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Message-ID: <CAJ9ii1GqdyW0iAAbzA_jei07RyxU0SiWZS-qAfdmVPweGa0UUA@mail.gmail.com>
Date: Thu, 4 Dec 2014 11:08:20 -0500
From: Matt Weir <cweir@...edu>
To: "john-users@...ts.openwall.com" <john-users@...ts.openwall.com>
Subject: Cracking multiple AES Zip files
AES encrypted zip files have a high number of false positives in them. From
the Winzip spec:
Password verification value
This two-byte value is produced as part of the process that derives the
encryption and decryption keys from the password. When encrypting, a
verification value is derived from the encryption password and stored with
the encrypted file. Before decrypting, a verification value can be derived
from the decryption password and compared to the value stored with the
file, serving as a quick check that will detect *most*, but not all,
incorrect passwords. There is a 1 in 65,536 chance that an incorrect
password will yield a matching verification value; therefore, a matching
verification value cannot be absolutely relied on to indicate a correct
password.
It also appears that the AES Zip verification value includes a salt. So my
question is, if I have multiple AES encrypted zip files that all use the
same password, would it be possible to have JtR see if a password matches
them all to reduce false positives?
Matt
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