django/docs/topics/auth/passwords.txt

=============================
Password management in Django
=============================

Password management is something that should generally not be reinvented
unnecessarily, and Django endeavors to provide a secure and flexible set of
tools for managing user passwords. This document describes how Django stores
passwords, how the storage hashing can be configured, and some utilities to
work with hashed passwords.

.. seealso::

    Even though users may use strong passwords, attackers might be able to
    eavesdrop on their connections. Use :ref:`HTTPS
    <security-recommendation-ssl>` to avoid sending passwords (or any other
    sensitive data) over plain HTTP connections because they will be vulnerable
    to password sniffing.

.. _auth_password_storage:

How Django stores passwords
===========================

Django provides a flexible password storage system and uses PBKDF2 by default.

The :attr:`~django.contrib.auth.models.User.password` attribute of a
:class:`~django.contrib.auth.models.User` object is a string in this format:

.. code-block:: text

    <algorithm>$<iterations>$<salt>$<hash>

Those are the components used for storing a User's password, separated by the
dollar-sign character and consist of: the hashing algorithm, the number of
algorithm iterations (work factor), the random salt, and the resulting password
hash.  The algorithm is one of a number of one-way hashing or password storage
algorithms Django can use; see below. Iterations describe the number of times
the algorithm is run over the hash. Salt is the random seed used and the hash
is the result of the one-way function.

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.

However, depending on your requirements, you may choose a different
algorithm, or even use a custom algorithm to match your specific
security situation. Again, most users shouldn't need to do this -- if
you're not sure, you probably don't.  If you do, please read on:

Django chooses the algorithm to use by consulting the
:setting:`PASSWORD_HASHERS` setting. This is a list of hashing algorithm
classes that this Django installation supports.

For storing passwords, Django will use the first hasher in
:setting:`PASSWORD_HASHERS`. To store new passwords with a different algorithm,
put your preferred algorithm first in :setting:`PASSWORD_HASHERS`.

For verifying passwords, Django will find the hasher in the list that matches
the algorithm name in the stored password. If a stored password names an
algorithm not found in :setting:`PASSWORD_HASHERS`, trying to verify it will
raise ``ValueError``.

The default for :setting:`PASSWORD_HASHERS` is::

    PASSWORD_HASHERS = [
        "django.contrib.auth.hashers.PBKDF2PasswordHasher",
        "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
        "django.contrib.auth.hashers.Argon2PasswordHasher",
        "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
        "django.contrib.auth.hashers.ScryptPasswordHasher",
    ]

This means that Django will use PBKDF2_ to store all passwords but will support
checking passwords stored with PBKDF2SHA1, argon2_, and bcrypt_.

The next few sections describe a couple of common ways advanced users may want
to modify this setting.

.. _argon2_usage:

Using Argon2 with Django
------------------------

Argon2_ is the winner of the 2015 `Password Hashing Competition`_, a community
organized open competition to select a next generation hashing algorithm. It's
designed not to be easier to compute on custom hardware than it is to compute
on an ordinary CPU. The default variant for the Argon2 password hasher is
Argon2id.

Argon2_ is not the default for Django because it requires a third-party
library. The Password Hashing Competition panel, however, recommends immediate
use of Argon2 rather than the other algorithms supported by Django.

To use Argon2id as your default storage algorithm, do the following:

#. Install the :pypi:`argon2-cffi` package. This can be done by running
   ``python -m pip install django[argon2]``, which is equivalent to
   ``python -m pip install argon2-cffi`` (along with any version requirement
   from Django's ``pyproject.toml``).

#. Modify :setting:`PASSWORD_HASHERS` to list ``Argon2PasswordHasher`` first.
   That is, in your settings file, you'd put::

        PASSWORD_HASHERS = [
            "django.contrib.auth.hashers.Argon2PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
            "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
            "django.contrib.auth.hashers.ScryptPasswordHasher",
        ]

   Keep and/or add any entries in this list if you need Django to :ref:`upgrade
   passwords <password-upgrades>`.

.. _bcrypt_usage:

Using ``bcrypt`` with Django
----------------------------

Bcrypt_ is a popular password storage algorithm that's specifically designed
for long-term password storage. It's not the default used by Django since it
requires the use of third-party libraries, but since many people may want to
use it Django supports bcrypt with minimal effort.

To use Bcrypt as your default storage algorithm, do the following:

#. Install the :pypi:`bcrypt` package. This can be done by running
   ``python -m pip install django[bcrypt]``, which is equivalent to
   ``python -m pip install bcrypt`` (along with any version requirement from
   Django's ``pyproject.toml``).

#. Modify :setting:`PASSWORD_HASHERS` to list ``BCryptSHA256PasswordHasher``
   first. That is, in your settings file, you'd put::

        PASSWORD_HASHERS = [
            "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
            "django.contrib.auth.hashers.Argon2PasswordHasher",
            "django.contrib.auth.hashers.ScryptPasswordHasher",
        ]

   Keep and/or add any entries in this list if you need Django to :ref:`upgrade
   passwords <password-upgrades>`.

That's it -- now your Django install will use Bcrypt as the default storage
algorithm.

.. _scrypt-usage:

Using ``scrypt`` with Django
----------------------------

scrypt_ is similar to PBKDF2 and bcrypt in utilizing a set number of iterations
to slow down brute-force attacks. However, because PBKDF2 and bcrypt do not
require a lot of memory, attackers with sufficient resources can launch
large-scale parallel attacks in order to speed up the attacking process.
scrypt_ is specifically designed to use more memory compared to other
password-based key derivation functions in order to limit the amount of
parallelism an attacker can use, see :rfc:`7914` for more details.

To use scrypt_ as your default storage algorithm, do the following:

#. Modify :setting:`PASSWORD_HASHERS` to list ``ScryptPasswordHasher`` first.
   That is, in your settings file::

        PASSWORD_HASHERS = [
            "django.contrib.auth.hashers.ScryptPasswordHasher",
            "django.contrib.auth.hashers.PBKDF2PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
            "django.contrib.auth.hashers.Argon2PasswordHasher",
            "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
        ]

   Keep and/or add any entries in this list if you need Django to :ref:`upgrade
   passwords <password-upgrades>`.

.. note::

    ``scrypt`` requires OpenSSL 1.1+.

Increasing the salt entropy
---------------------------

Most password hashes include a salt along with their password hash in order to
protect against rainbow table attacks. The salt itself is a random value which
increases the size and thus the cost of the rainbow table and is currently set
at 128 bits with the ``salt_entropy`` value in the ``BasePasswordHasher``. As
computing and storage costs decrease this value should be raised. When
implementing your own password hasher you are free to override this value in
order to use a desired entropy level for your password hashes. ``salt_entropy``
is measured in bits.

.. admonition:: Implementation detail

    Due to the method in which salt values are stored the ``salt_entropy``
    value is effectively a minimum value. For instance a value of 128 would
    provide a salt which would actually contain 131 bits of entropy.

.. _increasing-password-algorithm-work-factor:

Increasing the work factor
--------------------------

PBKDF2 and bcrypt
~~~~~~~~~~~~~~~~~

The PBKDF2 and bcrypt algorithms use a number of iterations or rounds of
hashing. This deliberately slows down attackers, making attacks against hashed
passwords harder. However, as computing power increases, the number of
iterations needs to be increased. We've chosen a reasonable default (and will
increase it with each release of Django), but you may wish to tune it up or
down, depending on your security needs and available processing power. To do so,
you'll subclass the appropriate algorithm and override the ``iterations``
parameter (use the ``rounds`` parameter when subclassing a bcrypt hasher). For
example, to increase the number of iterations used by the default PBKDF2
algorithm:

#. Create a subclass of ``django.contrib.auth.hashers.PBKDF2PasswordHasher``
   ::

        from django.contrib.auth.hashers import PBKDF2PasswordHasher


        class MyPBKDF2PasswordHasher(PBKDF2PasswordHasher):
            """
            A subclass of PBKDF2PasswordHasher that uses 100 times more iterations.
            """

            iterations = PBKDF2PasswordHasher.iterations * 100

   Save this somewhere in your project. For example, you might put this in
   a file like ``myproject/hashers.py``.

#. Add your new hasher as the first entry in :setting:`PASSWORD_HASHERS`::

        PASSWORD_HASHERS = [
            "myproject.hashers.MyPBKDF2PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2PasswordHasher",
            "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
            "django.contrib.auth.hashers.Argon2PasswordHasher",
            "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
            "django.contrib.auth.hashers.ScryptPasswordHasher",
        ]

That's it -- now your Django install will use more iterations when it
stores passwords using PBKDF2.

.. note::

    bcrypt ``rounds`` is a logarithmic work factor, e.g. 12 rounds means
    ``2 ** 12`` iterations.

Argon2
~~~~~~

Argon2 has the following attributes that can be customized:

#. ``time_cost`` controls the number of iterations within the hash.
#. ``memory_cost`` controls the size of memory that must be used during the
   computation of the hash.
#. ``parallelism`` controls how many CPUs the computation of the hash can be
   parallelized on.

The default values of these attributes are probably fine for you. If you
determine that the password hash is too fast or too slow, you can tweak it as
follows:

#. Choose ``parallelism`` to be the number of threads you can
   spare computing the hash.
#. Choose ``memory_cost`` to be the KiB of memory you can spare.
#. Adjust ``time_cost`` and measure the time hashing a password takes.
   Pick a ``time_cost`` that takes an acceptable time for you.
   If ``time_cost`` set to 1 is unacceptably slow, lower ``memory_cost``.

.. admonition:: ``memory_cost`` interpretation

    The argon2 command-line utility and some other libraries interpret the
    ``memory_cost`` parameter differently from the value that Django uses. The
    conversion is given by ``memory_cost == 2 ** memory_cost_commandline``.

``scrypt``
~~~~~~~~~~

scrypt_ has the following attributes that can be customized:

#. ``work_factor`` controls the number of iterations within the hash.
#. ``block_size``
#. ``parallelism`` controls how many threads will run in parallel.
#. ``maxmem`` limits the maximum size of memory that can be used during the
   computation of the hash. Defaults to ``0``, which means the default
   limitation from the OpenSSL library.

We've chosen reasonable defaults, but you may wish to tune it up or down,
depending on your security needs and available processing power.

.. admonition:: Estimating memory usage

    The minimum memory requirement of scrypt_ is::

        work_factor * 2 * block_size * 64

    so you may need to tweak ``maxmem`` when changing the ``work_factor`` or
    ``block_size`` values.

.. _password-upgrades:

Password upgrading
------------------

When users log in, if their passwords are stored with anything other than
the preferred algorithm, Django will automatically upgrade the algorithm
to the preferred one. This means that old installs of Django will get
automatically more secure as users log in, and it also means that you
can switch to new (and better) storage algorithms as they get invented.

However, Django can only upgrade passwords that use algorithms mentioned in
:setting:`PASSWORD_HASHERS`, so as you upgrade to new systems you should make
sure never to *remove* entries from this list. If you do, users using
unmentioned algorithms won't be able to upgrade. Hashed passwords will be
updated when increasing (or decreasing) the number of PBKDF2 iterations, bcrypt
rounds, or argon2 attributes.

Be aware that if all the passwords in your database aren't encoded in the
default hasher's algorithm, you may be vulnerable to a user enumeration timing
attack due to a difference between the duration of a login request for a user
with a password encoded in a non-default algorithm and the duration of a login
request for a nonexistent user (which runs the default hasher). You may be able
to mitigate this by :ref:`upgrading older password hashes
<wrapping-password-hashers>`.

.. _wrapping-password-hashers:

Password upgrading without requiring a login
--------------------------------------------

If you have an existing database with an older, weak hash such as MD5, you
might want to upgrade those hashes yourself instead of waiting for the upgrade
to happen when a user logs in (which may never happen if a user doesn't return
to your site). In this case, you can use a "wrapped" password hasher.

For this example, we'll migrate a collection of MD5 hashes to use
PBKDF2(MD5(password)) and add the corresponding password hasher for checking
if a user entered the correct password on login. We assume we're using the
built-in ``User`` model and that our project has an ``accounts`` app. You can
modify the pattern to work with any algorithm or with a custom user model.

First, we'll add the custom hasher:

.. code-block:: python
    :caption: ``accounts/hashers.py``

    from django.contrib.auth.hashers import (
        PBKDF2PasswordHasher,
        MD5PasswordHasher,
    )


    class PBKDF2WrappedMD5PasswordHasher(PBKDF2PasswordHasher):
        algorithm = "pbkdf2_wrapped_md5"

        def encode_md5_hash(self, md5_hash, salt, iterations=None):
            return super().encode(md5_hash, salt, iterations)

        def encode(self, password, salt, iterations=None):
            _, _, md5_hash = MD5PasswordHasher().encode(password, salt).split("$", 2)
            return self.encode_md5_hash(md5_hash, salt, iterations)

The data migration might look something like:

.. code-block:: python
    :caption: ``accounts/migrations/0002_migrate_md5_passwords.py``

    from django.db import migrations

    from ..hashers import PBKDF2WrappedMD5PasswordHasher


    def forwards_func(apps, schema_editor):
        User = apps.get_model("auth", "User")
        users = User.objects.filter(password__startswith="md5$")
        hasher = PBKDF2WrappedMD5PasswordHasher()
        for user in users:
            algorithm, salt, md5_hash = user.password.split("$", 2)
            user.password = hasher.encode_md5_hash(md5_hash, salt)
            user.save(update_fields=["password"])


    class Migration(migrations.Migration):
        dependencies = [
            ("accounts", "0001_initial"),
            # replace this with the latest migration in contrib.auth
            ("auth", "####_migration_name"),
        ]

        operations = [
            migrations.RunPython(forwards_func),
        ]

Be aware that this migration will take on the order of several minutes for
several thousand users, depending on the speed of your hardware.

Finally, we'll add a :setting:`PASSWORD_HASHERS` setting:

.. code-block:: python
    :caption: ``mysite/settings.py``

    PASSWORD_HASHERS = [
        "django.contrib.auth.hashers.PBKDF2PasswordHasher",
        "accounts.hashers.PBKDF2WrappedMD5PasswordHasher",
    ]

Include any other hashers that your site uses in this list.

.. _pbkdf2: https://en.wikipedia.org/wiki/PBKDF2
.. _nist: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-132.pdf
.. _bcrypt: https://en.wikipedia.org/wiki/Bcrypt
.. _argon2: https://en.wikipedia.org/wiki/Argon2
.. _scrypt: https://en.wikipedia.org/wiki/Scrypt
.. _`Password Hashing Competition`: https://www.password-hashing.net/

.. _auth-included-hashers:

Included hashers
----------------

The full list of hashers included in Django is::

    [
        "django.contrib.auth.hashers.PBKDF2PasswordHasher",
        "django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher",
        "django.contrib.auth.hashers.Argon2PasswordHasher",
        "django.contrib.auth.hashers.BCryptSHA256PasswordHasher",
        "django.contrib.auth.hashers.BCryptPasswordHasher",
        "django.contrib.auth.hashers.ScryptPasswordHasher",
        "django.contrib.auth.hashers.MD5PasswordHasher",
    ]

The corresponding algorithm names are:

* ``pbkdf2_sha256``
* ``pbkdf2_sha1``
* ``argon2``
* ``bcrypt_sha256``
* ``bcrypt``
* ``scrypt``
* ``md5``

.. _write-your-own-password-hasher:

Writing your own hasher
-----------------------

If you write your own password hasher that contains a work factor such as a
number of iterations, you should implement a
``harden_runtime(self, password, encoded)`` method to bridge the runtime gap
between the work factor supplied in the ``encoded`` password and the default
work factor of the hasher. This prevents a user enumeration timing attack due
to  difference between a login request for a user with a password encoded in an
older number of iterations and a nonexistent user (which runs the default
hasher's default number of iterations).

Taking PBKDF2 as example, if ``encoded`` contains 20,000 iterations and the
hasher's default ``iterations`` is 30,000, the method should run ``password``
through another 10,000 iterations of PBKDF2.

If your hasher doesn't have a work factor, implement the method as a no-op
(``pass``).

Manually managing a user's password
===================================

.. module:: django.contrib.auth.hashers

The :mod:`django.contrib.auth.hashers` module provides a set of functions
to create and validate hashed passwords. You can use them independently
from the ``User`` model.

.. function:: check_password(password, encoded, setter=None, preferred="default")
.. function:: acheck_password(password, encoded, asetter=None, preferred="default")

    *Asynchronous version*: ``acheck_password()``

    If you'd like to manually authenticate a user by comparing a plain-text
    password to the hashed password in the database, use the convenience
    function :func:`check_password`. It takes two mandatory arguments: the
    plain-text password to check, and the full value of a user's ``password``
    field in the database to check against. It returns ``True`` if they match,
    ``False`` otherwise. Optionally, you can pass a callable ``setter`` that
    takes the password and will be called when you need to regenerate it. You
    can also pass ``preferred`` to change a hashing algorithm if you don't want
    to use the default (first entry of ``PASSWORD_HASHERS`` setting). See
    :ref:`auth-included-hashers` for the algorithm name of each hasher.

.. function:: make_password(password, salt=None, hasher='default')

    Creates a hashed password in the format used by this application. It takes
    one mandatory argument: the password in plain-text (string or bytes).
    Optionally, you can provide a salt and a hashing algorithm to use, if you
    don't want to use the defaults (first entry of ``PASSWORD_HASHERS``
    setting). See :ref:`auth-included-hashers` for the algorithm name of each
    hasher. If the password argument is ``None``, an unusable password is
    returned (one that will never be accepted by :func:`check_password`).

.. function:: is_password_usable(encoded_password)

    Returns ``False`` if the password is a result of
    :meth:`.User.set_unusable_password`.

.. _password-validation:

Password validation
===================

.. module:: django.contrib.auth.password_validation

Users often choose poor passwords. To help mitigate this problem, Django
offers pluggable password validation. You can configure multiple password
validators at the same time. A few validators are included in Django, but you
can write your own as well.

Each password validator must provide a help text to explain the requirements to
the user, validate a given password and return an error message if it does not
meet the requirements, and optionally define a callback to be notified when
the password for a user has been changed. Validators can also have optional
settings to fine tune their behavior.

Validation is controlled by the :setting:`AUTH_PASSWORD_VALIDATORS` setting.
The default for the setting is an empty list, which means no validators are
applied. In new projects created with the default :djadmin:`startproject`
template, a set of validators is enabled by default.

By default, validators are used in the forms to reset or change passwords and
in the :djadmin:`createsuperuser` and :djadmin:`changepassword` management
commands. Validators aren't applied at the model level, for example in
``User.objects.create_user()`` and ``create_superuser()``, because we assume
that developers, not users, interact with Django at that level and also because
model validation doesn't automatically run as part of creating models.

.. note::

    Password validation can prevent the use of many types of weak passwords.
    However, the fact that a password passes all the validators doesn't
    guarantee that it is a strong password. There are many factors that can
    weaken a password that are not detectable by even the most advanced
    password validators.

Enabling password validation
----------------------------

Password validation is configured in the
:setting:`AUTH_PASSWORD_VALIDATORS` setting::

    AUTH_PASSWORD_VALIDATORS = [
        {
            "NAME": "django.contrib.auth.password_validation.UserAttributeSimilarityValidator",
        },
        {
            "NAME": "django.contrib.auth.password_validation.MinimumLengthValidator",
            "OPTIONS": {
                "min_length": 9,
            },
        },
        {
            "NAME": "django.contrib.auth.password_validation.CommonPasswordValidator",
        },
        {
            "NAME": "django.contrib.auth.password_validation.NumericPasswordValidator",
        },
    ]

This example enables all four included validators:

* ``UserAttributeSimilarityValidator``, which checks the similarity between
  the password and a set of attributes of the user.
* ``MinimumLengthValidator``, which checks whether the password meets a minimum
  length. This validator is configured with a custom option: it now requires
  the minimum length to be nine characters, instead of the default eight.
* ``CommonPasswordValidator``, which checks whether the password occurs in a
  list of common passwords. By default, it compares to an included list of
  20,000 common passwords.
* ``NumericPasswordValidator``, which checks whether the password isn't
  entirely numeric.

For ``UserAttributeSimilarityValidator`` and ``CommonPasswordValidator``,
we're using the default settings in this example. ``NumericPasswordValidator``
has no settings.

The help texts and any errors from password validators are always returned in
the order they are listed in :setting:`AUTH_PASSWORD_VALIDATORS`.

.. _included-password-validators:

Included validators
-------------------

Django includes four validators:

.. class:: MinimumLengthValidator(min_length=8)

    Validates that the password is of a minimum length.
    The minimum length can be customized with the ``min_length`` parameter.

    .. method:: get_error_message()

        .. versionadded:: 5.2

        A hook for customizing the ``ValidationError`` error message. Defaults
        to ``"This password is too short. It must contain at least <min_length>
        characters."``.

    .. method:: get_help_text()

        A hook for customizing the validator's help text. Defaults to ``"Your
        password must contain at least <min_length> characters."``.

.. class:: UserAttributeSimilarityValidator(user_attributes=DEFAULT_USER_ATTRIBUTES, max_similarity=0.7)

    Validates that the password is sufficiently different from certain
    attributes of the user.

    The ``user_attributes`` parameter should be an iterable of names of user
    attributes to compare to. If this argument is not provided, the default
    is used: ``'username', 'first_name', 'last_name', 'email'``.
    Attributes that don't exist are ignored.

    The maximum allowed similarity of passwords can be set on a scale of 0.1
    to 1.0 with the ``max_similarity`` parameter. This is compared to the
    result of :meth:`difflib.SequenceMatcher.quick_ratio`. A value of 0.1
    rejects passwords unless they are substantially different from the
    ``user_attributes``, whereas a value of 1.0 rejects only passwords that are
    identical to an attribute's value.

    .. method:: get_error_message()

        .. versionadded:: 5.2

        A hook for customizing the ``ValidationError`` error message. Defaults
        to ``"The password is too similar to the <user_attribute>."``.

    .. method:: get_help_text()

        A hook for customizing the validator's help text. Defaults to ``"Your
        password can’t be too similar to your other personal information."``.

.. class:: CommonPasswordValidator(password_list_path=DEFAULT_PASSWORD_LIST_PATH)

    Validates that the password is not a common password. This converts the
    password to lowercase (to do a case-insensitive comparison) and checks it
    against a list of 20,000 common password created by `Royce Williams
    <https://gist.github.com/roycewilliams/226886fd01572964e1431ac8afc999ce>`_.

    The ``password_list_path`` can be set to the path of a custom file of
    common passwords. This file should contain one lowercase password per line
    and may be plain text or gzipped.

    .. method:: get_error_message()

        .. versionadded:: 5.2

        A hook for customizing the ``ValidationError`` error message. Defaults
        to ``"This password is too common."``.

    .. method:: get_help_text()

        A hook for customizing the validator's help text. Defaults to ``"Your
        password can’t be a commonly used password."``.

.. class:: NumericPasswordValidator()

    Validate that the password is not entirely numeric.

    .. method:: get_error_message()

        .. versionadded:: 5.2

        A hook for customizing the ``ValidationError`` error message. Defaults
        to ``"This password is entirely numeric."``.

    .. method:: get_help_text()

        A hook for customizing the validator's help text. Defaults to ``"Your
        password can’t be entirely numeric."``.

Integrating validation
----------------------

There are a few functions in ``django.contrib.auth.password_validation`` that
you can call from your own forms or other code to integrate password
validation. This can be useful if you use custom forms for password setting,
or if you have API calls that allow passwords to be set, for example.

.. function:: validate_password(password, user=None, password_validators=None)

    Validates a password. If all validators find the password valid, returns
    ``None``. If one or more validators reject the password, raises a
    :exc:`~django.core.exceptions.ValidationError` with all the error messages
    from the validators.

    The ``user`` object is optional: if it's not provided, some validators may
    not be able to perform any validation and will accept any password.

.. function:: password_changed(password, user=None, password_validators=None)

    Informs all validators that the password has been changed. This can be used
    by validators such as one that prevents password reuse. This should be
    called once the password has been successfully changed.

    For subclasses of :class:`~django.contrib.auth.models.AbstractBaseUser`,
    the password field will be marked as "dirty" when calling
    :meth:`~django.contrib.auth.models.AbstractBaseUser.set_password` which
    triggers a call to ``password_changed()`` after the user is saved.

.. function:: password_validators_help_texts(password_validators=None)

    Returns a list of the help texts of all validators. These explain the
    password requirements to the user.

.. function:: password_validators_help_text_html(password_validators=None)

    Returns an HTML string with all help texts in an ``<ul>``. This is
    helpful when adding password validation to forms, as you can pass the
    output directly to the ``help_text`` parameter of a form field.

.. function:: get_password_validators(validator_config)

    Returns a set of validator objects based on the ``validator_config``
    parameter. By default, all functions use the validators defined in
    :setting:`AUTH_PASSWORD_VALIDATORS`, but by calling this function with an
    alternate set of validators and then passing the result into the
    ``password_validators`` parameter of the other functions, your custom set
    of validators will be used instead. This is useful when you have a typical
    set of validators to use for most scenarios, but also have a special
    situation that requires a custom set. If you always use the same set
    of validators, there is no need to use this function, as the configuration
    from :setting:`AUTH_PASSWORD_VALIDATORS` is used by default.

    The structure of ``validator_config`` is identical to the
    structure of :setting:`AUTH_PASSWORD_VALIDATORS`. The return value of
    this function can be passed into the ``password_validators`` parameter
    of the functions listed above.

Note that where the password is passed to one of these functions, this should
always be the clear text password - not a hashed password.

Writing your own validator
--------------------------

If Django's built-in validators are not sufficient, you can write your own
password validators. Validators have a fairly small interface. They must
implement two methods:

* ``validate(self, password, user=None)``: validate a password. Return
  ``None`` if the password is valid, or raise a
  :exc:`~django.core.exceptions.ValidationError` with an error message if the
  password is not valid. You must be able to deal with ``user`` being
  ``None`` - if that means your validator can't run, return ``None`` for no
  error.
* ``get_help_text()``: provide a help text to explain the requirements to
  the user.

Any items in the ``OPTIONS`` in :setting:`AUTH_PASSWORD_VALIDATORS` for your
validator will be passed to the constructor. All constructor arguments should
have a default value.

Here's a basic example of a validator, with one optional setting::

    from django.core.exceptions import ValidationError
    from django.utils.translation import gettext as _


    class MinimumLengthValidator:
        def __init__(self, min_length=8):
            self.min_length = min_length

        def validate(self, password, user=None):
            if len(password) < self.min_length:
                raise ValidationError(
                    _("This password must contain at least %(min_length)d characters."),
                    code="password_too_short",
                    params={"min_length": self.min_length},
                )

        def get_help_text(self):
            return _(
                "Your password must contain at least %(min_length)d characters."
                % {"min_length": self.min_length}
            )

You can also implement ``password_changed(password, user=None``), which will
be called after a successful password change. That can be used to prevent
password reuse, for example. However, if you decide to store a user's previous
passwords, you should never do so in clear text.