Advanced topics
===============

Configuration toggles
+++++++++++++++++++++

The following configuration elements can be defined (in your ``settings.py``)

CAPTCHA_FONT_PATH
-----------------

Full path and filename of a TrueType (TTF), OpenType, or pilfont font file used to render text.

Defaults to: ``fonts/Vera.ttf`` (included in the application, GPL font).

Note that your Pillow installation must support TTF and/or OpenFont if you want to use these kind of glyphs (most modern distributions of Pillow do.)

Note: as of version 0.4.6, ``CAPTCHA_FONT_PATH`` may be an iterable of font paths, in which case a font will be picked randomly from the list for each CAPTCHA.

CAPTCHA_FONT_SIZE
-----------------

Font-size in pixels of the rendered text.

Defaults to '22'.

CAPTCHA_IMAGE_SIZE
------------------

Image size in pixels of generated captcha, specified as a tuple (width, height)

Defaults to `None` (automatically calculated)

CAPTCHA_LETTER_ROTATION
-----------------------

A random rotation in this interval is applied to each letter in the challenge text.

Defaults to ``(-35,35)``.

New in version 0.1.6: set this to None to disable letter rotation.

CAPTCHA_BACKGROUND_COLOR
------------------------

Background-color of the captcha. Can be expressed as html-style #rrggbb, rgb(red, green, blue), or common html names (e.g. "red").

Defaults to: ``'#ffffff'``

CAPTCHA_FOREGROUND_COLOR
------------------------

Foreground-color of the captcha.

Defaults to ``'#001100'``

CAPTCHA_LETTER_COLOR_FUNCT
------------------------
A string representing a Python callable (i.e., a function) to determine the color of the letters in the CAPTCHA.

Defaults to ``'None'`` (uses CAPTCHA_FOREGROUND_COLOR) for all letters.

This function is called for each letter of the CAPTCHA string.
It takes two arguments: the first is the index of the current letter, and the second is the entire CAPTCHA string.

CAPTCHA_CHALLENGE_FUNCT
------------------------

String representing a python callable (i.e. a function) to use as challenge generator.

See Generators below for a list of available generators and a guide on how to write your own.

Defaults to: ``'captcha.helpers.random_char_challenge'``

CAPTCHA_MATH_CHALLENGE_OPERATOR
-------------------------------

When using the ``math_challenge``, lets you choose the multiplication operator. Use lowercase ``'x'`` for cross sign.

Defaults to: ``'*'`` (asterisk sign)

CAPTCHA_NOISE_FUNCTIONS
------------------------

List of strings of python callables that take a Pillow ``DrawImage`` object and an ``Image`` image as input, modify the ``DrawImage``, then return it.

Defaults to: ``('captcha.helpers.noise_arcs','captcha.helpers.noise_dots',)``

A null noise helper function useful when debugging issues is available at ``'captcha.helpers.noise_null'``.


CAPTCHA_FILTER_FUNCTIONS
------------------------

List of strings of python callables that take a PIL ``Image`` object as input, modify it and return it.

These are called right before the rendering, i.e. after the noise functions.

Defaults to: ``('captcha.helpers.post_smooth',)``


CAPTCHA_WORDS_DICTIONARY
------------------------

Required for the ``word_challenge`` challenge function only. Points a file containing a list of words, one per line.

Defaults to: ``'/usr/share/dict/words'``

CAPTCHA_FLITE_PATH
------------------------

Full path to the ``flite`` executable. When defined, will automatically add audio output to the captcha.

Defaults to: ``None`` (no audio output)

CAPTCHA_SOX_PATH
------------------------

Full path to the ``sox`` executable. If audio output is enabled via ``CAPTCHA_FLITE_PATH``, the generated output audio file is identical across multiple generations (unlike CAPTCHA images which get different random noise each time they are rendered). User appleorange1_ has shown_ that this could be used to pre-generate a "rainbow-table" of all possible input strings and a hash of the generated output soundfile, thus rendering an attack on audio CAPTCHAs trivial.

If sox_ is installed and used via this settings, random brown noise is injected into the generated audio file, rendering attacks via a rainbow table impossible.

Defaults to: ``None`` (no audio output)

.. _appleorange1: https://github.com/appleorange1
.. _shown: https://github.com/appleorange1/django-simple-captcha-cracker-poc
.. _sox: http://sox.sourceforge.net/


CAPTCHA_TIMEOUT
---------------

Integer. Lifespan, in minutes, of the generated captcha.

Defaults to: 5

CAPTCHA_LENGTH
------------------------

Sets the length, in chars, of the generated captcha. (for the ``'captcha.helpers.random_char_challenge'`` challenge)

Defaults to: 4

CAPTCHA_DICTIONARY_MIN_LENGTH
-----------------------------

When using the word_challenge challenge function, controls the minimum length of the words to be randomly picked from the dictionary file.

Defaults to: 0

CAPTCHA_DICTIONARY_MAX_LENGTH
-----------------------------

When using the word_challenge challenge function, controls the maximal length of the words to be randomly picked from the dictionary file.

Defaults to: 99

Note: it's perfectly safe to specify e.g. ``CAPTCHA_DICTIONARY_MIN_LENGTH = CAPTCHA_DICTIONARY_MAX_LENGTH = 6`` but it's considered an error to define ``CAPTCHA_DICTIONARY_MAX_LENGTH`` to be smaller than ``CAPTCHA_DICTIONARY_MIN_LENGTH``.


CAPTCHA_TEST_MODE
------------------------

New in version 0.3.6

When set to True, the string "PASSED" (any case) will be accepted as a valid response to any CAPTCHA.
Use this for testing purposes. Warning: do NOT set this to True in production.

Defaults to: False


CAPTCHA_GET_FROM_POOL
---------------------

By default, `django-simple-captcha` generates a new captcha when needed and stores it in the database. This occurs in a `HTTP GET request`, which may not be wished. This default behavior may also conflict with a load balanced infrastructure, where there is more than one database to read data from. If this setting is `True`, when a new captcha is needed, a random one will be just read from a pool of captchas saved previously in the database. In this case, the custom management command `captcha_create_pool` must be run regularly in intervals slightly shorter than `CAPTCHA_TIMEOUT`. A good value for `CAPTCHA_TIMEOUT` could be 1446 (24 hours and 6 minutes) when adding captchas to the pool every 24 hours, and setting `CAPTCHA_GET_FROM_POOL_TIMEOUT` (see below) to 5 minutes. This means that 6 minutes before the last captchas expires, new captchas will be created, and no captcha will be used whose expiration is less than 5 minutes. In this case, use a cronjob or similar to run `python manage.py captcha_create_pool` every 24 hours.

Defaults to: False


CAPTCHA_GET_FROM_POOL_TIMEOUT
-----------------------------

This is a timeout value in minutes used only if `CAPTCHA_GET_FROM_POOL` (see above) is `True`. When picking up randomly from the pool, this setting will prevent to pick up a captcha that expires sooner than `CAPTCHA_GET_FROM_POOL_TIMEOUT`.

Defaults to: 5


CAPTCHA_2X_IMAGE
----------------

When set to True, a double resolution version of the captcha image is made served by adding `@2` to the end of the image URL (`image/<key>@2/`). Note that this makes the image slightly more readable on e.g. HiDPI screens, but could also make the CAPTCHA potentially easier to break by a bot.

Defaults to: True


Rendering
+++++++++

``CaptchaTextInput`` supports the widget rendering using template introduced in Django 1.11.
To change the output HTML, change the ``template_name`` to a custom template or modify ``get_context`` method to provide further context.
See https://docs.djangoproject.com/en/dev/ref/forms/renderers/ for description of rendering API.
Keep in mind that ``CaptchaTextInput`` is a subclass of ``MultiWidget`` which affects the context, see https://docs.djangoproject.com/en/2.0/ref/forms/widgets/#multiwidget.

For example, you would::

    class CustomCaptchaTextInput(CaptchaTextInput):
        template_name = 'custom_field.html'

    class CaptchaForm(forms.Form):
        captcha = CaptchaField(widget=CustomCaptchaTextInput)


And then have a ``custom_field.html`` template::

    {% load i18n %}
    {% spaceless %}
    <div class="form-group">
      <label class="control-label">{{ label }}</label>
      <div class="form-group">
        <div class="input-group mb-3">
          <div class="input-group-prepend">
            {% if audio %}
                <a title="{% trans "Play CAPTCHA as audio file" %}" href="{{ audio }}">
            {% endif %}
            <img src="{{ image }}" alt="captcha" class="captcha" />
          </div>
          {% include "django/forms/widgets/multiwidget.html" %}
        </div>
      </div>
    </div>
    {% endspaceless %}


.. note:: For this to work, you MUST
   add ``django.forms`` to your ``INSTALLED_APPS`` and
   set ``FORM_RENDERER = 'django.forms.renderers.TemplatesSetting'`` in your settings.py.
   (See here_ for an explanation)

.. _here: https://docs.djangoproject.com/en/2.0/ref/forms/renderers/#django.forms.renderers.TemplatesSetting



Context
-------

The following context variables are passed to the three "individual" templates:

* ``image``: The URL of the rendered CAPTCHA image
* ``name``: name of the field (i.e. the name of your form field)
* ``key``: the hashed value (identifier) of this CAPTCHA: this is stored and passed in the hidden input
* ``id``: the HTML ``id`` attribute to be used

The ``captcha/field.html`` template receives the following context:

* ``image``: the rendered (HTML) image and optionally audio elements
* ``hidden_field``: the rendered hidden input
* ``text_field``: the rendered text input

Note: these elements have been marked as safe, you can render them straight into your template.

.. _generators_ref:

Generators and modifiers
++++++++++++++++++++++++

Random chars
------------

.. image:: _static/random_chars.png

Classic captcha that picks four random chars. This is case insensitive. ::

    CAPTCHA_CHALLENGE_FUNCT = 'captcha.helpers.random_char_challenge'


Simple Math
------------

.. image:: _static/math.png

Another classic, that challenges the user to resolve a simple math challenge by randomly picking two numbers between one and nine, and a random operator among plus, minus, times. ::

    CAPTCHA_CHALLENGE_FUNCT = 'captcha.helpers.math_challenge'


Dictionary Word
----------------

.. image:: _static/dict.png

Picks a random word from a dictionary file. Note, you must define ``CAPTCHA_WORDS_DICTIONARY`` in your configuration to use this generator. ::

    CAPTCHA_CHALLENGE_FUNCT = 'captcha.helpers.word_challenge'


Roll your own
-------------

To have your own challenge generator, simply point ``CAPTCHA_CHALLENGE_FUNCT`` to a function that returns a tuple of strings: the first one (the challenge) will be rendered in the captcha, the second is the valid response to the challenge, e.g. ``('5+10=', '15')``, ``('AAAA', 'aaaa')``

This sample generator that returns six random digits::

    import random

    def random_digit_challenge():
        ret = u''
        for i in range(6):
            ret += str(random.randint(0,9))
        return ret, ret
