Tag Archives: Elgg

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A new CAPTCHA approachOne thing we try and do when working on a new Elgg feature is – where we can – couple things together as loosely as possible and provide hooks for third party developers to extend Elgg and fill in any blanks.

A good example of where this has been done is the newly introduced Captcha functionality available in the latest nightly testing builds of Elgg.

The Captcha functionality is provided by a module which extends a view called “input/captcha“. This view is blank by default but is used in several places such as user registration and the lost password form.

This means two things; firstly that if a Captcha module isn’t installed or enabled then forms behave normally, and secondly it becomes a trivial matter for third party modules to provide their own Captcha functionality.

This same mechanism is how the URL shortener module works by the way.

Next, the Captcha module extends a number of actions to require a correctly validated Captcha code. This list itself is the product of a plugin hook which returns an array of actions which require Captcha validation:

$actions = array();
$actions = trigger_plugin_hook('actionlist', 'captcha', null, $actions);

...

function captcha_actionlist_hook($hook, $entity_type, $returnvalue, $params)
{
if (!is_array($returnvalue))
$returnvalue = array();

$returnvalue[] = 'register';
$returnvalue[] = 'user/requestnewpassword';

return $returnvalue;
}

The reason why the list of actions is provided this way is twofold, firstly it lets modules use Captcha functionality in their own code through a generic interface, and secondly it is harder to spoof than looking for some marker in the form code.

The Captcha itself injects a server generated token into the form, which together with the user’s response to the characters generated in an image are used to validate that the user is indeed human.

As we can see, Elgg asks to be provided with a Captcha if one is available by including a specific view, but is agnostic as to where (or indeed if) this functionality is supplied.

By using the techniques available to an Elgg programmer I was able to loosely couple the Captcha system to Elgg in such a way that a third party can easily use the same techniques to provide a more advanced module.

Happy coding!

Image “A new Captcha approach” by XKCD

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Those eagle-eyed developers who have been tracking the Elgg core SVN may have noticed that I have recently committed a bunch of captcha related changes, including a simple captcha module.

I just thought I’d write a quick post about it as this module makes use of a bit of Elgg functionality which has been around for a while, but that I know a number of plugin developers have missed.

Namely, the ability to extend actions.

When the Elgg framework calls an action the Action handler triggers a plugin hook called “action” before executing the action itself. This hook looks like this:

$event_result = true;
$event_result = trigger_plugin_hook('action', $action, null, $event_result);

Where $action is the action being called. If the hook returns false then the main action will not be executed.

The captcha module uses this to intercept the register and user/requestnewpassword actions and redirect them to a function which checks the captcha code. This check returns true if valid or false if not (which prevents the associated action from executing).

This is done as follows:

register_plugin_hook("action", "register", "captcha_verify_action_hook");
register_plugin_hook("action", "user/requestnewpassword", "captcha_verify_action_hook");

.
.
.

function captcha_verify_action_hook($hook, $entity_type, $returnvalue, $params)
{
$token = get_input('captcha_token');
$input = get_input('captcha_input');

if (($token) && (captcha_verify_captcha($input, $token)))
return true;

register_error(elgg_echo('captcha:captchafail'));

return false;
}

As you can see, this lets a plugin extend an existing action without the need to replace the action itself. In the case of the captcha plugin it allows the plugin to provide captcha support in a very loosely coupled way.

Happy coding!

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I just thought I’d write a quick blog post to wrap up the week before heading off to enjoy the May bank holiday sunshine… and what a week it has been!

The week began with a rather frustrating critical hardware failure on one of Curverider’s servers which took down our community site, SVN and bug tracker among a few others.

No data was lost, and we were able to get the main sites back online in fairly short order once the new server was hooked up and installed in our datacenter.

On a lighter note I am very pleased to welcome Leonard Lin to the Elgg team.  Leonard was previously senior hacker at Yahoo and creator of  Upcoming.org. Most recently he has also been involved in the Obama campaign.

Exciting times ahead!

And while we’re on the subject of Upcoming.org, I would like to take this opportunity to point you in the direction of the Barcamp Transparency entry on Upcoming. Go sign up now!