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Known plaintext attack (ARJ)


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ARJ files have relatively strong encryption algorithm. The password isn't stored anywhere in password protected archive. However, this algorithm isn't as strong as DES, RSA, IDEA and similar ones, and one of the ways of breaking ARJ protection is using known-plaintext attack.


Having encrypted file created by ARJ archiver, and the same file in unencrypted form, we can make some calculations and retrieve password. Usually, ARJ archive contains several files, and all of them has the same password. To perform plaintext attack, all you need is one file from archive, compressed with the same method as an encrypted one.


To perform plaintext attack you need to:


Find unencrypted file which also exists in password-protected archive.
Compress it with the same method as in encrypted archive. Note that this is strongly needed because ARCHPR checks file sizes and checksums of files.
Run ARCHPR, select encrypted archive, then select "plaintext" attack and browse for archive with unencrypted file.


After that, ARCHPR will check the files, and if there are matching ones, the password will be displayed immediately.

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