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Date: Sat, 20 Oct 2012 15:23:24 +0530
From: Dhiru Kholia <>
Subject: Re: cracking 40-bit RC4 keys of Office and PDF files

On Thu, Oct 18, 2012 at 1:23 PM, Dhiru Kholia <> wrote:
> On Thu, Oct 18, 2012 at 10:34 AM, Solar Designer <> wrote:
>> On Thu, Oct 18, 2012 at 10:22:29AM +0530, Dhiru Kholia wrote:
>> Huh?  GuaPDF cracks these in about the same time that GuaWord/GuaExcel
>> do, so I think that you're wrong.  Perhaps the MD5s are inside the loop
>> only when you're cracking passwords and not keys?
> My bad. You are right.

Update: brute-forcing RC4 keyspace for PDF files is a *lot* faster
than Office files since it doesn't involve extra MD5 step.

I have modified qpdf to do decryption using RC4 key. See

Now all we need are some kind of Rainbow Tables to make cracking faster.


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