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Message-ID: <20160513120157.GA10626@openwall.com>
Date: Fri, 13 May 2016 15:01:57 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: WinZip PBKDF2 use optimization
Jim, magnum -
atom just posted this:
Behind the WinZip KDF optimization
https://hashcat.net/forum/thread-5451.html
It's about only needing to compute some of the PBKDF2 output blocks for
AES key sizes larger than 128 bits.
I vaguely recalled that we already had it, and I went to check - to my
surprise, it looks like the code currently in jumbo is fully prepared
for this optimization, but does not actually include it for WinZip.
Specifically, pbkdf2_hmac_sha1.h says:
* simpler, AND contains an option to skip bytes, and only call the hashing
* function where needed (significant speedup for zip format).
Indeed, it accepts a parameter skip_bytes, but somehow zip_fmt_plug.c
passes 0 for that parameter all the time. Looking through commits
history for zip_fmt_plug.c, I found that the optimization was lost with:
commit 528e6bcfb1a59f068b70c63b3c0d7ffc62c32ce4
Author: JimF <jfoug@....net>
Date: Sun Jul 6 22:03:13 2014 -0500
zip2 format. #434 #691 Removed FMT_NOT_EXACT. Now fully detects passwords.
Can the two of you look into this, please, and likely reintroduce the
optimization? Also check the OpenCL format for the same.
Thanks,
Alexander
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