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Cracking Unix and Linux password files for beginners

Guess you're a newbie in pass-cracking like I was and you've probably started John

the Ripper full of enthusiasm, and got.... nothing. So the first thought you have is 'my god this must be hard, and I'm a newbie'. Forget it!!! You're always a newbie, and we all are... in pass cracking world, pardon, pass recovering world (or any world else) you always have something to learn. Sometimes, even if you are experienced in password cracking, you won't be able to crack the password or even get your own password. This is a pure technical manual and will give you only the recipe for cracking, but every password needs different approach...

 

OK, so a good way to get somewhere is to start getting somewhere... 
What you're about to learn is to crack *nix(Unix/Linux/etc.) password files. It does not mean that you need to have some Unix distribution on your box, but it means you'll have to stop clicking your ass off all around the screen... 'What this fool is trying to say', you'll probably ask... This fool is trying to say that john is a DOS program (there is also Linux/Unix version, but I guess that most of the people that read this tutorial have win boxes). I will try to put this tutorial through the examples so it wouldn't look like a boring script with incredible amount of switches. After reading this text it wouldn't be a bad idea to look at the texts you get with John. I learnt it all from there, but that, of course, was the hard way, and you want the easy way, right? Right.

First, it wouldn't be a bad idea to get yourself John the Ripper, I guess... if you don't have it you can find it at:

1) packetstorm.securify.com (look at archives, password cracking)
2) neworder.box.sk (do some searching by yourself)

John can be found practically anywhere. For example: try going to altavista.com and running a search for 'john the ripper'.

Second thing you'll need is.... a HUUUUGE amount of password dictionaries (I'll explain what these are in a minute). The best dictionary around is at www.theargon.com and packetstorm (look at the archives) and is called theargonlistserver1 and is about 20Mb packed, and over 200Mb 
unpacked... get it!!!! The people at theargon did a terrific job.

You should also get some smaller dictionary files (I'll explain why later).

2) Do we look like *nix?


 

So now you have john, loaded with that huuuuge pass dictionary, and you think that you can crack anything... If you plan to live for 100000 years, that wouldn't be a problem, but you only have some 80 years left in the best case scenario (unless, of course, scientists find a way to... oh, nevermind).

Now, the first thing is that you have to make sure your password file really looks like a Unix password file (were talking about the /etc/passwd file).

Let's see how Unix pass files look like

owner:Ejrt3EJUnh5Ms:510:102:Some free text:/home/subdir/owner:/bin/bash

The important part is the username and the encrypted password, which are the first and the second parts (each line is divided into seven parts by : symbols)

owner:Ejrt3EJUnh5Ms

Owner is the username and 'that other thing' is the crypted password (encrypted in altered DES (Data Encryption Standard) encryption). For the other part you can put anything that looks like that but the structure must be same so the john could recognize it as unix pass. In fact the other part

:510:102:Some free text:/home/subdir/owner:/bin/bash

Is just some information about the user, his home directory, etc...

Sometimes you'll have passes that have only the first and second part, such as password files that you got from a webboard running matt's web board script.

owner:Ejrt3EJUnh5Ms

You'll have to put the other part so that password would look like unix pass, and you can do a copy-paste from another pass, you can even use

:510:102:His name:/home/subdir/owner:/bin/bash

What you have now should look like:

owner:Ejrt3EJUnh5Ms:510:102:His name:/home/subdir/owner:/bin/bash

Hell, you can even put

owner:Ejrt3EJUnh5Ms:a:a:a:a:a

It won't matter to john at all.

3) We're getting somewhere... nowhere


 

Now you're ready to crack. Type in

john -w:words.lst password.file

Where words.lst is password dictionary and password file where you have your password or passwords. If you use it on example i gave to you you'll probably get password because it's really weak pass. You'd be surprised to see that people usually use really weak passes like their names, pet names, or even their username (for example: username=zalabuk, password=zalabuk).

Hint: Don't be stupid! Use strong passes like 
p4sswr!@ 
p@s$w11s 
with as many characters you can remember. Hint is to use special characters and numbers those passes are much harder to crack (I'll explain why in a minute).
The other hint is to use passes as long as you can remember, 8 characters are sometimes not enough... it depends what box that someone who cracks has... on dual alpha is certainly not enough... in other words... more than 10 characters will do fine, even more wouldn't hurt (like 16...). By the way, older *nix have fixed pass length of 8 chars... that is old DES crypted pass that uses a 64-bit key... now there are 128-bit keys, and some perverts use even more, so there is more fun now :) 
john -w:words.lst password.file

Wait wait wait! What am I doing here? 
Alright, listen up carefully. The DES encryption that Unix uses CANNOT be reversed. Some encryptions can be reversed using a sometimes simple or sometimes incredibly complicated algorithm (in the 3rd century AD, Ceasar used to send encrypted letters which used a formula of "shift by three", which means that d stands for a, e stands for b etc'. At that time, such an algorithm was just fine. Today, it isn't). 
So anyway, the altered DES encryption that Unix uses for it's password files cannot be reversed. Why? Because it's a key-based encryption. The encryption algorithm uses a bunch of letters (lowercase and uppercase), numbers and symbols within the algorithm. So, in other words, to run the decryption algorithm you will need this key, which you simply cannot just have, because the key is the password! You see, when a user picks a password, the system generates an encrypted password for him, called a hash (which is what you get when you somehow acquire a password file), which is created by running this altered DES algorithm using the user's password as a key. If you try to decrypt the password using standard reversable DES encryption, you get a null string. 
So how do John and other password crackers do it? Easy. They try to recreate this process by taking passwords out of these dictionary files (or wordlists) and using them as keys for this altered DES algorithm process. Then, they compare the result to all the encrypted passwords within the password file you've given them. If the two strings match - there you have it! The password is yours!

If the first step doesn't work, the next step would be to do this:

john -w:words.lst -rules password.file

This switch turn on not only browsing through the dictionary, but it uses some modifications of the words that are word dictionary (like adding a number at the end of pass - fool -> fool1, etc' etc'). This one will take long with huge pass dictionary, but it may give better results... For a start you could do a try with a small pass dictionary, and if it doesn't works you can try it with a huge pass dictionary.

Sometimes people are not stupid when they choose passwords and basic rules won't do a job... aaargh. As you've seen it takes more and more time for your CPU to crack this thing out as we go further. Now you can leave your computer on and go to sleep....

If you want to get even more possible passwords out of your password file, try typing

john -i password.file

This -i stands for incremental cracking, not a really good word for it, but... 
Okay, what the hell does it do? It uses the default incremental mode parameters, which are defined in john.ini.
What does this mean? Do you remember -rules? Yes, well, of course you do, unless you're either incredibly senile or you've stopped reading after this part and only came back, like... a couple of years later. That is very much like rules, but much much more powerful than -rules, and it takes much, much more time.

4) So where are we now (dictionary vs. brute-force)?


 

You can see that in all cases you use so-called dictionary cracking... but hell, why not just run John on a mode where it tried all possible combinations of lowercase and uppercase letters, numbers and symbols? I mean, this would be much more efficient, right? ... WROOOOOOONGG!!! 
This method is called 'brute-force' attack (basically, dictionary attack is also sort of brute-force attack, but most people use the word brute-force for this specific attack). 
What are the differences? First and most important, with dictionary you go through the selected words that could be passwords and their modifications, and with brute force cracking you use ALL possible combinations. That means you have 
comb=nrch^let

where:

comb - number of possible combinations 
nrch - number of chars 
let - number of letters used

In case you're dealing with john's default -i 95 character set and, presume, a 6 letter password you have possible 735091890625 combinations! OUCH!! 
Sure, this is useful for passwords like 2405v7, but still... with the computational powers of today's modern PC, I'd just give up, unless I had access to some University's supercomputer, which I'd bet noone would ever give me (well, at least not for free, and certainly not to run a password cracker on it). 
As you can see it can take a looooong time until you crack a single one pass, do a little math and try to calculate how many possible combinations there are for 10, 12 and 16 chars. 
I don't think you'll like the answer :) 
Of course, sometimes dictionary attacks are not enough, but john has very powerful 'thinking'. In 'incremental' mode john will do all possible combinations from 0 to 8 characters (by zero password length is considered a hashed empty string, this sometimes happens). So incremental mode is one sort of brute-force attack in some way...

If you want to fire all weapons at one then you use

john password.file

this will do first basic dictionary attack, then -rules, then -i

5) What if...


 

Ok, you have to turn off your box from time to time, don't you? If you're doing that haaard password that will take more than 20 hours of cracking you can set john with ctrl+c and then resume with

john -restore

If your box crashes or if there's a power failure, you won't be able to restore your cracking sessions (sometimes)... well that's just too bad. Hell, it happened 
to me once :-(

John is modular, and that is the most powerful thing about john the ripper, and that is what makes john the most advanced password cracker. John is very, very modular. John uses modes that are described in john.ini (do you still remember that incremental cracking i was talking about? Modes for rules and incremental are described in john.ini). 
If you're some inventive guy then you may change the parameters in john.ini.

Here is example how some default parameters for -i look like:

# Incremental modes 
[Incremental:All] 
File = ~/all.chr 
MinLen = 0 
MaxLen = 8 
CharCount = 95

Ok... what do we have here?

[Incremental:All] - this stands for the beginning of the definition for the -i:all switch 
File - filename of file that has characters used in mode -i:all (whole character 
set) 
MinLen - logically, minimum length of password that john -i:all would try 
MaxLen - even more logical, maximum length of password that will john -i:all try 
CharCount - number of chars used by john when you 'turn on' this switch

So, there are some more switches... heh 
Yes there are and down there are all default modes pasted from john the ripper's documents:

John the Ripper's Command Line Options


 

You can list any number of password files on John's command line, and also 
specify some of the following options (all of them are case sensitive, but 
can be abbreviated; you can also use the GNU-style long options syntax):

  • single "single crack" mode Enables the "single crack" mode, using rules from [List.Rules:Single].
  • wordfile:FILE wordlist mode, read words from FILE,
  • stdin or from stdin These are used to enable the wordlist mode.
  • rules enable rules for wordlist mode Enables wordlist rules, that are read from [List.Rules:Wordlist].
  • incremental[:MODE] incremental mode [using section MODE] Enables the incremental mode, using the specified ~/john.ini definition (section [Incremental:MODE], or [Incremental:All] by default).
  • external:MODE external mode or word filter Enables an external mode, using external functions defined in ~/john.ini's [List.External:MODE] section.
  • stdout[:LENGTH] no cracking, write words to stdout When used with a cracking mode, except for "single crack", makes John print the words it generates to stdout instead of cracking. While applying 
    wordlist rules, the significant password length is assumed to be LENGTH, or unlimited by default.
  • restore[:FILE] restore an interrupted session Continues an interrupted cracking session, reading point information from the specified file (~/restore by default).
  • session:FILE set session file name to FILE Allows you to specify another point information file's name to use for this cracking session. This is useful for running multiple instances of John in parallel, or just to be able to recover an older session later, not always continue the latest one.
  • status[:FILE] print status of a session [from FILE] Prints status of an interrupted or running session. To get an up to date status information of a detached running session, send that copy of John a SIGHUP before using this option.
  • makechars:FILE make a charset, overwriting FILE Generates a charset file, based on character frequencies from ~/john.pot, for use with the incremental mode. The entire ~/john.pot will be used for
    the charset file unless you specify some password files. You can also use an external filter() routine with this option.
  • show show cracked passwords Shows the cracked passwords in a convenient form. You should also specify the password files. You can use this option while another John is cracking, to see what it did so far.
  • test perform a benchmark Benchmarks all the enabled ciphertext format crackers, and tests them for 
    correct operation at the same time.
  • users:[-]LOGIN|UID[,..] load this (these) user(s) only Allows you to filter a few accounts for cracking, etc. A dash before the list can be used to invert the check (that is, load all the users that aren't listed).
  • groups:[-]GID[,..] load this (these) group(s) only Tells John to load users of the specified group(s) only.
  • shells:[-]SHELL[,..] load this (these) shell(s) only This option is useful to load accounts with a valid shell only, or not to load accounts with a bad shell. You can omit the path before a shell name, so '-shells:csh' will match both '/bin/csh' and '/usr/bin/csh', while '-shells:/bin/csh' will only match '/bin/csh'.
  • salts:[-]COUNT set a passwords per salt limit This feature sometimes allows to achieve better performance. For example you can crack only some salts using '-salts:2' faster, and then crack the 
    rest using '-salts:-2'. Total cracking time will be about the same, but you will get some passwords cracked earlier.
  • format:NAME force ciphertext format NAME 
    Allows you to override the ciphertext format detection. Currently, valid 
    format names are DES, BSDI, MD5, BF, AFS, LM. You can use this option when 
    cracking or with '-test'. Note that John can't crack password files with 
    different ciphertext formats at the same time.
  • savemem:LEVEL enable memory saving, at LEVEL 1..3 
    You might need this option if you don't have enough memory, or don't want 
    John to affect other processes too much. Level 1 tells John not to waste 
    memory on login names, so you won't see them while cracking. Higher levels 
    have a performance impact: you should probably avoid using them unless John 
    doesn't work or gets into swap otherwise.

 

6) Tips


 

I) A good schedule to do your cracking job is

john -w:words.lst password.file 
john -w:words.lst -rules password.file 
john -w:words.lst password.file 
john -i:digits password.file 
john -i:all password.file

II) If you have a file that has only passes that look like

owner:*:510:102:His name:/home/subdir/owner:/bin/bash 
you have a shadowed passwords file. 
Go to the Byte-Me page at blacksun.box.sk and try to find out more about 
password files (I'll leave it up to you to do this. It's important that you'll 
learn how to find things by yourself).

III) You have some little tools that you get with john, they are all 
listed below (from john's docs)

unshadow PASSWORD-FILE SHADOW-FILE 
Combines the passwd and shadow files (when you already have access to 
both) for use with John. You might need this since if you only used your 
shadow file, the GECOS information wouldn't be used by the "single crack" 
mode, and also you wouldn't be able to use the '-shells' option. You'll 
usually want to redirect the output of 'unshadow' to a file.

unafs DATABASE-FILE CELL-NAME 
Gets password hashes out of the binary AFS database, and produces a file 
usable by John (again, you should redirect the output yourself).

unique OUTPUT-FILE 
Removes duplicates from a wordlist (read from stdin), without changing 
the order. You might want to use this with John's '-stdout' option, if 
you got a lot of disk space to trade for the reduced cracking time.

mailer PASSWORD-FILE 
A shell script to send mail to all the users who got weak passwords. You 
should edit the message inside before using.


 

So, that was about it... hope you've got something from this text. 
Further readings: try reading ALL the documentation you get with john in the docs 
directory. Maybe it's a little bit chaotic, but.... man those are the docs :)

Ohh, wait, wait!! 
Remember, not all password files can be cracked! Smart admins alter the 
encryption that they are using, especially when it comes to root passwords. 
But there are always other ways to get passwords. These are covered in other 
BSRF tutorials. Collect them all (lol) at http://blacksun.box.sk.

 

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