A simple math-based captcha for Django forms
A textual captcha for Django using simple decorator syntax.
So What Does it DO?
simplecaptcha provides an easy decorator syntax to add a textual captcha to your Django forms. The captcha is a simple arithmetic question: Either add, subtract, or multiply two numbers between 1 and 10. No server-side context is needed, as the captcha uses cryptographic signatures to securely pass the context to the client, and then validate the supplied answer on the back end.
In order to mitigate replay attacks, the signatures expire after a configurable amount of time (default 5 minutes): enough time to fill out and submit the form, but short enough to reduce the ability to reuse signatures with known answers.
Why Make Another One?
There’s lots of Django captchas out there, including more than one that uses arithmetic questions just like this one. So why do we need another?
Simply put, the others all lack in flexibility. When I set out to find one for my form, I needed one that would allow me to manually render my fields; the first few I found, however, hardcoded the question (as a label) into the format_output() method, or even directly in the render() method itself. This meant I couldn’t separately render the label where I need it for my design. I kept digging, and found another that offered the flexibility I needed in the layout, but put the captcha generation logic in the field’s __init__() method. While this sounds great, Django’s method of using class objects – rather than instance objects – means that you get only a single captcha question per server thread, period.
So I sat down to write a captcha that would give me the flexibility I needed to fit into my front-end design, but that also would reliably generate a fresh captcha question each time the page was loaded.
This is that captcha.
(Recommended) Install from PyPi with a simple pip install django-simplecaptcha.
Download the source from GitHub, and simply make the simplecaptcha module available to Python in some way; on *nix systems, a simple symlink in the root of your Django project to the simplecaptcha directory is probably the most straightforward solution.
Using the Captcha
Using simplecaptcha is simple:
from simplecaptcha import captcha @captcha class MyForm(Form): pass
This will add a field named “captcha” to MyForm. However, nothing else need be done: the decorator takes care of adding the field and ensuring it is always updated when a new form instance is created, as well as validating bound forms and providing useful error messages for users.
simplecaptcha, as its name implies, is simple. It works straight out of the box without any need to add any configuration in your Django project. However, if you do want to modify its behavior, you can do that as well, by simply adding any of these settings to your Django project’s settings module:
- SIMPLECAPTCHA_DURATION: Defines how long (in seconds) a captcha is considered valid for; default: 300 seconds (5 minutes)
- SIMPLECAPTCHA_ITERATIONS: The cryptographic signature passed to the client and used to validate the captcha is hashed multiple times for security. You can change the number of iterations used with this setting; default: 1024
- SIMPLECAPTCHA_DEFAULT_FIELD_NAME: The default field name used in the captcha decorator; default: ‘captcha’
Controlling Field Order
The decorator will always add the captcha field to the end of your form. If this is undesirable for any reason, you can of course always manually render your form fields as decribed in the Django docs.
Another option is to simply add a “dummy” field to your form with the same name as that used by the decorator. The decorator would then effectively replace the field in your form:
from simplecaptcha import captcha from simplecaptcha.fields import CaptchaField @captcha class MyForm(Form): field1 = CharField() field2 = CharField() captcha = CaptchaField() field3 = CharField()
(NOTE: Since the decorator will replace the field of the same name, it does not matter what type of field you specify when using this approach. Because of the way Django processes Form classes, however, you must specify a Django field, or else Django will ignore it and you won’t get the desired effect.)
Now when you render MyForm in your template, fields will be ordered precisely as they are in your source: field1, then field2, followed by captcha, and finally field3.
Specifying the Field Name
If for any reason you don’t want your captcha field to be named “captcha”, and you don’t want to set SIMPLECAPTCHA_DEFAULT_FIELD_NAME in your Django settings module, you can use the @captchaform decorator and supply the desired field name as an argument, like so:
from simplecaptcha import captchaform @captchaform('securitycheck') class MyForm(Form): pass
This will add a field named “securitycheck” to MyForm that will contain the form’s captcha.
If you wish to do this and use the method in the previous section to specify the field order, note that the “dummy” field you add must match the name you passed into the decorator.
Multiple Captcha Fields
It is possible to add multiple captcha fields to your form simply by decorating your form multiple times. However note that field order in your form will be the reverse of the order that you write your decorators:
from simplecaptcha import captchaform @captchaform('captcha') @captchaform('captcha2') class MyForm(Form): pass
In this example, when MyForm is rendered in your template, “captcha2” will appear first, and then “captcha”. This is a consequence of how decorators in Python are processed; you simply have to remember that the last captcha decorated into your form is the first one that will appear in your templates.
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