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231 lines
10 KiB
Plaintext
=============================
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Password management in Django
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=============================
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Password management is something that should generally not be reinvented
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unnecessarily, and Django endeavors to provide a secure and flexible set of
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tools for managing user passwords. This document describes how Django stores
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passwords, how the storage hashing can be configured, and some utilities to
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work with hashed passwords.
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.. _auth_password_storage:
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How Django stores passwords
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===========================
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Django provides a flexible password storage system and uses PBKDF2 by default.
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The :attr:`~django.contrib.auth.models.User.password` attribute of a
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:class:`~django.contrib.auth.models.User` object is a string in this format::
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<algorithm>$<iterations>$<salt>$<hash>
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Those are the components used for storing a User's password, separated by the
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dollar-sign character and consist of: the hashing algorithm, the number of
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algorithm iterations (work factor), the random salt, and the resulting password
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hash. The algorithm is one of a number of one-way hashing or password storage
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algorithms Django can use; see below. Iterations describe the number of times
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the algorithm is run over the hash. Salt is the random seed used and the hash
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is the result of the one-way function.
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By default, Django uses the PBKDF2_ algorithm with a SHA256 hash, a
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password stretching mechanism recommended by NIST_. This should be
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sufficient for most users: it's quite secure, requiring massive
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amounts of computing time to break.
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However, depending on your requirements, you may choose a different
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algorithm, or even use a custom algorithm to match your specific
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security situation. Again, most users shouldn't need to do this -- if
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you're not sure, you probably don't. If you do, please read on:
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Django chooses the algorithm to use by consulting the
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:setting:`PASSWORD_HASHERS` setting. This is a list of hashing algorithm
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classes that this Django installation supports. The first entry in this list
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(that is, ``settings.PASSWORD_HASHERS[0]``) will be used to store passwords,
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and all the other entries are valid hashers that can be used to check existing
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passwords. This means that if you want to use a different algorithm, you'll
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need to modify :setting:`PASSWORD_HASHERS` to list your preferred algorithm
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first in the list.
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The default for :setting:`PASSWORD_HASHERS` is::
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PASSWORD_HASHERS = (
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'django.contrib.auth.hashers.PBKDF2PasswordHasher',
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'django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher',
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'django.contrib.auth.hashers.BCryptSHA256PasswordHasher',
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'django.contrib.auth.hashers.BCryptPasswordHasher',
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'django.contrib.auth.hashers.SHA1PasswordHasher',
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'django.contrib.auth.hashers.MD5PasswordHasher',
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'django.contrib.auth.hashers.CryptPasswordHasher',
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)
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This means that Django will use PBKDF2_ to store all passwords, but will support
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checking passwords stored with PBKDF2SHA1, bcrypt_, SHA1_, etc. The next few
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sections describe a couple of common ways advanced users may want to modify this
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setting.
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.. _bcrypt_usage:
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Using bcrypt with Django
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------------------------
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Bcrypt_ is a popular password storage algorithm that's specifically designed
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for long-term password storage. It's not the default used by Django since it
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requires the use of third-party libraries, but since many people may want to
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use it Django supports bcrypt with minimal effort.
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To use Bcrypt as your default storage algorithm, do the following:
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1. Install the `bcrypt library`_. This can be done by running ``pip install
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django[bcrypt]``, or by downloading the library and installing it with
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``python setup.py install``.
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2. Modify :setting:`PASSWORD_HASHERS` to list ``BCryptSHA256PasswordHasher``
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first. That is, in your settings file, you'd put::
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PASSWORD_HASHERS = (
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'django.contrib.auth.hashers.BCryptSHA256PasswordHasher',
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'django.contrib.auth.hashers.BCryptPasswordHasher',
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'django.contrib.auth.hashers.PBKDF2PasswordHasher',
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'django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher',
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'django.contrib.auth.hashers.SHA1PasswordHasher',
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'django.contrib.auth.hashers.MD5PasswordHasher',
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'django.contrib.auth.hashers.CryptPasswordHasher',
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)
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(You need to keep the other entries in this list, or else Django won't
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be able to upgrade passwords; see below).
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That's it -- now your Django install will use Bcrypt as the default storage
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algorithm.
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.. admonition:: Password truncation with BCryptPasswordHasher
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The designers of bcrypt truncate all passwords at 72 characters which means
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that ``bcrypt(password_with_100_chars) == bcrypt(password_with_100_chars[:72])``.
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The original ``BCryptPasswordHasher`` does not have any special handling and
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thus is also subject to this hidden password length limit.
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``BCryptSHA256PasswordHasher`` fixes this by first first hashing the
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password using sha256. This prevents the password truncation and so should
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be preferred over the ``BCryptPasswordHasher``. The practical ramification
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of this truncation is pretty marginal as the average user does not have a
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password greater than 72 characters in length and even being truncated at 72
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the compute powered required to brute force bcrypt in any useful amount of
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time is still astronomical. Nonetheless, we recommend you use
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``BCryptSHA256PasswordHasher`` anyway on the principle of "better safe than
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sorry".
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.. admonition:: Other bcrypt implementations
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There are several other implementations that allow bcrypt to be
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used with Django. Django's bcrypt support is NOT directly
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compatible with these. To upgrade, you will need to modify the
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hashes in your database to be in the form ``bcrypt$(raw bcrypt
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output)``. For example:
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``bcrypt$$2a$12$NT0I31Sa7ihGEWpka9ASYrEFkhuTNeBQ2xfZskIiiJeyFXhRgS.Sy``.
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.. _increasing-password-algorithm-work-factor:
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Increasing the work factor
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--------------------------
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The PBKDF2 and bcrypt algorithms use a number of iterations or rounds of
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hashing. This deliberately slows down attackers, making attacks against hashed
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passwords harder. However, as computing power increases, the number of
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iterations needs to be increased. We've chosen a reasonable default (and will
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increase it with each release of Django), but you may wish to tune it up or
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down, depending on your security needs and available processing power. To do so,
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you'll subclass the appropriate algorithm and override the ``iterations``
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parameters. For example, to increase the number of iterations used by the
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default PBKDF2 algorithm:
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1. Create a subclass of ``django.contrib.auth.hashers.PBKDF2PasswordHasher``::
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from django.contrib.auth.hashers import PBKDF2PasswordHasher
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class MyPBKDF2PasswordHasher(PBKDF2PasswordHasher):
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"""
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A subclass of PBKDF2PasswordHasher that uses 100 times more iterations.
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"""
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iterations = PBKDF2PasswordHasher.iterations * 100
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Save this somewhere in your project. For example, you might put this in
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a file like ``myproject/hashers.py``.
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2. Add your new hasher as the first entry in :setting:`PASSWORD_HASHERS`::
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PASSWORD_HASHERS = (
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'myproject.hashers.MyPBKDF2PasswordHasher',
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'django.contrib.auth.hashers.PBKDF2PasswordHasher',
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'django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher',
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'django.contrib.auth.hashers.BCryptSHA256PasswordHasher',
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'django.contrib.auth.hashers.BCryptPasswordHasher',
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'django.contrib.auth.hashers.SHA1PasswordHasher',
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'django.contrib.auth.hashers.MD5PasswordHasher',
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'django.contrib.auth.hashers.CryptPasswordHasher',
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)
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That's it -- now your Django install will use more iterations when it
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stores passwords using PBKDF2.
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.. _password-upgrades:
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Password upgrading
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------------------
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When users log in, if their passwords are stored with anything other than
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the preferred algorithm, Django will automatically upgrade the algorithm
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to the preferred one. This means that old installs of Django will get
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automatically more secure as users log in, and it also means that you
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can switch to new (and better) storage algorithms as they get invented.
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However, Django can only upgrade passwords that use algorithms mentioned in
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:setting:`PASSWORD_HASHERS`, so as you upgrade to new systems you should make
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sure never to *remove* entries from this list. If you do, users using
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unmentioned algorithms won't be able to upgrade. Passwords will be upgraded
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when changing the PBKDF2 iteration count.
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.. _sha1: http://en.wikipedia.org/wiki/SHA1
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.. _pbkdf2: http://en.wikipedia.org/wiki/PBKDF2
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.. _nist: http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf
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.. _bcrypt: http://en.wikipedia.org/wiki/Bcrypt
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.. _`bcrypt library`: https://pypi.python.org/pypi/bcrypt/
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Manually managing a user's password
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===================================
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.. module:: django.contrib.auth.hashers
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The :mod:`django.contrib.auth.hashers` module provides a set of functions
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to create and validate hashed password. You can use them independently
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from the ``User`` model.
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.. function:: check_password(password, encoded)
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If you'd like to manually authenticate a user by comparing a plain-text
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password to the hashed password in the database, use the convenience
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function :func:`check_password`. It takes two arguments: the plain-text
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password to check, and the full value of a user's ``password`` field in the
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database to check against, and returns ``True`` if they match, ``False``
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otherwise.
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.. function:: make_password(password, salt=None, hasher='default')
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Creates a hashed password in the format used by this application. It takes
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one mandatory argument: the password in plain-text. Optionally, you can
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provide a salt and a hashing algorithm to use, if you don't want to use the
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defaults (first entry of ``PASSWORD_HASHERS`` setting).
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Currently supported algorithms are: ``'pbkdf2_sha256'``, ``'pbkdf2_sha1'``,
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``'bcrypt_sha256'`` (see :ref:`bcrypt_usage`), ``'bcrypt'``, ``'sha1'``,
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``'md5'``, ``'unsalted_md5'`` (only for backward compatibility) and ``'crypt'``
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if you have the ``crypt`` library installed. If the password argument is
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``None``, an unusable password is returned (a one that will be never
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accepted by :func:`check_password`).
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.. function:: is_password_usable(encoded_password)
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Checks if the given string is a hashed password that has a chance
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of being verified against :func:`check_password`.
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