John Tool In Kali

[Channing Arther]

Passwordcrackers can be online or offline. Online password crackers, such as Hydra, are used when brute-forcing online network protocols and HTML forms. Situations where online brute forcing might be impractical include a remote system that limits the rate of sign-in attempts or a system that locks users out indefinitely after a predefined number of invalid login attempts.

In these scenarios, an offline password cracker attempts to gain access to a password where it is stored instead of using a brute-force attack strategy. Since systems and applications rarely store passwords without cryptographic protection, passwords must be cracked to make use of them.

A popular offline password cracker is John the Ripper. This tool enables security practitioners to crack passwords, regardless of encrypted or hashed passwords, message authentication codes (MACs) and hash-based MACs (HMACs), or other artifacts of the authentication process.

Editor's note: It is possible to use John the Ripper -- and any password cracker -- lawfully and unlawfully. It is up to you to ensure your usage is lawful. Get the appropriate permission and approval before attempting to crack passwords, and handle the information obtained ethically. If you are unsure whether a given usage is lawful, do not proceed until you have confirmed that it is -- for example, by discussing and validating your planned usage with your organization's counsel.

The tool is also notable for its ubiquity and accessibility. It's included in the default repositories for many Linux distributions, including Debian and Ubuntu, and installed by default in most penetration testing distributions, including Kali and BlackArch. A snap install of it is available, along with multiple container options in Docker Hub.

Simply put, John cracks passwords. It attempts to replicate the password-handling functionality of a target storage mechanism so that numerous password values can be attempted without relying on password validation.

Before using John the Ripper, one important note: We're using VMs to illustrate usage. In the case of this tool specifically, performance is important. Consider the relative advantages and disadvantages of using virtualization and the context. For example, you might find an IaaS where paying for CPU time can be an expensive proposition. Or you might get better performance running the tool on a host OS rather than a guest.

That aside, John is simple to use. At a minimum, you need to specify the stored password values you want to crack. If you provide no information other than that, the tool tries to figure out what format the stored values are in and assumes a set of constraints about what you might want to do. That said, in most cases, you should provide more information to allow the tool to operate most effectively.

While not necessary in every case, it's helpful to know the format passwords are stored in. This is because applications might manipulate formats in ways John might not expect -- a web application, for example, might Base64- or URL-encode values before storing them. It's also helpful because the built-in mechanisms used by John to determine type and format can be wrong. Consider two similar-in-format but vastly different values: an MD5 hash and an MD5 keyed hash. The outputs are identical in format, but the processes that led to the outputs are anything but. John wouldn't know the difference, unless you provide the missing information.

Figure 2 illustrates using the unshadow command. This is distributed with John the Ripper in most packages. It combines the contents of /etc/passwd and /etc/shadow on a Linux VM, in this case, Kali. Then, we used the john command and specified the format -- in this case, the crypt mechanism. Since we haven't told it what cracking mode to use, John begins with single crack and then proceeds to wordlist -- none was specified, so it used the default. Ultimately, it will move to incremental mode.

In a more complicated example, Figure 3 shows an attempt at cracking Microsoft Windows passwords. As with the Linux example, passwords must be put into a format John the Ripper can understand. To accomplish that, we used PwDump v8.2.

These values are fed into John the Ripper to crack in Figure 4. We specified wordlist mode and instructed it to use rockyou.txt, one of the built-in wordlists that comes by default with most security-focused Linux distributions. Note that you may need to extract the text file first. The passwords were set to weak values -- dictionary words in all cases -- to enable the cracking to complete quickly. These two examples demonstrate the most universally applicable and most basic usage of John the Ripper. For more advanced usage, you may need to get a little creative. Sometimes, the format of stored password values you can obtain are not in one of the formats supported by John out of the box. With a few clever modifications, however, you can convert to one of those with a little research.

So, the first time you run the latter, it will show the password on the screen, after which point, it will save it into a file called john.pot. In my case, the folder john.pot was located at /home/kali/.john/john.pot.

John the Ripper can retrieve the password using one of two methods: the dictionary attack or the bruteforce attack. A file with a list of potential passwords is compared to the hash at hand in a dictionary attack. If a match is found, it will chuck it out. On the other hand, if a password is not found, you can use a different password file or use a different method.

In this article we are going to show how we can crack /etc/shadow file using John the Ripper. It is common in CTF like events to somehow get access to the shadow file or part of it and having to crack it so you can get the password of a user.

The process involves two basic steps, the first is called unshadowing while the second is the cracking itself. Unshadowing is a process where we combine the /etc/passwd file along with the /etc/shadow in order for John to be able to understand what we are feeding to it. Unshadow is a tool that handles this task and it is part of the John package. In order to unshadow the shadow file we need to also have the equivalent line from the passwd for the user of our interest. An example is the following:

Next and final step is to actually start the cracking with John. It is up to you which cracking method you will chose, though a bruteforcing using a wordlist is usually enough for CTFs. An example attack using a wordlist would be launched like below

Where as we see John managed to crack the password of the user root as it was included in the wordlist used.

If you would like to print all the passwords John managed to crack you may run john --show unshadowed.txt and you will get something like:

In this article we showed how John the Ripper can be used to crack the hashed password of a user that can be found in the /etc/shadow file. The process is pretty simple and straightforward yet if you find yourself stuck somewhere please feel free to reach out to me.

This page will walk through some basic password cracking with John the Ripper. We'll go from wanting to test certain passwords to being able to generate a stream of them with John the Ripper. This is important to be able to do, so that we don't need to devote gigabytes of disk space to word files. Intelligent use of patterns can save us a whole lot of headaches.

Note that if you're using Kali 2.0, you'll need to install John jumbo 1.8 from source, instead of using the Kali 2.0 repository version of John, if you want to use John the Ripper to guess passwords but not actually crack them. This is useful for sending John's password guesses to a file to see how John works, or for piping John password guesses to another cracking program like Aircrack.

This page is going to cover some basic rules and modes for guessing passwords in John. We'll figure out how to start with low-hanging fruit, in terms of password guesses, and implementing those in John the Ripper.

Now, we want to be able to use some password cracking rules, so that our wordlists don't have to be terabytes to be effective. We can do that by telling John how to create password variations from the wordlist. This is where the rules come in.

KoreLogic provided a pretty amazingly comprehensive set of John the Ripper rules form the Defcon 2010 crack me if you can contest, with the set of rules being released as part of the terms of victory: _media/john/korelogic-rules-20100801.txt

To use the rule, for exmaple the KoreLogicRulesAppendYears rule, we can call john with the --rules=KoreLogicRulesAppendYears argument, or with the -rules:KoreLogicRulesAppendYears argument (etiher works, but be consistent):

where we specify the wordlist, the encryption format of the john_password_file, the location of the john_password_file, and the rules to use. We'll cover how to get password files in a format John likes from programs like Airodump-ng and Aircrack below. But let's finish talking about the rules.

It's pretty straightforward to script with John the Ripper. I find that the easiest way, since John the Ripper jobs can get pretty enormous, is to use a modular approach: keeping track of what password wordlists and what variations have been tried for a given password file manually, rather than trying to maintain consistency by using one enormous John command.

I use Python to iterate through combinations and parse pieces and commands together into scripts. I don't have complex or fancy Bash scripts with nice menus and quick prompts. Too bad. Maybe soon. (The problem, though, is that every good library has to be installed first. And I want a tool that deploys without hassle on a bare box. No "oops, forgot to install pandas, now I can't use any fo my scripts.")

John the Ripper is a powerful password cracking tool that can be used to crack passwords in Kali Linux. To install John the Ripper, open a terminal window and type the following command: sudo apt-get install john. This will install the John the Ripper package on your system. Once the installation is complete, you can start using John the Ripper to crack passwords.

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