John the Ripper

John the Ripper is a widely used open-source password cracking tool. It's designed to detect weak passwords by trying to crack hashed passwords recovered from a system's shadow file or obtained from other sources. John the Ripper uses a variety of techniques, including brute force and dictionary attacks, to guess passwords.

It typically operates on password hashes. When you enter a password into a system, that password is usually not stored directly; instead, it's processed through a cryptographic hash function, and the resulting hash is stored. John the Ripper first tries a dictionary attack, where it runs through a list of common passwords (from a wordlist) and their precomputed hash values.

If the dictionary attack fails, it employs brute-force attacks, generating every possible password combination until it finds a match. It also applies various mangling rules to dictionary words (like adding numbers, capitalizing letters, or reversing characters) to guess more complex passwords that might be derived from simple words.

Suppose you have a file containing password hashes named hashes.txt. To attempt cracking these passwords with John the Ripper, you would use the following syntax in the command line:

john hashes.txt

This command tells John the Ripper to start cracking the hashes in hashes.txt using its default settings and wordlists. You can also specify different modes and options. For example, to use a specific wordlist, you can use the --wordlist option followed by the path to your wordlist file.

john --wordlist=/path/to/wordlist.txt hashes.txt

To enable rules (which apply various transformations to wordlist entries), you can use the --rules option.

john --wordlist=/path/to/wordlist.txt --rules hashes.txt

If you want to use brute-force mode, you can specify it with --incremental.

john --incremental hashes.txt

To display the passwords that have been successfully cracked, use the --show option.

john --show hashes.txt

If you know the type of hash algorithm used (e.g., MD5, SHA-256), you can specify it with the --format option:

john --format=md5crypt hashes.txt

Info

This command tells John the Ripper to specifically use the MD5 hash algorithm when attempting to crack the hashes in hashes.txt.