John the Ripper


John the Ripper is a free password cracking software tool. Originally developed for the Unix operating system, it can run on fifteen different platforms. It is among the most frequently used password testing and breaking programs as it combines a number of password crackers into one package, autodetects password hash types, and includes a customizable cracker. It can be run against various encrypted password formats including several crypt password hash types most commonly found on various Unix versions, Kerberos AFS, and Windows NT/2000/XP/2003 LM hash. Additional modules have extended its ability to include MD4-based password hashes and passwords stored in LDAP, MySQL, and others.

Sample output

Here is a sample output in a Debian environment.

  1.  cat pass.txt
user:AZl.zWwxIh15Q
  1.  john -w:password.lst pass.txt
Loaded 1 password hash 
example 
guesses: 1 time: 0:00:00:00 100% c/s: 752 trying: 12345 - pookie

The first line is a command to expand the data stored in the file "pass.txt". The next line is the contents of the file, i.e. the user and the hash associated with that user. The third line is the command for running John the Ripper utilizing the "-w" flag. "password.lst" is the name of a text file full of words the program will use against the hash, pass.txt makes another appearance as the file we want John to work on.
Then we see output from John working. Loaded 1 password hash — the one we saw with the "cat" command — and the type of hash John thinks it is. We also see that the attempt required one guess at a time of 0 with a 100% guess rate.

Attack types

One of the modes John can use is the dictionary attack. It takes text string samples, encrypting it in the same format as the password being examined, 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 and checks the variations against the 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 then 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 takes a long time to run.