ARJ files feature relatively strong encryption. However, one of the ways of breaking ARJ protection is using the known-plaintext attack. This attack is instant.

If you have access to the encrypted file created by ARJ archiver, and the same file in unencrypted form, you may be able to retrieve the original password. To perform the plaintext attack, you need at least one file from the encrypted archive, compressed but unencrypted.

To perform the plaintext attack:

•Find unencrypted file which also exists in the password-protected archive.

•Compress it with the same method as in the encrypted archive.

•Run ARCHPR, select encrypted the archive, then select "plaintext" attack and browse for the archive with the unencrypted file.

After that, ARCHPR will check the files, and if a match is found, the password will be displayed instantly.