What's up CAPTCHA?: a CAPTCHA based on image orientation
Proceedings of the 18th international conference on World wide web
Breaking a 3d-based CAPTCHA scheme
ICISC'11 Proceedings of the 14th international conference on Information Security and Cryptology
A survey and analysis of current CAPTCHA approaches
Journal of Web Engineering
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It has become a necessity these days to differentiate between a human and a bot trying to access some particular computer resource to prevent its misuse. The Turing Test is a test for determining whether or not machine intelligence can converse like a human. This test has three participants -- two subjects and judge. One of the subjects is a person and the other is a computer. Both subjects are hidden from the view of the judge. They communicate with the judge via text-only channels. The role of the judge is to determine which text channel corresponds to the human and which corresponds to the computer. If the judge cannot determine this, then the computer passes the test. Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) is similar to Turing Test but the judge is a computer. The basic idea is that the distorted image of a sequence of letters and numbers is shown to a user and asked to type that sequence. The user will have no problem to recognize all letters and numbers, but the computer programs or bots cannot recognize them. The CAPTCHA is usually a simple visual test or puzzle that a human can complete without much difficulty, but an automated program cannot understand.