How It Works

John works great with other programs, since it does many things well, and each of those capabilities are valuable on their own and can be combined with other programs. For an example, see Aircrack and John the Ripper

Password Generation

First, John has a powerful password generation mechanism. It does not simply take wordlists. It can generate many, many more passwords from those wordlists. For example, starting with a list of the 10,000 most common passwords, John can create additional lists with 400,000 or 8 million additional password variations to try. You can carefully control how those variations are generated to target your password search as you go.

Password Testing and Encryption

Second, John has support for many encryption mechanisms built-in, so it can handle a wide variety of password files. If you have a Unix password file with a list of users and encrypted passwords, John can brute force it.

Using John on Shadow Files

Unix stores password hashes in the /etc/shadow file, and user information in the /etc/passwd file. John the Ripper can use this information to crack the passwords of Unix users.

John the Ripper/Shadow File - a guide to using John to crack passwords from an /etc/shadow file.

Using John as a Password Generator

You can feed John the Ripper a wordlist, and use it to generate a slew of variations on that wordlist, using rules. (For example, from a wordlist containing "password" you can generate the variants "p4ssw0rd", "Password", "password0000", etc.)

See John the Ripper/Password Generation

This can even be used in conjunction with the pw-inspector tool, which will take a list of passwords as inputs and return only those passwords that meet certain criteria. More info: http://tools.kali.org/password-attacks/hydra