One of the features added in Joomla! 2.5 version in reCaptcha in core. If you have installed Joomla! in 2.5 (or Joomla! 3.2) version reCaptcha plugin should be visible in Plugin Manger view, but before you start configuring plugin settings you have to get captcha keys.
reCaptcha mechanism demands two keys: public and private and both you can generate on The service is free but you will need a Google account to get keys for your domain.
Once you have keys all that you need is login to your Joomla! administration panel and go to Extensions > Plugin Manager. Find on list Captcha – ReCaptcha plugin, select it, paste your keys and set Status to Enabled. Remember to save your settings by clicking ‘Save’ button. Additionally, plugin settings allows you to choose color theme for the captcha form, ‘clean’ option is almost the best style for all occasions.
Captcha settings in Joomla
If you want to protect contact form you should go to Site > Global Configuration > Site (tab) and from the Default Captcha list, select Captcha – ReCaptcha. If you want to use captcha also to prevent no human registartion go Users > User Manager and click Options icon on the right side. From Capcha list choose reCaptcha plugin, save changes – and this is all you have to do. This reCaptcha security images will also work with reset password and remind username options.
Important! If your reset password/remind username pages shows message ‘ReCaptcha plugin needs a public key to be set in its parameters. Please contact a site administrator.’ it is strictly related with reCaptcha mechanism limitations.
Currently, the Google captcha mechanism offer only one captcha form per page so when the mod_gk_register module is enabled on mentioned pages this cause some problems. In this case, captcha plugin try to put two forms in the same page, one in register pop-up window and second in remind/reset area. Solution for this problem is very simply, mod_gk_register module should be unpublished on this pages.