Encrypt and decrypt Django model primary key values (useful for publicly viewable unique identifiers) [python3 version]

models.py

# This code is under the MIT license.
# Inspired by this StackOverflow question:
# http://stackoverflow.com/questions/3295405/creating-django-objects-with-a-random-primary-key
# Python 3.x version
import struct
from Crypto.Cipher import DES
from django.db import models


def base36encode(number):
    # encode number to string of alphanumeric characters (0 to z)
    if not isinstance(number, int):
        raise TypeError('number must be an integer')
    if number < 0:
        raise ValueError('number must be positive')

    alphabet = '0123456789abcdefghijklmnopqrstuvwxyz'
    base36 = ''
    while number:
        number, i = divmod(number, 36)
        base36 = alphabet[i] + base36

    return base36 or alphabet[0]


def base36decode(value):
    # convert a base-36 alphanumeric string to its numeric value
    return int(value, 36)


class EncryptedPKModelManager(models.Manager):
    """
    This manager allows models to be identified based on their encrypted_pk value.
    """
    def get(self, *args, **kwargs):
        encrypted_pk = kwargs.pop('encrypted_pk', None)
        if encrypted_pk:
            # decrypt encrypted_pk argument and set pk argument to the appropriate value
            key = bytes(getattr(self.model, 'PK_SECRET_KEY'), 'utf8')
            encryption_obj = DES.new(key, DES.MODE_ECB)
            kwargs['pk'] = struct.unpack('<Q', encryption_obj.decrypt(
                struct.pack('<Q', base36decode(encrypted_pk))
            ))[0]

        return super().get(*args, **kwargs)


class EncryptedPKModel(models.Model):
    """
    Adds encrypted_pk property to children which returns the encrypted value of the primary key.
    """
    PK_SECRET_KEY = 'default1'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        setattr(
            self.__class__,
            "encrypted_%s" % (self._meta.pk.name,),
            property(self.__class__._encrypted_pk)
        )

    def _encrypted_pk(self):
        key = bytes(getattr(self, 'PK_SECRET_KEY'), 'utf8')
        encryption_obj = DES.new(key, DES.MODE_ECB)
        return base36encode(struct.unpack('<Q', encryption_obj.encrypt(
            struct.pack('<Q', self.pk)
        ))[0])

    encrypted_pk = property(_encrypted_pk)

    class Meta:
        abstract = True


# class ExampleModelManager(EncryptedPKModelManager):
#     pass


# class ExampleModel(EncryptedPKModel):
#     PK_SECRET_KEY = '8charkey'
#     objects = ExampleModelManager()
#     example_field = models.CharField(max_length=32)


# Example usage:
# example_instance = ExampleModel.objects.get(pk=1)
# url_pk = example_instance.encrypted_pk
# ExampleModel.objects.get(encrypted_pk=url_pk)