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+====== Audio Captcha ======
+Notice: this is a draft of unpublished working paper by Takuya Nishimoto.
+Part of the work is published in Japanese.
+===== References =====
+  * Luis von Ahn, Benjamin Maurer, Colin McMillen, David Abraham, Manuel Blum. "reCAPTCHA: Human-Based Character Recognition via Web Security Measures," SCIENCE, VOL 321, 12, 1465, Sept 2008.
+  * Inaccessibility of CAPTCHA, Alternatives to Visual Turing Tests on the Web, W3C Working Group Note 23 November 2005. http://www.w3.org/TR/turingtest/
+  * Jennifer Tam, Jiri Simsa, Sean Hyde, Luis Von Ahn: "Breaking Audio CAPTCHAs," Proc. of NIPS, 2008.
+===== Introduction =====
+CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart) are popular security techniques on the World Wide Web that prevent automated programs from abusing online services.
+A typical CAPTCHA is an image containing several distorted characters that appears at the Web forms.
+Users are asked to type the distorted dirty characters to let the system know that they are human.
+Image-based CAPTCHAs are, however, preventing Web use of persons with visual disability.
+Audio CAPTCHAs were created to solve this accessibility issue.
+Within the CAPTCHA tasks, the difference of recognition performance between human and machine can be used as a criterion of the safeness.
+Viewied from another side, mental workload for human in listening audio CAPTCHA tasks has not been investigated very much.
+Design method of CAPTCHA task in consideration of safety and ease-of-use is described in this paper.