xorxorxor — HackTheBox

Description

"Who needs AES when you have XOR?"

Given an encryption program in Python and an encrypted flag.

Analysis

Encryption Source Code

#!/usr/bin/python3
import os
flag = open('flag.txt', 'r').read().strip().encode()

class XOR:
    def __init__(self):
        self.key = os.urandom(4)
    def encrypt(self, data: bytes) -> bytes:
        xored = b''
        for i in range(len(data)):
            xored += bytes([data[i] ^ self.key[i % len(self.key)]])
        return xored
    def decrypt(self, data: bytes) -> bytes:
        return self.encrypt(data)

def main():
    global flag
    crypto = XOR()
    print ('Flag:', crypto.encrypt(flag).hex())

if __name__ == '__main__':
    main()

Encrypted Flag

Flag: 134af6e1297bc4a96f6a87fe046684e8047084ee046d84c5282dd7ef292dc9

Vulnerabilities

1. Short key (4 bytes) — the key repeats cyclically
2. Known flag format — the flag starts with HTB{ (exactly 4 bytes)
3. XOR property — A ⊕ B ⊕ B = A (self-inverse)

Since the length of known plaintext (4 bytes) equals the key length, we can fully recover the key!

Solution

Attack Mathematics

ciphertext = plaintext ⊕ key
key = ciphertext ⊕ plaintext

Knowing the first 4 bytes of plaintext (HTB{) and the first 4 bytes of ciphertext, we compute the key:

Ciphertext (hex): 13 4a f6 e1
Plaintext (hex):  48 54 42 7b  (HTB{)
Key (hex):        5b 1e b4 9a

Solution Script

#!/usr/bin/env python3
"""
HackTheBox: xorxorxor
Known Plaintext Attack on repeating-key XOR
"""

encrypted_hex = "134af6e1297bc4a96f6a87fe046684e8047084ee046d84c5282dd7ef292dc9"
encrypted = bytes.fromhex(encrypted_hex)

# Known plaintext: HTB{ (first 4 bytes = key length)
known_plaintext = b"HTB{"

# Recover key: key = encrypted XOR plaintext
key = bytes([encrypted[i] ^ known_plaintext[i] for i in range(4)])
print(f"Recovered key: {key.hex()}")

# Decrypt the entire flag
decrypted = b''
for i in range(len(encrypted)):
    decrypted += bytes([encrypted[i] ^ key[i % 4]])

print(f"Flag: {decrypted.decode()}")

Output

Recovered key: 5b1eb49a
Flag: HTB{REDACTED}

Theory

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