def fixed(b1, b2):
    """
    XORs two given sets of bytes, of equal length, together.
    """
    # Ensure that byte arrays are of equal length.
    if len(b1) != len(b2):
        raise Exception('Byte arrays are not of equal length.')

    # XOR byte arrays together and return.
    return bytearray([x ^ y for x, y in zip(b1, b2)])