Symmetrically encrypt the data using the optional iv.

def Encrypt(self, data, iv=None):
    """Symmetrically encrypt the data using the optional iv."""
    if iv is None:
      iv = rdf_crypto.EncryptionKey.GenerateKey(length=128)
    cipher = rdf_crypto.AES128CBCCipher(self.cipher.key, iv)
    return iv, cipher.Encrypt(data)