Didier Stevens

I’ve been asked many times to support 32-bit keys with my XORSearch tool. But the problem is that a 32-bit bruteforce attack would take too much time.

Now I found a solution that doesn’t take months or years: a 32-bit dictionary attack.

I assume that the 32-bit XOR key is inside the file as a sequence of 4 consecutive bytes (MSB or LSB).

If you use the new option -k, XORSearch will perform a 32-bit dictionary attack to find the XOR key. The standard bruteforce attacks are disabled when you choose option -k.

XORSearch will extract a list of keys from the file: all unique sequences of 4 consecutive bytes (MSB and LSB order). Key 0x00000000 is excluded. Then it will use this list of keys to perform an XOR dictionary attack on the file, searching for the string you provided. Each key will be tested with an offset of 0, 1, 2 and 3.

It is not unusual to find the 32-bit XOR key inside the file itself. If it is a self-decoding executable, it can contain an XOR x86 instruction with the 32-bit key as operand. Or if the original file contains a sequence of 0x00 bytes (4 consecutive 0x00 bytes at least), then the encoded file will also contain the 32-bit XOR key.

Here is a test where XORSearch.exe searches a 0xDEADBEEF XOR encoded copy of itself. With only 74KB, there are still 100000+ keys to test, taking almost 10 minutes on my machine:

XORSearch_V1_9_2.zip (https)
MD5: BF1AC6CAA325B6D1AF339B45782B8623
SHA256: 90793BEB9D429EF40458AE224117A90E6C4282DD1C9B0456E7E7148165B8EF32