Google Recaptcha ref: http://qiita.com/anhtuank7c/items/6edb2b332d79136192e1

file0.txt

composer require crabstudio/recaptcha

file1.txt

"require": {
    "crabstudio/recaptcha": "^1.0"
}

file2.txt

composer update

file3.txt

    public function forgotPassword() {
        if($this->request->is('post')){
            if($this->Recaptcha->verify()) { // if configure enable = false, always return true
                //do something here
            }
            $this->Flash->error(__('Please pass Google Recaptcha first'));
        }
    }

file4.txt

$config = [
    'Recaptcha' => [
        'sitekey' => 'the site key',
        'secret' => 'the secret',
        'theme' => 'light',
        'type' => 'image',
        'enable' => true
    ]
];
Cake\Core\Configure::config('default', $config);
Cake\Core\Configure::load('recaptcha', 'default', false);

file5.txt

public function initialize() {
    parent::initialize();
    if ($this->request->action === 'contact') {
        $this->loadComponent('Crabstudio/Recaptcha.Recaptcha');
    }
}

file6.txt

public function contact() {
    if ($this->request->is('post')) {
        if ($this->Recaptcha->verify()) {
            if ($contact->execute($this->request->data)) {
                $this->Flash->success(__('We will get back to you soon.'));
                return $this->redirect($this->referer());
            } else {
                $this->Flash->error(__('There was a problem submitting your form.'));
            }
        } else {
            $this->Flash->error(__('Please check your Recaptcha Box.'));
        }
    }
}

file7.txt

<?= $this->Form->create() ?>
<?= $this->Recaptcha->display() ?>

<?= $this->Form->input('name', [
  'label' => __('Your Name'),
  // 'default' => $this->request->query('name'); // in case you add the Prg Component
]) ?>
<?= $this->Form->input('message', [
  'type' => 'textarea',
  // 'default' => $this->request->query('message'); // in case you add the Prg Component
  'label' => __('Your Message')
]) ?>

<?= $this->Form->button(__('OK')) ?>
<?= $this->Form->end() ?>