[TOOL] PKZip Plaintext Attack Using Pkcrack (Step by Step)

From: support@securiteam.com
Date: 04/12/03

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Date: 12 Apr 2003 21:59:40 +0200

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      PKZip Plaintext Attack Using Pkcrack (Step by Step)
    ------------------------------------------------------------------------

    DETAILS

    True story:
    We found on one of our backup CDs an old password protected zip archive,
    containing several important documents. Since the password for this file
    were long forgotten, we used elcomSoft's ( <http://www.elcomsoft.com/>
    http://www.elcomsoft.com/) excellent recovery tool (azpr) to brute force
    the password for up to 7 characters. After seeing we were facing 50 days
    of brute forcing, we decided to give known-plaintext attack a shot. Here's
    what happened.

    Ingredients:
    To decrypt a password protected .zip file, you will need the following:
    1) A PKZip password protected file.
    2) pkcrack toolkit, which can be downloaded from:
    <http://www.unix-ag.uni-kl.de/~conrad/krypto/pkcrack.html>
    http://www.unix-ag.uni-kl.de/~conrad/krypto/pkcrack.html
    2) Zip compressing software, same that was used to encrypt the original
    file (see note that follows).
    3) Some known plaintext from the encrypted file.

    Preperation:
    "Why would we want to decrypt a password protected file if we already have
    its contents in plaintext?"
    You don't have to have the entire archive contents in plaintext - you only
    need a part of it.

    Recipe:
    PKZip allows us to list the file's contents (even if it is password
    protected) so we can look at this example file:

    Stuff 1 WinZip file: Documents.zip (password protect):

    Document1.doc
    Document2.doc

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