Known-Plaintext XOR Key Recovery (Crib Dragging)

Updated 13 Sep 2026

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What it is
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  A technique for breaking a repeating-key (or otherwise structured) XOR cipher when you know or can guess a fragment of the plaintext — a "crib." Since XOR is invertible, XORing the crib against the aligned ciphertext bytes reveals the key bytes at that position directly.
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When to apply
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  Ciphertext is XORed with an unknown key, and the plaintext format is predictable — e.g. a CTF flag guaranteed to start with crypto{ and end with }.
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Math
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  $c_i \oplus p_i = k_{i \mod \text{keylen}}$ . Knowing $p_i$ at a given offset recovers the key byte at that offset directly.
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Worked example
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  Ciphertext byte at offset 0 is 0x0e; the flag must start with c (0x63). Key byte = 0x0e ^ 0x63 = 0x6d ('m') — matching the recovered key myXORkey from the actual challenge.

Python

def crack_byte(ct_hex, expected_char):
    for k in range(256):
        if chr(int(ct_hex, 16) ^ k) == expected_char:
            return k