THE GARDEN

CBC Padding Oracle Attack (Vaudenay)

Updated 19 Sep 2026

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  What it is

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    Vaudenay's classic attack: if a service decrypts attacker-supplied CBC ciphertext and reveals — even just through a Boolean "valid/invalid" signal — whether the resulting plaintext has correct PKCS#7 padding, an attacker can recover the entire plaintext without ever knowing the key. Working one byte at a time from the last byte of a block backward, the attacker tweaks a single byte of the previous ciphertext block (or the IV, for the first block) and brute-forces all 256 values until the oracle reports valid padding — from which the true plaintext byte at that position follows directly.

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  When to apply

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    A CBC decryption endpoint reveals (directly, via an error message, or via any behavioral difference) whether decrypted padding was valid, and lets you submit arbitrary ciphertext/IV pairs.

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  Math

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    For the last byte of a block, a forged previous-block byte $c'$ makes padding valid when $D_K(C_i)[15] \oplus c' = 0x01$. Since $P_i[15] = D_K(C_i)[15] \oplus C_{i-1}[15]$, once $c'$ is found, $P_i[15] = c' \oplus C_{i-1}[15] \oplus 0x01$. Each earlier byte repeats this against a higher target padding value (0x02, 0x03, ...), holding the already-recovered suffix fixed to that new padding value.

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  Worked example

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    For the final byte, a valid response suggests $P[15]=\text{guess}$. To distinguish genuine 0x01 padding from an unchanged longer valid padding, perturb byte 14 and query again. Continue only if validity survives, then force the recovered suffix to 0x02, 0x03, and so on.

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  Python

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    def recover_block(oracle, prev_block, target_block):
        recovered = bytearray(16)
        crafted = bytearray(prev_block)
        for idx in range(15, -1, -1):
            pad = 16 - idx
            for j in range(15, idx, -1):
                crafted[j] = prev_block[j] ^ recovered[j] ^ pad
            for guess in range(256):
                crafted[idx] = prev_block[idx] ^ guess ^ pad
                if oracle(bytes(crafted) + target_block):
                    recovered[idx] = guess
                    break
        return bytes(recovered)
    

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  Related

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    AES CBC Bit-Flipping

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    Noisy Padding Oracle Attack (Statistical Amplification)

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    AES-IGE Padding Oracle Attack (CBC-Equivalence)

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  Cards

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    What single piece of information does the classic padding oracle attack need from the server?

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      A Boolean signal (however indirect) telling you whether the decrypted plaintext had valid PKCS#7 padding.

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    How do you recover a plaintext byte once you've found a "crafted" previous-block byte that makes padding valid?

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      XOR that crafted byte with the corresponding real previous-ciphertext byte and the target padding value.

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