Math Comment Spam Protection asks the visitor making the comment to answer a simple math question. This is intended to prove that the visitor is a human being and not a spam robot.
Example of such question: Sum of 9 + 9 ?
This plugin does not require JavaScript or cookies. It uses a special encryption function; the result of the question is being passed via a hidden field. The result is being compared with the value entered by the visitor – after encrypring this value as well.
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the official website for further details and the latest information on this plugin.