Linux Security Cookbook

9.2. Testing Login Passwords (CrackLib)

Problem

You want assurance that your login passwords are secure.

Solution

Write a little program that calls the FascistCheck function from CrackLib:

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <crack.h>
#define DICTIONARY "/usr/lib/cracklib_dict"
int main(int argc, char *argv[]) {
        char *password;
        char *problem;
        int status = 0;
        printf("\nEnter an empty password or Ctrl-D to quit.\n");
        while ((password = getpass("\nPassword: ")) != NULL && *password ) {
                if ((problem = FascistCheck(password, DICTIONARY)) != NULL) {
                        printf("Bad password: %s.\n", problem);
                        status = 1;
                } else {
                        printf("Good password!\n");
                }
        }
        exit(status);
}

Compile and link it thusly:

$ gcc cracktest.c -lcrack -o cracktest

Run it (the passwords you type will not appear on the screen):

$ ./cracktest
Enter an empty password or Ctrl-D to quit.
Password: xyz
Bad password: it's WAY too short.
Password: elephant
Bad password: it is based on a dictionary word.
Password: kLu%ziF7
Good password!

Discussion

CrackLib is an offshoot of Alec Muffet’s password cracker, Crack. It is designed to be embedded in other programs, and hence is provided only as a library (and dictionary). The FascistCheck function subjects a password to a variety of tests, to ensure that it is not vulnerable to guessing.

See Also

Learn more about CrackLib at http://www.crypticide.org/users/alecm.

Perl for System Administration (O’Reilly), section 10.5, shows how to make a Perl module to use CrackLib. ...