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Date: Tue, 14 Jan 2014 21:37:40 +0100
From: magnum <>
Subject: Re: Cracking MSChap v2

On 2014-01-14 20:17, Rich Rumble wrote:
> On Tue, Jan 14, 2014 at 1:37 PM, magnum <> wrote:
>> The optimized version brute forces the last third of the MD4 (NT hash) once
>> and for all when loading the ciphertext, a DES key space of just 2^16. The
>> inner loop just do an MD4 from the candidate and if the last third doesn't
>> match the bruted one, it's rejected without wasting more time on it. For any
>> number of salts, we still only need that single MD4 and that's why the many
>> salts speed is so good.
> How does that look in the pot file? I'm assuming it's the same as the
> input hash+challenge in the pot, and not the "striped" hash and challenge.

This optimization doesn't affect the pot entry. It's all about early 
rejection, that's all.

> Would it be easy to output the hash after the last 1/3rd is
> BF'd? Maybe a verbose setting or some such with that format? I think,
> since JtR is doing that method, the OP (and now me:) would like that
> option, put it on a To-Do/Wishlist. Again someone would have to write
> a patch, but sounds possible. Thanks for the info also!

It would be easy to output the last two octets (of 16), but that would 
be pointless. Brute forcing the full NT hash *instead* of guessing 
passwords would mean up to 2^56 rounds of DES, perhaps roughly 2^55 on 
average. I think that would take a day or so on an eight core. If the 
password is uncrackable it might be meaningful but it's out of scope for 

Like bruting the 40-bit RC4 key of an old Office document, it would be 
curious though. Personally I wouldn't mind including such separate tools 
in the JtR tree (reusing existing code) but I think Solar would mind. We 
could start a separate tree though. "JtRtools" perhaps? We have the bits 
and pieces for doing that Office thing on GPU with fair speed but I 
never put it together.


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