[diminish]

Can anyone do a rough cryptoanalysis of the code? It uses AES block cipher in CBC mode with a random iv. Which attacks is this open to?

First, I suspect it's lacking a secure integrity check (MAC), so is weak against chosen ciphertext attacks.

    def encrypt(self, plaintext):
        plaintext = self.pad(plaintext)
        iv = Random.new().read(AES.block_size)
        cipher = AES.new(self.key, AES.MODE_CBC, iv)
        return iv + cipher.encrypt(plaintext)

I'm also not sure about his padding of zeros to attain the AES block size - was there a more secure padding?

    def pad(self, s):
        return s + b"\0" * (AES.block_size - len(s) % AES.block_size)