Captcha/Luis von Ahn/Mayer-Schönberger: Ahn's original captcha represents random characters and digits that had to be typed correctly. His later re-captcha, on the other hand, brought combinations of two words from failed attempts by computers to read texts automatically. (1)
Both times it is about the user demonstrating by typing that he/she is a human being and not a machine. To ensure security, the system presents the same random words to about 5 people at a time.(2) ((s) Then unanimity decides.)
1. Story of Luis von Ahn—Based on Cukier interviews with von Ahn from 2010. See also Clive Thompson, “For Certain Tasks, the Cortex Still Beats the CPU,” Wired, June 25, 2007 (http://www.wired.com/techbiz/it/magazine/15-07/ff_humancomp?currentPage=all); Jessie Scanlon, “Luis von Ahn: The Pioneer of ‘Human Computation,’” Businessweek, November 3, 2008 (http://www.businessweek.com/stories/2008-11-03/luis-von-ahn-the-pioneer-of-human-computation-businessweek-business-news-stock-market-and-financial-advice).
2. His technical description of reCaptchas is at Luis von Ahn et al., “reCAPTCHA: Human-Based Character Recognition via Web Security Measures,” Science 321 (September 12, 2008), pp. 1465–68 (http://www.sciencemag.org/content/321/5895/1465.abstract)._____________Explanation of symbols: Roman numerals indicate the source, arabic numerals indicate the page number. The corresponding books are indicated on the right hand side. ((s)…): Comment by the sender of the contribution. The note [Author1]Vs[Author2] or [Author]Vs[term] is an addition from the Dictionary of Arguments. If a German edition is specified, the page numbers refer to this edition.
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