Problem: Crack

Questions? Feel free to head to CS50 on Reddit, CS50 on StackExchange, or the CS50 Facebook group.


Implement a program that cracks passwords, per the below.

$ ./crack 50fkUxYHbnXGw

Academic Honesty

This course’s philosophy on academic honesty is best stated as "be reasonable." The course recognizes that interactions with classmates and others can facilitate mastery of the course’s material. However, there remains a line between enlisting the help of another and submitting the work of another. This policy characterizes both sides of that line.

The essence of all work that you submit to this course must be your own. Collaboration on problems is not permitted (unless explicitly stated otherwise) except to the extent that you may ask classmates and others for help so long as that help does not reduce to another doing your work for you. Generally speaking, when asking for help, you may show your code or writing to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on quizzes and tests is not permitted at all. Collaboration on the final project is permitted to the extent prescribed by its specification.

Below are rules of thumb that (inexhaustively) characterize acts that the course considers reasonable and not reasonable. If in doubt as to whether some act is reasonable, do not commit it until you solicit and receive approval in writing from your instructor. If a violation of this policy is suspected and confirmed, your instructor reserves the right to impose local sanctions on top of any disciplinary outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.


  • Communicating with classmates about problems in English (or some other spoken language).

  • Discussing the course’s material with others in order to understand it better.

  • Helping a classmate identify a bug in his or her code, such as by viewing, compiling, or running his or her code, even on your own computer.

  • Incorporating snippets of code that you find online or elsewhere into your own code, provided that those snippets are not themselves solutions to assigned problems and that you cite the snippets' origins.

  • Reviewing past years' quizzes, tests, and solutions thereto.

  • Sending or showing code that you’ve written to someone, possibly a classmate, so that he or she might help you identify and fix a bug.

  • Sharing snippets of your own solutions to problems online so that others might help you identify and fix a bug or other issue.

  • Turning to the web or elsewhere for instruction beyond the course’s own, for references, and for solutions to technical difficulties, but not for outright solutions to problems or your own final project.

  • Whiteboarding solutions to problems with others using diagrams or pseudocode but not actual code.

  • Working with (and even paying) a tutor to help you with the course, provided the tutor does not do your work for you.

Not Reasonable

  • Accessing a solution to some problem prior to (re-)submitting your own.

  • Asking a classmate to see his or her solution to a problem before (re-)submitting your own.

  • Decompiling, deobfuscating, or disassembling the staff’s solutions to problems.

  • Failing to cite (as with comments) the origins of code, writing, or techniques that you discover outside of the course’s own lessons and integrate into your own work, even while respecting this policy’s other constraints.

  • Giving or showing to a classmate a solution to a problem when it is he or she, and not you, who is struggling to solve it.

  • Looking at another individual’s work during a quiz or test.

  • Paying or offering to pay an individual for work that you may submit as (part of) your own.

  • Providing or making available solutions to problems to individuals who might take this course in the future.

  • Searching for, soliciting, or viewing a quiz’s questions or answers prior to taking the quiz.

  • Searching for or soliciting outright solutions to problems online or elsewhere.

  • Splitting a problem’s workload with another individual and combining your work (unless explicitly authorized by the problem itself).

  • Submitting (after possibly modifying) the work of another individual beyond allowed snippets.

  • Submitting the same or similar work to this course that you have submitted or will submit to another.

  • Using resources during a quiz beyond those explicitly allowed in the quiz’s instructions.

  • Viewing another’s solution to a problem and basing your own solution on it.


Your work on this problem set will be evaluated along four axes primarily.


To what extent does your code implement the features required by our specification?


To what extent is your code consistent with our specifications and free of bugs?


To what extent is your code written well (i.e., clearly, efficiently, elegantly, and/or logically)?


To what extent is your code readable (i.e., commented and indented with variables aptly named)?

To obtain a passing grade in this course, all students must ordinarily submit all assigned problems unless granted an exception in writing by the instructor.


On most systems running Linux these days is a file called /etc/shadow, which contains usernames and passwords. Fortunately, the passwords therein aren’t stored "in the clear" but are instead encrypted using a "one-way hash function." When a user logs into these systems by typing a username and password, the latter is encrypted with the very same hash function, and the result is compared against the username’s entry in /etc/shadow. If the two hashes match, the user is allowed in. If you’ve ever forgotten some password, you might have been told that tech support can’t look up your password but can change it for you. Odds are that’s because tech support can only see, if anything at all, a hash of your password, not your password itself. But they can create a new hash for you.

Even though passwords in /etc/shadow are hashed, the hash function is not always that strong. Quite often are adversaries, upon obtaining that file somehow, able to guess (and check) users' passwords or crack them using brute force (i.e., trying all possible passwords). Below is what /etc/shadow might look like, albeit simplified, wherein each line is formatted as username:hash.



Design and implement a program, crack, that cracks passwords.

  • Implement your program in a file called crack.c in a directory called crack (inside chapter2).

  • Your program should accept a single command-line argument: a hashed password.

  • If your program is executed without any command-line arguments or with more than one command-line argument, your program should print an error (of your choice) and exit immediately, with main returning 1 (thereby signifying an error).

  • Otherwise, your program must proceed to crack the given password, ideally as quickly as possible, ultimately printing the password in the clear followed by \n, nothing more, nothing less, with main returning 0.

  • Assume that each password has been hashed with C’s DES-based (not MD5-based) crypt function.

  • Assume that each password is no longer than (gasp) four characters

  • Assume that each password is composed entirely of alphabetical characters (uppercase and/or lowercase).



Your program should behave per the examples below. Assumed that the underlined text is what some user has typed.

$ ./crack
Usage: ./crack hash
$ ./crack 50fkUxYHbnXGw


No check50 for this one! But odds are, if you can crack all ten passwords above, you’re in good shape!


Be sure to read up on crypt, taking particular note of its mention of "salt":

man crypt

Per that man page, you’ll likely want to put

#include <unistd.h>

at the top of your file in order to use crypt. Moreover, you’ll want to link with -lcrypt, as by compiling not with make but with:

clang -ggdb3 -O0 -std=c11 -Wall -Werror -Wshadow crack.c -lcrypt -lcs50 -lm -o crack

How to Submit

Step 1 of 3

Execute update50 again to ensure that your IDE is up-to-date.

Step 2 of 3

  • Recall that you were asked to implement crack.

    • Be sure that all relevant files for this problem are in ~/workspace/pset2/crack/, as with:

      cd ~/workspace/chapter2/crack/

Step 3 of 3

  • To submit crack, execute

    cd ~/workspace/chapter2/crack/
    submit50 cs50/2017/ap/crack

    inputting your GitHub username and GitHub password as prompted.

If you run into any trouble, email!

You may resubmit any problem as many times as you’d like.

Your submission should be graded for correctness within 2 minutes, at which point your score will appear at!

This was Crack.