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django-scrypt 0.2.3

A Scrypt-enabled password hasher for Django 1.4/1.5

Django-Scrypt is a Scrypt-enabled password hasher for Django ver. 1.4/1.5.

Warning

The encoded hash format has changed in version 0.2.0. This change is backwards incompatible. Please read the notice in the Caveats section.

Scrypt is a sequential memory-hard key derivation function. This software allows Django to access low-level bindings of the Scrypt key derivation function via Py-Scrypt.

Installation

Warning

This is alpha software under active development and as such it is not suitable for production use. It was tested only on Python 2.6/2.7 on a 32-bit Mac and 64-bit CentOS 6. It probably will not run on Python 2.5 since Py-Scrypt doesn't run on interpreters earlier than Python 2.6.

Installing Django-Scrypt into your system-wide Python's site-packages directory is not recommended. Instead, use virtualenv and virtualenvwrapper to create isolated virtual Python environments and then install this software into the isolated virtualenv's site-packages directory.

Note

You should install Django 1.4 and py-scrypt prior to installing Django-Scrypt

Using source tarballs

  1. Download the Django-Scrypt source tarball from Pypi (Python Package Index)

    http://pypi.python.org/pypi/django-scrypt/

  2. Decompress it and make it your working directory:

    $ tar zxvf django-scrypt-0.2.3.tar.gz
    $ cd django-scrypt-0.2.3
    
  3. Install it into your site-packages directory (if you install to the system's site-packages you will probably need to be root or you will probably need to use sudo to copy into protected directories):

    $ python setup.py install
    
  4. Test your installation:

    $ python setup.py test
    

Using Pip and Pypi

If you are installing to the system-wide site-packages then you will probably need to be root or you will probably need to use sudo.

  1. Use the pip command to install from Pypi:

    $ pip install django-scrypt
    

Using a Clone from BitBucket

Since this is nascent software, you may want to get the most recent development version.

  1. Use Mercurial to clone the repository:

    $ hg clone https://bitbucket.org/kelvinwong_ca/django-scrypt
    $ cd django-scrypt
    
  2. Install it into your site-packages (if you install it in your system's site-packages you will probably need to be root or you will probably need to use sudo):

    $ python setup.py install
    
  3. Test your installation (seriously, please do this):

    $ python setup.py test
    

Keep in mind that the development tip will always be the least stable and the least tested version of the software. Please excuse the mess.

Usage

Basic Usage

Warning

This software depends on Py-Scrypt version 0.5.5 to reveal the Scrypt hashing function. Unfortunately, it contains a bug that can result in incorrect hashing when run on 64-bit Linux systems. View the Py-Scrypt issue tracker for the latest information on this issue. [1]

[1]See Py-Scrypt Issue 6

To use Scrypt as your default password storage algorithm in Django 1.4/1.5, install it and make the following changes. In your Django 1.4/1.5 application's settings.py file, modify the PASSWORD_HASHERS tuple (or add it if it is missing) to include ScryptPasswordHasher as the first hasher in the tuple. It needs to be at the very top.

For example:

PASSWORD_HASHERS = (
  'django_scrypt.hashers.ScryptPasswordHasher',
  'django.contrib.auth.hashers.PBKDF2PasswordHasher',
  'django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher',
  'django.contrib.auth.hashers.SHA1PasswordHasher',
  'django.contrib.auth.hashers.MD5PasswordHasher',
  'django.contrib.auth.hashers.CryptPasswordHasher',
)

You have now changed your app to use Scrypt as the default storage algorithm. As users login to your system they will automatically upgrade their hashed passwords to Scrypt hashes.

Note

You need to keep the other hasher entries in this list or else Django won't be able to upgrade the passwords!

Advanced Usage

If you use this software in a resource constrained environment or if you want a higher degree of protection, you can create custom ScryptPasswordHashers by subclassing the provided ScryptPasswordHasher. Subclassing will allow you to tune the Scrypt parameters to meet your needs.

The first thing to do is create a new custom hasher. Let's assume that you create a new file named my_hashers.py and you put it into your application root (my_app). In that file you can subclass the default hasher:

from django_scrypt.hashers import ScryptPasswordHasher

class BigMemNScryptHasher(ScryptPasswordHasher):
    """This hasher is tuned to use lots of memory
    (128 * 2 ** 15 * 8) == 33554432 or ~32mb
    """
    algorithm = "bigN"
    N = 15

You can change any (or all) of the class variables N, r, or p. The algorithm class variable must also be changed to a short unique string since it is used to identify and upgrade the hashing scheme in the stored password hash.

To use BigMemNScryptHasher as your default password storage algorithm make the following changes. In your application's settings.py file, modify the PASSWORD_HASHERS tuple (or add it if it is missing) to include BigMemNScryptHasher as the first hasher in the tuple. It needs to be at the very top:

PASSWORD_HASHERS = (
  'my_app.my_hashers.BigMemNScryptHasher',
  'django.contrib.auth.hashers.PBKDF2PasswordHasher',
  'django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher',
  'django.contrib.auth.hashers.SHA1PasswordHasher',
  'django.contrib.auth.hashers.MD5PasswordHasher',
  'django.contrib.auth.hashers.CryptPasswordHasher',
)

If you want to change the parameters again in the future, simply repeat the process with another subclass and another unique algorithm class variable. Add it to the top of the tuple and your users will have their password hashes migrated to the new scheme as they log in.

Scrypt Parameters

Scrypt takes three tuning parameters: N, r and p. They affect memory usage and running time. Memory usage is approximately 128 * r * N bytes. [2] These are the default values:

Nexp = lb(N) = 14, r = 8 and p = 1
where lb is logarithm base 2

Django-Scrypt stores Nexp in the encoded hash, but not N. The positive integer Nexp is the exponent used to generate N which is calculated as needed (N = 2 ** Nexp). Doing this saves space in the database row. These default values lead to Scrypt using 128 * 8 * 2 ** 14 = 16M bytes of memory.

The values of N, r and p affect running time proportionately; however, p can be used to independently tune the running time since it has a smaller influence on memory usage.

The final parameter buflen has been proposed for Py-Scrypt but is not implemented. The value will be used to change the size of the returned hash. Currently, Py-Scrypt's hash function returns a message digest of length 64-bytes or 512-bits.

[2]Adapted from Falko Peters' Crypto.Scrypt package for Haskell

Caveats

Hash Format Changes As 'N' Removed

In an attempt to shorten the length of the encoded hash, I removed the N-value and replaced it with an N-exponent value named Nexp. The reason for this is that N must be a power of two {2, 4, 8, ... 16384, ...etc...} and those digits take up room in a 128 character hash storage space. It makes more sense to me to store the exponent and just make the actual integer on the fly.

N == 16384 == 2 ** 14 therefore Nexp == 14

The old encoded hash format that got stored in Django's database was

scrypt$salt$16384$8$1$64$Base64Hash==

The new and shorter encoded hash format is

scrypt$salt$14$8$1$64$Base64Hash==

The good news is that "14" is three characters shorter than "16384". The bad news is that this introduces a backwards incompatible change as of version 0.2.0.

If you see your application generating HTTP 500 Server Errors with an Exception raised with error: 'hash parameters are wrong (r*p should be < 2**30, and N should be a power of two > 1)' then you should suspect that an old hash is telling Scrypt to use N = 2 ** 16384 which is way, way, way too large. The solution is to replace the 16384 in the old hashes with 14. You might have to alter your database manually or write some custom code to fix this change.

Django Password Field Character Length Limits

By default, Django limits password field lengths to 128 characters. Using the default settings in Django-Scrypt with Django's salting implementation should yield encoded hashes less than 128 characters (approx 119 characters); however, if you override the ScryptPasswordHasher class variables you might end up overflowing the default password field.

The solution is to increase the size of the password field using SQL. You should consult your database documentation for the correct commands necessary to alter your database.

Bugs! Help!!

If you find any bugs in this software please report them via the BitBucket issue tracker [3] or send an email to code@kelvinwong.ca. Any serious security bugs should be reported via email only.

[3]Django-Scrypt issue tracker https://bitbucket.org/kelvinwong_ca/django-scrypt/issues

Thank-you

Thank-you for taking the time to evaluate this software. I appreciate receiving feedback on your experiences using it and I welcome code contributions and development ideas.

http://www.kelvinwong.ca/coders

Thanks to Dr Colin Percival for his original Scrypt software [4], also to Magnus Hallin for the Py-Scrypt Python module [5].

[4]Visit http://www.tarsnap.com/scrypt.html
[5]Visit http://pypi.python.org/pypi/scrypt/
 
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