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::

    <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. The first entry in this list
(that is, ``settings.PASSWORD_HASHERS[0]``) will be used to store passwords,
and all the other entries are valid hashers that can be used to check existing
passwords.  This means that if you want to use a different algorithm, you'll
need to modify :setting:`PASSWORD_HASHERS` to list your preferred algorithm
first in the list.

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.BCryptPasswordHasher',
    ]

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.

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 Argon2 as your default storage algorithm, do the following:

1. Install the `argon2-cffi library`_.  This can be done by running ``pip
   install django[argon2]``, which is equivalent to ``pip install argon2-cffi``
   (along with any version requirement from Django's ``setup.py``).

2. 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.BCryptPasswordHasher',
        ]

   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:

1. Install the `bcrypt library`_. This can be done by running ``pip install
   django[bcrypt]``, which is equivalent to  ``pip install bcrypt`` (along with
   any version requirement from Django's ``setup.py``).

2. 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.BCryptPasswordHasher',
            'django.contrib.auth.hashers.PBKDF2PasswordHasher',
            'django.contrib.auth.hashers.PBKDF2SHA1PasswordHasher',
            'django.contrib.auth.hashers.Argon2PasswordHasher',
        ]

   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.

.. admonition:: Password truncation with BCryptPasswordHasher

    The designers of bcrypt truncate all passwords at 72 characters which means
    that ``bcrypt(password_with_100_chars) == bcrypt(password_with_100_chars[:72])``.
    The original ``BCryptPasswordHasher`` does not have any special handling and
    thus is also subject to this hidden password length limit.
    ``BCryptSHA256PasswordHasher`` fixes this by first hashing the
    password using sha256. This prevents the password truncation and so should
    be preferred over the ``BCryptPasswordHasher``. The practical ramification
    of this truncation is pretty marginal as the average user does not have a
    password greater than 72 characters in length and even being truncated at 72
    the compute powered required to brute force bcrypt in any useful amount of
    time is still astronomical. Nonetheless, we recommend you use
    ``BCryptSHA256PasswordHasher`` anyway on the principle of "better safe than
    sorry".

.. admonition:: Other bcrypt implementations

   There are several other implementations that allow bcrypt to be
   used with Django. Django's bcrypt support is NOT directly
   compatible with these. To upgrade, you will need to modify the
   hashes in your database to be in the form ``bcrypt$(raw bcrypt
   output)``. For example:
   ``bcrypt$$2a$12$NT0I31Sa7ihGEWpka9ASYrEFkhuTNeBQ2xfZskIiiJeyFXhRgS.Sy``.

.. _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``
parameters. For example, to increase the number of iterations used by the
default PBKDF2 algorithm:

1. 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``.

2. 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.BCryptPasswordHasher',
        ]

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

Argon2
~~~~~~

Argon2 has three 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``.

.. _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 or
bcrypt rounds.

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 or SHA1,
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 SHA1 hashes to use
PBKDF2(SHA1(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:

.. snippet::
    :filename: accounts/hashers.py

    from django.contrib.auth.hashers import (
        PBKDF2PasswordHasher, SHA1PasswordHasher,
    )


    class PBKDF2WrappedSHA1PasswordHasher(PBKDF2PasswordHasher):
        algorithm = 'pbkdf2_wrapped_sha1'

        def encode_sha1_hash(self, sha1_hash, salt, iterations=None):
            return super(PBKDF2WrappedSHA1PasswordHasher, self).encode(sha1_hash, salt, iterations)

        def encode(self, password, salt, iterations=None):
            _, _, sha1_hash = SHA1PasswordHasher().encode(password, salt).split('$', 2)
            return self.encode_sha1_hash(sha1_hash, salt, iterations)

The data migration might look something like:

.. snippet::
    :filename: accounts/migrations/0002_migrate_sha1_passwords.py

    from django.db import migrations

    from ..hashers import PBKDF2WrappedSHA1PasswordHasher


    def forwards_func(apps, schema_editor):
        User = apps.get_model('auth', 'User')
        users = User.objects.filter(password__startswith='sha1$')
        hasher = PBKDF2WrappedSHA1PasswordHasher()
        for user in users:
            algorithm, salt, sha1_hash = user.password.split('$', 2)
            user.password = hasher.encode_sha1_hash(sha1_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:

.. snippet::
    :filename: mysite/settings.py

    PASSWORD_HASHERS = [
        'django.contrib.auth.hashers.PBKDF2PasswordHasher',
        'accounts.hashers.PBKDF2WrappedSHA1PasswordHasher',
    ]

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

.. _sha1: https://en.wikipedia.org/wiki/SHA1
.. _pbkdf2: https://en.wikipedia.org/wiki/PBKDF2
.. _nist: https://dx.doi.org/10.6028/NIST.SP.800-132
.. _bcrypt: https://en.wikipedia.org/wiki/Bcrypt
.. _`bcrypt library`: https://pypi.python.org/pypi/bcrypt/
.. _`argon2-cffi library`: https://pypi.python.org/pypi/argon2_cffi/
.. _argon2: https://en.wikipedia.org/wiki/Argon2
.. _`Password Hashing Competition`: https://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.SHA1PasswordHasher',
        'django.contrib.auth.hashers.MD5PasswordHasher',
        'django.contrib.auth.hashers.UnsaltedSHA1PasswordHasher',
        'django.contrib.auth.hashers.UnsaltedMD5PasswordHasher',
        'django.contrib.auth.hashers.CryptPasswordHasher',
    ]

The corresponding algorithm names are:

* ``pbkdf2_sha256``
* ``pbkdf2_sha1``
* ``argon2``
* ``bcrypt_sha256``
* ``bcrypt``
* ``sha1``
* ``md5``
* ``unsalted_sha1``
* ``unsalted_md5``
* ``crypt``

.. _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 password. You can use them independently
from the ``User`` model.

.. function:: check_password(password, encoded)

    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 arguments: the plain-text
    password to check, and the full value of a user's ``password`` field in the
    database to check against, and returns ``True`` if they match, ``False``
    otherwise.

.. 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. 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 (a one that
    will be never accepted by :func:`check_password`).

.. function:: is_password_usable(encoded_password)

   Checks if the given string is a hashed password that has a chance
   of being verified against :func:`check_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 it's
simple to 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 receive passwords that have been set.
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 simple set of validators is enabled.

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 simply 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
  1000 common passwords.
* ``NumericPasswordValidator``, which checks whether the password isn't
  entirely numeric.

For ``UserAttributeSimilarityValidator`` and ``CommonPasswordValidator``,
we're simply 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 validators
-------------------

Django includes four validators:

.. class:: MinimumLengthValidator(min_length=8)

    Validates whether the password meets a minimum length.
    The minimum length can be customized with the ``min_length`` parameter.

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

    Validates whether 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 minimum similarity of a rejected password can be set on a scale of 0 to
    1 with the ``max_similarity`` parameter. A setting of 0 rejects all
    passwords, whereas a setting of 1 rejects only passwords that are identical
    to an attribute's value.

.. class:: CommonPasswordValidator(password_list_path=DEFAULT_PASSWORD_LIST_PATH)

    Validates whether the password is not a common password. By default, this
    checks against a list of 1000 common password created by
    `Mark Burnett <https://web.archive.org/web/20150315154609/https://xato.net/passwords/more-top-worst-passwords/>`_.

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

.. class:: NumericPasswordValidator()

    Validates whether the password is not 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 are fairly simple classes. 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, simply 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 ugettext as _

    class MinimumLengthValidator(object):
        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.