CSCI 3500: Lab 3

crack - Brute Force Password Cracking

Modern computers overwhelmingly use multi-core architectures. Exploiting this hardware for parallel programming (creating a program that executes simultaneously on more than one processor core) requires the use of multiple threads. We will use one of the most basic threading interfaces, Pthreads, to create a multi-threaded password cracking program.

In this lab, you will:

  1. Use crypt() and crypt_r() to guess password hashes
  2. Iterate over aribtrary length strings with characters from 'a' to 'z'
  3. Create and wait for threads with pthread_create() and pthread_join()
  4. Divide work between multiple parallel threads


This is a solo assignment. Please do not collaborate with other students on this assignment.


crack <threads> <keysize> <target>


crack should attempt to find the password associated to the target DES hash. It does this by trying all possible lowercase alphabetic (a-z) passwords of length up to keysize. The program should run with threads concurrent threads for speed.

Linux/Unix user passwords are never stored on the system. Instead, a function called a hash is applied to the password. Then, the hashed password is stored, traditionally in /etc/passwd or more recently in /etc/shadow. The classic hash function on Unix systems is crypt(3). To make things harder on crackers, crypt also uses a two character string called a salt which it combines with the password to create the hash. Schematically:

password + salt => crypt() => hash

The salt is visible in the hash as the first two characters. As an example, a password 'apple' and salt 'na' become the hash 'na3C5487Wz4zw'.

The crack program should extract the salt from the first two characters of target, then repeatedly call crypt() using all possible passwords built of up to keysize lowercase alphabetic characters. For example, if the given keysize is four then your program would iterate over all strings 'aaaa', 'aaab', 'aaac', ..., 'zzzz' looking for one that hashes to the given input. If a match is not found, it would also look through all strings of length less than keysize.

When a match to target is found, the program should print the cracked password and exit immediately. If the entire space of passwords is searched with no match, the program should exit with no output.


Start by writing a short program that uses crypt() to encrypt a given password and salt. You can also encrypt using the one line perl command: perl -e 'print crypt("apple","na")'

Don't forget to compile with the -lpthread and -lcrypt options.

The maximum allowed keysize is 8 (since crypt only uses the first 8 characters of the password anyway).

You might want to write this first as a single threaded program. Just remember that crypt() won't work with multiple threads - you need to switch to crypt_r().

You need to check passwords of length keysize, but don't forget you also need to check the shorter ones. The simplest way is probably to write a function that checks all passwords which are exactly a given length, and then have main call it in a loop from 1 to keysize.

Useful man pages



  1. As the answer to the first exercise, list your name.

  2. What four-letter password corresponds to the following target: 'abccBcrPOxnLU'?

  3. What five-letter password corresponds to the following target: 'xyxMB6gxuiBZg'?

  4. The following hash does not correspond to any password, so your program will exhaust the entire search space without finding any match. Use the time command to measure how long your program takes to search the entire space of five-letter passwords. Hash 'nomatchingpass'

  5. Re-run the previous experiment but with two threads, four threads, eight threads, thirteen threads, and twenty-six threads. Record how long each run takes.

  6. Calculate the hash rate your program gets as a result of using multiple threads. The hash rate is defined to be the number of hashes computed divided by the execution time. Do this for each of the two, four, eight, thirteen, and twenty-six threads.

  7. Interpret your results. Does your program run twice as fast with two threads? Does it run eight times faster with eight threads? How does this relate to the way you have written your program?

  8. What was the maximum hash rate you recorded?

  9. Assuming each password only uses the 26 lowercase characters, compute the total number of hashes needed to exhaustively search the keyspace for keysizes of one through eight.

  10. Using the last two answers, estimate how long it should take to exhaust each keyspace at your maximum rate? Describe the benefit of increasing key length.

  11. Instead of just using lowercase characters, estimate how long it takes to exhaustively search a keyspace of size six when you are allowed to use lowercase and uppercase (52 characters), digits (62 characters), and common keyboard symbols (93 characters). Use your maximum hash rate.

  12. Describe any known bugs or ways that your submission deviates from the above specification.

  13. Indicate which, if any, extra credit exercises you have attempted.

Optional Enrichment Exercises


Upload your program to the git repository in the appropriate directory. Your submission must include a makefile that will compile your program by simply issuing the command make. You must also include a text file with your answers to the required exercises. Please include your name and the names of any partners in the body of your email.