XOR Keystream Cracking: Known-Plaintext Attack

Repeating-Key XOR — The Classic Mistake

XOR encryption is seductively simple: each byte of plaintext is XORed with a byte from a cycling key. When the key is short and reused, patterns emerge that make the cipher trivially breakable. Many real-world schemes (early game DRM, firmware blobs, malware payloads) have fallen to exactly this attack.

How the Cipher Works

Given a key like "wolf" (4 bytes) and a plaintext, byte i of ciphertext = plaintext[i] XOR key[i % 4]. The key wraps around indefinitely. If you know even a few bytes of plaintext at a known position, you can XOR them against the ciphertext bytes to recover the corresponding key bytes directly.

Your Attack Strategy

All flags in this lab start with the standard prefix FLAG{. That five-character known plaintext is your skeleton key. XOR those five ASCII bytes against the first five bytes of the hex-encoded ciphertext blob to immediately recover the first (up to) five bytes of the repeating key. Once you have the full key, decrypt the entire blob.

In Python: key_byte = ciphertext_byte ^ ord(known_char). In CyberChef, use From Hex → XOR (set to Key mode with your recovered key in hex). Alternatively, openssl or a simple one-liner: python3 -c "import sys; ct=bytes.fromhex('BLOB'); key=b'wolf'; print(bytes(c^key[i%len(key)] for i,c in enumerate(ct)).decode())".

Step-by-Step

1. Grab your personal ciphertext hex blob from the lab console.
2. XOR the first 4 bytes of the blob against F, L, A, G to extract the 4-byte key.
3. Apply the full repeating XOR with that key to the entire blob.
4. Read the plaintext — it is your flag. Submit it.