Coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford, CAPTCHA is short for “Completely Automated Public Turing test to tell Computers and Humans Apart”. It is a type of challenge-response test used by most websites to distinguish between a human user and a robot as a security measure to keep spam away. CAPTCHA can be just letters, pictures, numbers or a combination of two or more of these.
This CAPTCHA type relies on a human’s ability to generalize and recognize novel patterns based on variable past experience. In contrast, bots can often only follow set patterns or input randomized characters. This limitation makes it unlikely that bots will correctly guess the right combination.
Since CAPTCHA was introduced, bots that use machine learning have been developed. These bots are better able to identify traditional CAPTCHAs with algorithms trained in pattern recognition. Due to this development, newer CAPTCHA methods are based on more complex tests. For example, reCAPTCHA requires clicking in a specific area and waiting until a timer runs out.
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