OverTheWire Maze Level 4 → 5 tutorial!!

int main(void) {
  char user[9];
  char pass[9];

  printf(" Username: ");
  scanf("%8s", user);

  printf("      Key: ");
  scanf("%8s", pass);

  if (strlen(user)==8 && strlen(pass)==8) {
    if (ptrace(PTRACE_TRACEME,0,0,0) == 0) {
      if (foo(user, pass)) {
        puts("Yeh, here's your shell");
        system("/bin/sh");
      } else {
        puts("Nah, wrong.");
      }
    } else {
      puts("nahnah..."); // anti-debug
    }
  } else {
    puts("Wrong length you!");
  }
}

int foo(char* user, char* pass){
  char p[9] = {0x70, 0x72, 0x69, 0x6e, 0x74, 0x6c, 0x6f, 0x6c}; 
  // ASCII: "printlol"

  for(int i = 0; i < 8; i++){
    p[i] -= user[i] + 2*i - 0x41;
  }

  do {
    i--;
    if(i == 0) return 1; // success if all matched
  } while(pass[i] == p[i]);

  return 0; // fail
}