Daniel Carrera wrote:
How do they verify if you entered the correct word? A captcha is only useful if the server can verify the answer.
To answer my own question: The captcha actually gives you two words. One for which the answer is known and one for which it isn't. The captcha is valid if the known word is typed correctly. So the security of the captcha depends only on the word that is known. I don't know whether the known word is taken from the words that the OCR could read or from the words that other users have already confirmed. The latter would be more effective at stopping spam.
Daniel.
