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jvdsn
GitHub Repository: jvdsn/crypto-attacks
 Path: blob/master/attacks/ctr/separator_oracle.py
2589 views
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def _find_separator_positions(separator_oracle, c):

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    separator_positions = []

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    c = bytearray(c)

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    for i in range(len(c)):

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        c[i] ^= 1

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        valid = separator_oracle(c)

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        c[i] ^= 1

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        if not valid:

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            c[i] ^= 2

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            valid = separator_oracle(c)

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            c[i] ^= 2

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            if not valid:

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                separator_positions.append(i)

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    return separator_positions

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def attack(separator_oracle, separator_byte, c):

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    """

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    Recovers the plaintext using the separator oracle attack.

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    :param separator_oracle: the separator oracle, returns True if the separators are correct, False otherwise

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    :param separator_byte: the separator which is used in the separator oracle

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    :param c: the ciphertext

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    :return: the plaintext

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    """

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    separator_positions = _find_separator_positions(separator_oracle, c)

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    c = bytearray(c)

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    # Ensure that at least 1 separator is missing.

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    c[separator_positions[0]] ^= 1

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    p = bytearray(len(c))

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    for i in range(len(c)):

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        if i in separator_positions:

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            p[i] = separator_byte

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        else:

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            c_i = c[i]

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            # Try every byte until an additional separator is created.

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            for b in range(256):

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                c[i] = b

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                if separator_oracle(c):

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                    p[i] = c_i ^ c[i] ^ separator_byte

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                    break

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            c[i] = c_i

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    return p

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