Patent ID: 8307407

Claim:
A method comprising: producing, by a computer, a first set of Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHAs) whose images are based partially on an alphanumeric sequence, wherein each CAPTCHA in the first set has a common value for a CAPTCHA property, wherein the common value for the CAPTCHA property associated with a linguistic characteristic of the alphanumeric sequence; producing, by the computer, a second set of CAPTCHAs whose images are based partially on an alphanumeric sequence, wherein none of the CAPTCHAs in the second set has the common value for the CAPTCHA property shared by the CAPTCHAs in the first set; displaying, by the computer, the first set of CAPTCHAs and the second set of CAPTCHAs; and determining whether a user passes a CAPTCHA challenge to distinguish the first set of CAPTCHAs from the second set of CAPTCHAs based on a user selection of all the CAPTCHAs belonging to either the first set of CAPTCHAs or the second set of CAPTCHAs from the displayed CAPTCHAs, wherein the CAPTCHA challenge is associated with a required skill set such that: a person with the required skill set passes the CAPTCHA challenge; a person without the required skill set does not pass the CAPTCHA challenge; and an automated tool does not acquire the required skill set.