crackme

crackme

This challenge is a simple crackme that asks for a flag and checks if it is correct.

Disassembling and reversing with Ghidra

int main(int argc,char **argv) {
  code *pcVar1;
  int iVar2;

  signal(5,catch_function);
  if (argc < 2) {
    puts("USAGE: ./crackme FLAG!");
    return 1;
  }
  input = argv[1];
  pcVar1 = (code *)swi(3);
  iVar2 = (*pcVar1)();
  return iVar2;
}

The swi(3) is doing an int3 to trigger a SIGTRAP signal. This is used to prevent the program from being debugged. We can patch this out with a hex editor or gdb. Or we can just reverse engineer the catch_function and see what it does. This is what the flag_cracker.c program does. In fact, it is a very simple function: the input is xored with a key and the result is compared with another array of bytes. So we need just to xor the key array with the result bytes array to get the flag.

The complete cracking program is in flag_cracker.c.