A.P. Lawrence

2005/03/26 John the Ripper

Password security. John the Ripper attempts to crack passwords by comparing the hashed version with hashes computed from common words, permutations of common words, pointless additions ("alex123", "betty222" and the like).

On any modern system, no non-administrative user should be able to read the hashed passwords at all, so you should understand that this is not a black-hat tool used to crack your passwords (though similar tools can be used to generate brute force password guessing). If somebody can read your shadow password file, you are already in trouble.

No, this is a tool for administrators to test for weak passwords. For Linux shadow passwords, you use the included "unshadow" to create a file to run against:

./unshadow /etc/passwd /etc/shadow > pw1
./john pw1

and sit back to wait.. of course this is one of the few times where you (the administrator) are actually hoping that John takes a long time and finds nothing. Even if the passwords aren't all that good, John is probably going to run a goodly period of time. If you use it in incremental mode ("john -i"), it might run forever - seriously. As the docs explain:

This will use the default incremental mode parameters, which are defined
in ~/john.ini's [Incremental:All] section. In the configuration file
supplied with John these parameters are to use the full 95 character set,
and to try all possible password lengths, from 0 to 8. (By zero password
length I mean a hashed empty string, this sometimes happens.) Don't expect
this to terminate in a reasonable time (unless all the passwords were weak
and got cracked), read doc/MODES for an explanation.

You really need to read the documentation here. It isn't difficult to understand, but you need to read it.

What you DON'T want to see is something like this:

# ./john pw1
Loaded 1 password (FreeBSD MD5 [32/32])
fred123          (fred)
guesses: 1  time: 0:00:00:00 100% (1)  c/s: 625  trying: fred123
#

That popped back almost before my finger bounced off the ENTER key. When I ran it with -i, it took a bit longer (actually about two hours with MaxLen set to 12), but when you consider how many combinations it had to try before getting to that, it's gets there pretty darn fast. John never did crack my real passwords, though of course it would if had I left it running long enough. Don't forget to set MaxLen in john.ini if your passwords exceed its default of 8 (and they should!).

If you are curious about what John is up to, press ENTER while it is running:

guesses: 1  time: 0:00:00:31 8% (2)  c/s: 1624  trying: dexter1
guesses: 1  time: 0:00:00:32 8% (2)  c/s: 1624  trying: Mustang1
guesses: 1  time: 0:00:00:34 9% (2)  c/s: 1624  trying: Stacey1
guesses: 1  time: 0:00:00:35 9% (2)  c/s: 1625  trying: Gunner1
guesses: 1  time: 0:00:01:13 20% (2)  c/s: 1625  trying: ariel!

When John cracks a password, it gets stored in "john.pot". For example, after cracking Fred's password, john.pot contained:

$1$2wf30E0F$zBGoSoo0aS9C0eEi/HqoH1:fred123

The "fred123" was, in fact, the actual password used, but you have no absolute knowledge that is the case. It's pretty likely, but it is at least in the realm of possibility that two different words can hash to the same string. You are quite safe in assuming that did not happen, and would never go broke betting against it. If you want to read more about that sort of thing, see this FAQ on hash collisions.

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