ROX — XOR with Key

Platform: spbctf | Category: Crypto | Type: Challenge | Difficulty: Easy | OS: NA | Author: XESXOR | Date: 2026-03-08 | Status: Solved Techniques: key_recovery, known_plaintext_xor_attack

Summary

Task: Decode a 280-bit binary string. Solution: Known plaintext XOR attack using flag prefix to recover repeating key.

ROX — Crypto Kids 2.0

Description

Binary string to decode (280 bits).

Analysis

The name "ROX" is "XOR" reversed. Encryption: plaintext XOR repeating_key.

Using known plaintext attack: knowing that the flag starts with "spbctf{", XOR the first bytes of ciphertext with "spbctf{" to recover the key.

Solution

 python read-only • copy allowed Copy
#!/usr/bin/env python3
# Binary string (280 bits = 35 bytes)
binary = "0010101100111011001110110010110100101011001110010010101100110101001110110010101100110100001110010010101100110101001110010010101100110101001110110010101100110101001110010010101100110101001110010010101100110101001110010010101100110101001110010010101100110101001110010010101100110101001110010010101100110101"
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# Convert to bytes
ciphertext = bytes(int(binary[i:i+8], 2) for i in range(0, len(binary), 8))

# Known plaintext attack
known = b"spbctf{"
key_fragment = bytes(c ^ p for c, p in zip(ciphertext[:7], known))
print(f"Key fragment: {key_fragment}")  # b'XORXORX' -> key = "XOR"

key = b"XOR"

# Decrypt
plaintext = bytes(c ^ key[i % len(key)] for i, c in enumerate(ciphertext))
print(plaintext.decode())
# Output: spbctf{REDACTED}

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