DISTINGUISH VALID USERS FROM BOTS, OCRs AND THIRD PARTY SOLVERS WHEN PRESENTING CAPTCHA

A method and system of administering a CAPTCHA challenge comprising: providing a CAPTCHA challenge to the client computer, the CAPTCHA challenge comprising an image and browser code for displaying the image, the image comprising a character string in which a first sub string smaller than said character string corresponds to a solution to the CAPTCHA challenge and a second substring smaller than said character string corresponds to extra characters not part of the solution, displaying the image on the client computer wherein only the first substring is visible in the browser, receiving a response from the client computer, comparing the response with the second substring, and determining, if the response includes the second substring, that the client computer used a CAPTCHA bot or relay.