Explicit Authentication Response Considered Harmful
Automated online password guessing attacks are facilitated by the fact that most user authentication techniques provide a yes/no answer as the result of an authentication attempt. These attacks are somewhat restricted by Automated Turing Tests (ATTs, e.g., captcha challenges) that attempt to mandate human assistance. ATTs are not very difficult for legitimate users, but always pose an inconvenience. Several current ATT implementations are also found to be vulnerable to improved image processing algorithms. ATTs can be made more complex for automated software, but that is limited by the trade-off between user-friendliness and effectiveness of ATTs.