Captcha In The Layout (Global Captcha)

Hi!
It gets a hard long time to find out that every Captcha belongs to a controller. Now I have a little contact form in footer of site. ContactController is responsible for this form. How to generate a Captcha that can be validate by this controller, but render in every single action of other controllers? I've found a 'controller' property in CCaptcha but it's read-only...