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Date: Tue, 18 Aug 2015 23:39:15 +0200
From: magnum <>
Subject: Re: 7z's KDF is unsalted

On 2015-08-18 23:25, Solar Designer wrote:
> On Tue, Aug 18, 2015 at 10:59:43PM +0200, magnum wrote:
>> On 2015-08-18 10:52, magnum wrote:
>>> On another note, it seems 7z is actually unsalted within the KDF. So one
>>> could make extremely effective Rainbow tables for it.
>> Issue #1679, PR #1681.
> I went to:
> and there you mention similarity to WPA-PSK.  But we don't have this for

BTW I dropped that comment before reading this.

> [ run]$ GOMP_CPU_AFFINITY=0-31 ./john -test -form=wpapsk
> Will run 32 OpenMP threads
> Benchmarking: wpapsk, WPA/WPA2 PSK [PBKDF2-SHA1 128/128 AVX 4x]... (32xOMP) DONE
> Raw:    10645 c/s real, 334 c/s virtual
> ... or do we?  And folks are generating per-SSID rainbow tables for
> WPA-PSK, treating it as salted for that purpose.

For WPA-PSK the SSID is the salt (and we do take advantage when 
possible). For 7-zip the KDF is completely unsalted. You won't see the 
boost in WPA-PSK benchmarks but in real cracks against same-SSID pcaps. 
I guess we could add a same-SSID test vector to WPA-PSK to show the 
boost but it's not as fair a figure.

> And doesn't the AES step prevent rainbow tables for 7-Zip (even if it
> doesn't prevent the speedup you've now implemented)?

I'm not quite sure. Does it? It's a later step, just like the 
post-processing for WPA-PSK.


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