ctf-writeups

Badmode

For our software license protection, we've implemented a secret string that complements the license file for authentication. We consider this system to be extremely secure and invincible, and we're seeking your validation. Are we in badmode?

Decompile in Ghidra:

iVar3 = strcmp(&stack0xffffffffffffffa0 + lVar2,&secret);
if (iVar3 == 0) {
  sVar4 = strlen(&secret);
  printf("Congratulations! Here is your flag: ASIS{");
  for (local_10 = 0; (local_10 < sVar4 && (local_10 < 0x1c)); local_10 = local_10 + 1) {
    putchar((uint)(byte)((&secret)[local_10] ^ (&DAT_00102a20)[local_10]));
  }
  puts("}");
  DAT_00120190 = 1;
  DAT_0012018c = 1;
}

It calls strcmp to compare secret with some data. Use ltrace to find the arguments:

$ ltrace -o ltrace.log ./badmode license
Please enter the SECRET string to validate license: secret
Your license is NOT valid!
$ cat ltrace.log | grep strcmp
strcmp("8d45ac2d9904d910613f94ba81b5", "secret")                        = -118
$ ./badmode license
Please enter the SECRET string to validate license: 8d45ac2d9904d910613f94ba81b5
Congratulations! Here is your flag: ASIS{ADSB_3nc0din9S_iN__R3vEr5Es!}

Flag: ASIS{ADSB_3nc0din9S_iN__R3vEr5Es!}.