For Joomla registration and contact page, it is necessary to protect them from spam bots. Joomla 2.5 comes with built-in ReCpatcha plugin. This allows you to protect the Joomla registration and contact forms from spam bots.
The plugin can be managed from the Joomla 2.5 admin area -> Extensions -> Extension Manager -> Plug-in Manager ->Captcha – ReCaptcha.
You can obtain the API keys from http://google.com/recaptcha and enter them in the Public and Private Key fields. Save the configuration.