Various programming stuff

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Using hashids to hide ids of objects in Django

A common pattern in Django urls is to have the following setup for CRUD operations of your objects. Let’s suppose we have a Ship object. It’s CRUD urls would be something like:

  • /ships/create/ To add a new object
  • /ships/list/ To display a list of your objects
  • /ships/detail/id/ To display the particular object with that id (primary key)
  • /ships/update/id/ To update/edit the particular object with that id (primary key)
  • /ships/delete/id/ To delete the particular object with that id (primary key)

This is very easy to implement using class based views. For example for the detail view add the following to your views.py:

class ShipDetailView(DetailView):
    model = models.Ship

and then in your urls.py add the line:

urlpatterns = [
  # ...

This path means that it expects an integer (int) which will be used as the primary key of the ship (pk).

Now, a common requirement if you are using integers as primary keys is to not display them to the public. So you shouldn’t allow the users to write something like /ships/detail/43 to see the details of ship 43. Even if you have add proper authorization (each user only sees the ids he has access to) you are opening a window for abuse. Also you don’t want the users to be able to estimate how many objects there are in your database (if a user creates a new ship he’ll get the latest id and know approximately how many ships are in your database).

One simple requirement is to use some encryption mechanism to encode the ids to some string and display that string to the public urls. When you receive the string you’ll then decode it to get the id.

Thankfully, not only there’s a particular library that makes this whole encode/decode procedure very easy but Django has functionality to make trivial to integrate this functionality to an existing project with only miniman changes!

The library I propose for this is called hashids-python. This is the python branch of the hashids library that works for many languages. If you take a look at the documentation you’ll see that it can be used like this:

from hashids import Hashids
hashids = Hashids()
hashid = hashids.encode(123) # 'Mj3'
ints = hashids.decode('xoz') # (456,)

This library offers two useful utilities: Define a random salt so that the generated hashids will be unique for your app and add a minimum hash length so that the real length of the id will be obfuscated. I’ve found out that a length of 8 characters will be more than enough to encode all possible ids up to 99 billion:

hashids = Hashids(min_length=8)
len(hashids.encode(99_999_999_999)) # 8
This is more than enough since by default django will use an integer to store the primary keys which is around 4 billion (you actually can
use 7 characters to encode up to 5 billion but I prefer even numbers.

Finally, you can use a different alphabet, for example to use all greek characters:

hashids = Hashids(alphabet='ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡΣΤΥΦΧΨΩ')
hashids.encode(123) # 'ΣΝΦ'

This isn’t recommended though for our case because not all characters are url-safe.

To integrate the hashids with Django we are going to use a custom path converter. The custom path converter is similar to the int portion of the "detail/<int:pk>/" of the url i.e it will retrieve something and convert it to a python object. To implement your custom path converter just add a file named utils.py in one of your applications with the following conents:

from hashids import Hashids
from django.conf import settings

hashids = Hashids(settings.HASHIDS_SALT, min_length=8)

def h_encode(id):
    return hashids.encode(id)

def h_decode(h):
    z = hashids.decode(h)
    if z:
        return z[0]

class HashIdConverter:
    regex = '[a-zA-Z0-9]{8,}'

    def to_python(self, value):
        return h_decode(value)

    def to_url(self, value):
        return h_encode(value)

The above will generate a hashids global object with a min length of 8 as discussed above and retrieving a custom salt from your settings (just add HASHIDS_SALT=some_random_string to your project settings). The HashIdConverter defines a regex that will match the default aplhabet that hasid uses and two methods to convert from url to python and vice versa. Notice that hashids.decode returns an array so we’ll retrieve the first number only.

To use that custom path converter you will need to add the following lines to your urls.py to register your HashIdConverter as hashid:

from core.utils import HashIdConverter

register_converter(HashIdConverter, "hashid")

and then use it in your urls.py like this:

urlpatterns = [
  # ...

That’s it! Your CBVs do not need any other changes! The hashid will match the hashid in the url and convert it to the model’s pk using the to_python method we defined above!

Of course you should also add the opposite direction (i.e convert from the primary key to the hashid). To do that we’ll add a get_absolute_url method to our Ship model, like this:

class Ship(models.Model):
  def get_hashid(self):
      return h_encode(self.id)

  def get_absolute_url(self):
      return reverse("ship_detail", args=[self.id])

Notice that you just call the reverse function passing self.id; everything else will be done automatically from the hashid custom path generator to_url method. I’ve also added a get_hashid method to my model to have quick access to the id in case I need it.

Now you don’t have any excuses to not hide your database ids from the public!