Gods of XOR

Platform: spbctf | Category: Crypto | Type: Challenge | Difficulty: Easy | OS: NA | Author: XESXOR | Date: 2026-03-09 | Status: Solved Techniques: known_plaintext_attack, repeating_key_xor, xor_key_recovery

Summary

Task: XOR-encrypted files with known plaintext. Solution: recover repeating key via XOR of plaintext and ciphertext, then decrypt the second file.

Gods of XOR — CryptoKids 2.0

Description

Given three files:

• task1.jpg — known plaintext (image)
• enc1 — encrypted file of the same size as task1.jpg
• enc2 — encrypted file of larger size

Analysis

Classic XOR challenge with known plaintext. If enc1 = task1.jpg XOR key, then we can recover the key:

key = enc1 XOR task1.jpg

Since enc1 and task1.jpg are the same size, the key is either equal to the file length or repeats. After XOR we get a repeating pattern — this is the key. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Solution

Step 1: Key Recovery

 python read-only • copy allowed Copy
#!/usr/bin/env python3

# Read known plaintext and encrypted file
with open('task1.jpg', 'rb') as f:
    plaintext = f.read()

with open('enc1', 'rb') as f:
    ciphertext1 = f.read()

# Recover key via XOR
key_stream = bytes([p ^ c for p, c in zip(plaintext, ciphertext1)])

# Find repeating pattern in the key
def find_key_length(data, max_len=256):
    for key_len in range(1, max_len):
        is_repeating = True
        for i in range(len(data)):
            if data[i] != data[i % key_len]:
                is_repeating = False
                break
        if is_repeating:
            return key_len
    return len(data)
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

key_len = find_key_length(key_stream)
key = key_stream[:key_len]
print(f"Key length: {key_len}")
print(f"Key: {key}")

Step 2: Decrypting enc2

 python read-only • copy allowed Copy
with open('enc2', 'rb') as f:
    ciphertext2 = f.read()

# Decrypt with repeating key
decrypted = bytes([c ^ key[i % len(key)] for i, c in enumerate(ciphertext2)])

with open('decrypted.jpg', 'wb') as f:
    f.write(decrypted)

print("Decrypted file saved as decrypted.jpg")

Step 3: Result

The decrypted file is an image with a scene from Norse mythology (Fenrir, northern chaos gods). The flag is visible in the image. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Notes

• "Northern chaos gods" — reference to Norse mythology (Fenrir, Loki)
• The attack works because XOR is reversible: A XOR B XOR B = A
• With known plaintext, any XOR cipher is trivially broken ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍