intperm 1.1.1

Usage

Create a new Permutation instance by passing in an optional seed.

>>> fromo intperm import Permutation
>>> perm = Permutation(42)
>>> perm.map_to(37)
13750393542137160527L
>>> perm.map_from(13750393542137160527)
37

Not providing a seed will create a random permutation:

>>> perm = Permutation()
>>> perm.map_from(perm.map_to(37)) == 37
True

Use cases

Use cases may vary, but an example that I find useful is generating hard-to-guess, random-looking tokens based on IDs stored in a database. The IDs can be used together with the seed to decode the original ID, but their cardinality is the same as that of the IDs themselves. When used smartly, this can save you from having to index those tokens in the database.

Another good example is randomising IDs of private objects that are available via some sort of an API. Let’s say the user accounts on your website are accessible via the path /user/:id, where :id is the user’s ID. Someone could track the growth of your user base just by enumerating the URLs and keeping track of the status codes (e.g. 403 vs. 404).

Using this simple permutation, user IDs can be kept unpredictable, rendering these kinds of attacks practically useless.

See also

This library is also implemented in Ruby and Go.