simple django 16 byte aes field

aesfield.py

from django.db import models
from django.utils.encoding import smart_str
from Crypto.Cipher import AES
import base64


class AESField(models.TextField):
    __metaclass__ = models.SubfieldBase


    def __init__(self, key="", iv="", aes_prefix="aes:", *args, **kwargs):
        self.key = smart_str(key).rjust(16)
        self.iv = smart_str(iv).rjust(16)
        self.aes_prefix = aes_prefix
        assert len(self.key) == 16, 'key must less then 16 byte string'
        assert len(self.iv) == 16, 'iv must less then 16 byte string'
        return super(AESField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        try:
            assert value.startswith(self.aes_prefix), 'source'
            value = base64.b64decode(value[len(self.aes_prefix):])
            value = self.aes.decrypt(value).strip()
            value = base64.b64decode(value)
        except:
            pass
        finally:
            return value

    def get_prep_value(self, value):
        value = base64.b64encode(value)
        value = value + " " * (16 - len(value) % 16)
        value = self.aes.encrypt(value)
        value = "{}{}".format(self.aes_prefix, base64.b64encode(value))
        return value

    @property
    def aes(self):
        return AES.new(self.key, AES.MODE_CBC, self.iv)


try:
    from south.modelsinspector import add_introspection_rules
except ImportError:
    pass
else:
    add_introspection_rules([], ["^fields\.AESField"])