Password Hashing in Django

This technique can be useful when migrating applications from Django to ASP.NET or PHP, keeping usernames and passwords intact.

Passwords in Django are stored in two ways (in the core_vipassuser table of the database):

1. When accounts have been created without a valid password, or using Facebook or any other OAuth2 provider, the passwords are stored like this: !mafzMhEywOfhrFMvFJ16JhB1uQiAvHRaN4KuEqfg which is a random string prefixed with !. This is shown in the make_password() function in django.contrib.auth.hashers when the password parameter is None. This prevents usage of the system with an empty password.

UNUSABLE_PASSWORD_PREFIX = '!'
UNUSABLE_PASSWORD_SUFFIX_LENGTH = 40

def is_password_usable(encoded):
    """
    Return True if this password wasn't generated by
    User.set_unusable_password(), i.e. make_password(None).
    """
    return encoded is None or not encoded.startswith(UNUSABLE_PASSWORD_PREFIX)

def make_password(password, salt=None, hasher='default'):
    if password is None:
        return UNUSABLE_PASSWORD_PREFIX + get_random_string(UNUSABLE_PASSWORD_SUFFIX_LENGTH)

OAuth2 tokens are stored in the social_auth_usersocialauth table.

2. Django passwords are hashed using PBKDF2 with test vectors here. There are implementations of this algorithm in several languages. Hashed passwords in a Django DB look like this: pbkdf2_sha256$36000$5LjfzfBwQAVI$sbEcyHm7a27GFKOgOOymu+mauqVLhS2QKQE4yLk8B9Y= where the following elements are separated by $:

   1. pbkdf2_sha256 indicates the hashing algorithm
   2. 36000 is the number of iterations
   3. 5LjfzfBwQAVI is the salt
   4. sbEcyHm7a27GFKOgOOymu+mauqVLhS2 is the actual hash.

This format is referred as <algorithm>$<iterations>$<salt>$<hash> in the Django documentation:

By default, Django uses the PBKDF2 algorithm with a SHA256 hash, a password stretching mechanism recommended by NIST. This should be sufficient for most users: it’s quite secure, requiring massive amounts of computing time to break.

Validating Django Users from PHP

Using that information, the following code takes a password (given by the user) and creates the same hash as in point 2.4 above; that can be used to validate that the user is effectively whoever they claim to be.

<?php
hash_pbkdf2('sha256', 'password', 'salt', 36000, 32, true);

The hash_pbkdf2() function is available since PHP 5.5.

Validating Django Users from C#

using System;
using System.Text;
using System.Security.Cryptography;

namespace App
{
    class Program
    {
        static string PBKDF2(string password, string salt, int iterations, int size)
        {
            byte[] saltBytes = Encoding.UTF8.GetBytes(salt);
            using (Rfc2898DeriveBytes pbkdf2 = new Rfc2898DeriveBytes(password, saltBytes, iterations, HashAlgorithmName.SHA256))
            {
                byte[] resultBytes = pbkdf2.GetBytes(size);
                return Convert.ToBase64String(resultBytes);
            }
        }

        static void Main(string[] args)
        {
            Console.WriteLine(PBKDF2("password", "salt", 36000, 32));
        }
    }
}

More information

Password management in Django, and GitHub - defuse/password-hashing: Password hashing code., where the code above was adapted from.