The original recaptcha library can be downloaded from here
Comments or suggestions: brianpow [AT] gmail [DOT] com
First, register a recaptcha account to have the private key and public key.
Then, edit the config/recaptcha.php and add your keys.
To check the answer in Controller:
$recaptcha=new Recaptcha();
if($recaptcha->check($_POST))
$message='Answer Correct!";
else
$message='Are you robot?";
You may also get the original error message by
$error=$recaptcha->error();
In case you want to use more than one key in your project, you may add your new key as parameter when creating a new object
$another_recaptcha=new Recaptcha($my_other_keys_array);
To show the input field in View:
$body=View::factory('YOUR_VIEW_FILE')->set('recaptcha', $recaptcha->html());