Computer Network Security

The reCAPTCHA project is an attempt to make good use of the effort humans put into solving CAPTCHAs [322]. In reCAPTCHA, a user is shown two distorted words, where one of the words is an actual CAPTCHA, but the other is a word—distorted to look like a CAPTCHA—that an optical character recognition (OCR) program was unable to recognize. If the real CAPTCHA is solved correctly, then the reCAPTCHA program assumes that the other word was also solved correctly. Since humans are good at correcting OCR errors, reCAPTCHA can be used, for example, to improve the accuracy of digitized books.

a. It is estimated that about 200,000,000 CAPTCHAs are solved daily. Suppose that each of these is a reCAPTCHA and each requires about 10 seconds to solve. Then, in total, about how much time would be spent by users solving OCR problems each day? Note that we assume two CAPTCHAs are solved for one reCAPTCHA, so 200,000,000 CAPTCHAs represents 100,000,000 reCAPTCHAs.

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b. Suppose that when digitizing a book, on average, about 10 hours of human effort is required to fix OCR problems. Under the assumptions in part a, how long would it take to correct all of the OCR problems created when digitizing all books in the Library of Congress? The Library of Congress has about 32,000,000 books, and we assume that every CAPTCHA in the world is a reCAPTCHA focused on this specific problem.

c. How could Trudy attack a reCAPTCHA system? That is, what could Trudy do to make the results obtained from a reCAPTCHA less reliable?

d. What could the reCAPTCHA developer do to minimize the effect of attacks on the system?

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