this RC4 first, and then do a more standard candidate search aiming for that RC4 key. But it would not help a lot. Perhaps there's code in OpenOffice or LibreOffice that can be ripped for making an "unlocker" but as already discussed, this is not in JtR's scope.

>> Anyway, the try_key() would be trivial to implement in OpenCL. I wonder how fast it would be on a good GPU? A couple of minutes on average? Only the two outermost loops in host code then. I think I'll do this soon just for phun.
> I am working on a similar key brute-forcer for PDF files where the key
> calculation is expensive (50 rounds of MD5). OpenCL implementation
> will be useful there.

It turns out that while RC4 is indeed trivial to implement, it's not GPU friendly at all. Still, my first take, not very optimised, is 3x faster on 7970 than on a quad i7 [my OpenCL code is almost exactly as fast on CPU as your OpenMP code] so with a lot of tweaks it should be usable for composite stuff like mskrb5 (which is MD4 + HMAC-MD5 + RC4). There are some tricks possible that helps on GPU. I'll play some more with this mostly for coming up with an RC4 for future JtR stuff.


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