fake-cbc.py

def xor(x, y):
    ''' XOR two lists of bytes.
    '''
    return map(lambda (xx, yy) : xx ^ yy, zip(x, y))

def random_vector(n):
    ''' Generate a random byte list with n elements.
    '''
    return [random.randint(0, 255) for i in range(n)]

def encrypt_cbc(key, iv, vectors):
    ''' "Encrypt" a list of plaintext blocks using the given key and IV in CBC mode.
    We don't really encrypt here... we just XOR the plaintext with the key. This has
    no impact on the PO attack.
    '''

    # Pad, if necessary
    if len(vectors[-1]) < 16:
        vectors[-1] = pad_pkcs7(vectors[-1])
    else:
        vectors.append(pad_pkcs7([]))

    result = []
    state = iv
    for i, v in enumerate(vectors):
        input_block = xor(state, v)
        state = xor(key, input_block) # replacement for AES
        result.append(state)
    return result

def decrypt_cbc(key, iv, vectors):
    ''' Invert our "encryption" in CBC mode.
    '''
    pt = []
    state = iv
    for i, v in enumerate(vectors):
        next_state = v
        output_block = xor(key, v) # replacement for AESi
        xor_result = xor(state, output_block)
        pt.append(xor_result)
        state = next_state

    if not is_valid_pad_pkcs7(pt[-1]):
        raise Exception("Invalid padding")
    return pt