Attack Types
One of the modes John can use is the dictionary attack. It takes text string samples (usually from a file, called a wordlist, containing words found in a dictionary), encrypting it in the same format as the password being examined (including both the encryption algorithm and key), and comparing the output to the encrypted string. It can also perform a variety of alterations to the dictionary words and try these. Many of these alterations are also used in John's single attack mode, which modifies an associated plaintext (such as a username with an encrypted password) and checks the variations against the encrypted hashes.
John also offers a brute force mode. In this type of attack, the program goes through all the possible plaintexts, hashing each one and comparing it to the input hash. John uses character frequency tables to try plaintexts containing more frequently used characters first. This method is useful for cracking passwords which do not appear in dictionary wordlists, but it does take a long time to run.
Read more about this topic: John The Ripper
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