CAPTCHA
A CAPTCHA (/ˈkæp.tʃə/ KAP-chə) is a type of challenge–response test used in computing to determine whether the user is human in order to deter bot attacks and spam.
[11] A normal CAPTCHA test only appears if the user acts like a bot, such as when they request webpages, or click links too fast.
[12] In 2001, PayPal used such tests as part of a fraud prevention strategy in which they asked humans to "retype distorted text that programs have difficulty recognizing.
[17] CAPTCHAs are automated, requiring little human maintenance or intervention to administer, producing benefits in cost and reliability.
[18] Modern text-based CAPTCHAs are designed such that they require the simultaneous use of three separate abilities—invariant recognition, segmentation, and parsing to complete the task.
According to an article by Ahn, Blum and Langford,[23] "any program that passes the tests generated by a CAPTCHA can be used to solve a hard unsolved AI problem.
[23] CAPTCHAs based on reading text—or other visual-perception tasks—prevent blind or visually impaired users from accessing the protected resource.
The use of CAPTCHA thus excludes a small percentage of users from using significant subsets of such common Web-based services as PayPal, Gmail, Orkut, Yahoo!, many forum and weblog systems, etc.
Although these are much easier to defeat using software, they are suitable for scenarios where graphical imagery is not appropriate, and they provide a much higher level of accessibility for blind users than the image-based CAPTCHAs.
[32] Two main ways to bypass CAPTCHA include using cheap human labor to recognize them, and using machine learning to build an automated solver.
[33] According to former Google "click fraud czar" Shuman Ghosemajumder, there are numerous services which solve CAPTCHAs automatically.
[22] As a result, there were many instances in which CAPTCHAs were of a fixed length and therefore automated tasks could be constructed to successfully make educated guesses about where segmentation should take place.
Modern CAPTCHAs like reCAPTCHA rely on present variations of characters that are collapsed together, making them hard to segment, and they have warded off automated tasks.
[35] In October 2013, artificial intelligence company Vicarious claimed that it had developed a generic CAPTCHA-solving algorithm that was able to solve modern CAPTCHAs with character recognition rates of up to 90%.
[37] In August 2014 at Usenix WoOT conference, Bursztein et al. presented the first generic CAPTCHA-solving algorithm based on reinforcement learning and demonstrated its efficiency against many popular CAPTCHA schemas.
[35] In October 2018 at ACM CCS'18 conference, Ye et al. presented a deep learning-based attack that could consistently solve all 11 text captcha schemes used by the top-50 popular websites in 2018.
[48] Microsoft (Jeremy Elson, John R. Douceur, Jon Howell, and Jared Saul) claim to have developed Animal Species Image Recognition for Restricting Access (ASIRRA) which ask users to distinguish cats from dogs.
