Checking System Account Passwords on GNU/Linux

check-password.c

#include <stdlib.h>
#include <string.h>

#include <crypt.h>

#include <shadow.h>

int check_password(const char *plain_password, const char *crypt_password)
{
	return strcmp(crypt(plain_password, crypt_password), crypt_password) == 0;
}

int main(int argc, char* argv[])
{
	struct spwd* spwd;

	if (argc != 3) {
  		// invalid number of arguments
		return EXIT_FAILURE;
	}

	spwd = getspnam(argv[1]);

	if (spwd == NULL) {
		// we're either not running as root
		// or the user doesn't exist
		return EXIT_FAILURE;
	}

	if (!check_password(argv[2], spwd->sp_pwdp)) {
		// the provided password is incorrect
		return EXIT_FAILURE;
	}

	return EXIT_SUCCESS;
}