CAPTCHA challenge strings: problems and improvements

Jon Bentley; Colin Mallows

doi:10.1117/12.650644

16 January 2006 CAPTCHA challenge strings: problems and improvements

Jon Bentley, Colin Mallows

Author Affiliations +

Proceedings Volume 6067, Document Recognition and Retrieval XIII; 60670H (2006) https://doi.org/10.1117/12.650644
Event: Electronic Imaging 2006, 2006, San Jose, California, United States

Abstract

A CAPTCHA is a Completely Automated Public Test to tell Computers and Humans Apart. Typical CAPTCHAs present a challenge string consisting of a visually distorted sequence of letters and perhaps numbers, which in theory only a human can read. Attackers of CAPTCHAs have two primary points of leverage: Optical Character Recognition (OCR) can identify some characters, while nonuniform probabilities make other characters relatively easy to guess. This paper uses a mathematical theory of assurance to characterize the probability that a correct answer to a CAPTCHA is not just a lucky guess. We examine the three most common types of challenge strings, dictionary words, Markov text, and random strings, and find substantial weaknesses in each. We therefore propose improvements to Markov text, and new challenges based on the consonant-vowel-consonant (CVC) trigrams of psychology. Theory and experiment together quantify problems in current challenges and the improvements offered by modifications.

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Jon Bentley and Colin Mallows "CAPTCHA challenge strings: problems and improvements", Proc. SPIE 6067, Document Recognition and Retrieval XIII, 60670H (16 January 2006); https://doi.org/10.1117/12.650644

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