This program takes a password hash from command line, then generates every possible 5 letter password combination using upper and lowercase letters, then uses crypt() to hash the generated word, and finally compares the hash to the user hash input from command line. If they match, password has been cracked.

crack.c

/*-- This program takes a password hash from command line, then generates

every possible 5 letter password combination using upper and lowercase

letters, then uses crypt() to hash the generated word, and finally compares the hash

to the user hash input from command line.  If they match, password has been cracked --*/

#define _XOPEN_SOURCE

#include <unistd.h>

#include <stdio.h>

#include <cs50.h>

#include <string.h>

#include <ctype.h>

#include <stdlib.h>

// first, get hashed password from command line argument

int main(int argc, string argv[])

{

    // only accept a single command-line argument

    if (argc == 2)

    {

        // create an array (alpha[]) of all upper and lowercase letters

        char alpha[53];

        char c;

        // add uppercase alphabet to array

        int upperChar = 65;

        int arrayIndex = 0;

        for (int i = 0; i < 26; i++)

        {

            c = upperChar;

            alpha[i] = c;

            upperChar++;

            arrayIndex++;

        }

        // add lowercase alphabet to array

        int lowerChar = 97;

        for (int j = 0; j < 26; j++)

        {

            c = lowerChar;

            alpha[arrayIndex + j] = c;

            lowerChar++;

        }

        // assign hash variable from hash received from user at command line

        string hash = argv[1];

        char a = hash[0];

        char b = hash[1];

        // assign salt variable to pass to crypt() function

        char salt[3];

        salt[0] = a;

        salt[1] = b;

        // assign variable to hold hash from generated passwords

        string cryptHash;

        // assign password variables

        char password[6] = {0};

        char d;

        char e;

        char f;

        char g;

        // assign varialbe to hold return value of compared strings

        int ret;

        // generate all one letter passwords

        for (int n = 0; n < strlen(alpha); n++)

        {

            g = alpha[n];

            password[0] = g;

            //validate

            //if validates, print password and return 0

            cryptHash = crypt(password, salt);

            ret = strncmp(cryptHash, hash, 13);

            if (ret == 0)

            {

                printf("%s\n", password);

                return 0;

            }

        }

        // if hash didn't validate yet, try two letter combinations

        for (int m = 0; m < strlen(alpha); m++)

        {

            f = alpha[m];

            password[0] = f;

            for (int n = 0; n < strlen(alpha); n++)

            {

                g = alpha[n];

                password[1] = g;

                //validate

                // if validates, print password and return 0

                cryptHash = crypt(password, salt);

                ret = strncmp(cryptHash, hash, 13);

                if (ret == 0)

                {

                    printf("%s\n", password);

                    return 0;

                }

            }

        }

        // if hash didn't validate yet, try three letter combinations

        for (int l = 0; l < strlen(alpha); l++)

        {

            e = alpha[l];

            password[0] = e;

            for (int m = 0; m < strlen(alpha); m++)

            {

                f = alpha[m];

                password[1] = f;

                for (int n = 0; n < strlen(alpha); n++)

                {

                    g = alpha[n];

                    password[2] = g;

                    //validate

                    //if validates, print password and return 0

                    cryptHash = crypt(password, salt);

                    ret = strncmp(cryptHash, hash, 13);

                    if (ret == 0)

                    {

                        printf("%s\n", password);

                        return 0;

                    }

                }

            }

        }

        // if hash didn't validate yet, try 4 letter combinations

        for (int k = 0; k < strlen(alpha); k++)

        {

            d = alpha[k];

            password[0] = d;

            for (int l = 0; l < strlen(alpha); l++)

            {

                e = alpha[l];

                password[1] = e;

                for (int m = 0; m < strlen(alpha); m++)

                {

                    f = alpha[m];

                    password[2] = f;

                    for (int n = 0; n < strlen(alpha); n++)

                    {

                        g = alpha[n];

                        password[3] = g;

                        //validate

                        //if validates, print password and return 0

                        cryptHash = crypt(password, salt);

                        ret = strncmp(cryptHash, hash, 13);

                        if (ret == 0)

                        {

                            printf("%s\n", password);

                            return 0;

                        }

                    }

                }

            }

        }

        // if hash didn't validate yet, try 5 letter combinations

        for (int j = 0; j < strlen(alpha); j++)

        {

            c = alpha[j];

            password[0] = c;

            for (int k = 0; k < strlen(alpha); k++)

            {

                d = alpha[k];

                password[1] = d;

                for (int l = 0; l < strlen(alpha); l++)

                {

                    e = alpha[l];

                    password[2] = e;

                    for (int m = 0; m < strlen(alpha); m++)

                    {

                        f = alpha[m];

                        password[3] = f;

                        for (int n = 0; n < strlen(alpha); n++)

                        {

                            g = alpha[n];

                            password[4] = g;

                            //validate

                            //if validates, print password and return 0

                            cryptHash = crypt(password, salt);

                            ret = strncmp(cryptHash, hash, 13);

                            if (ret == 0)

                            {

                                printf("%s\n", password);

                                return 0;

                            }

                        }

                    }

                }

            }

        }

        // if none of the password hashes validaded, we did not generate the right password

        printf("All passwords were tried and hashed. No hashes matched hash input by user. Sorry!");

        return 0;



    }

    /*--- if program is executed without any comand-line argument or more than one argument

    is entered, print error message and return from main value of 1 ---*/

    else

    {

        printf("ERROR!! You must enter a hashed password. Only one argument is accepted.\n");

        return 1;

    }

}