Four-Dimensional Usability Investigation of Image CAPTCHA

TL;DR

This study systematically evaluates the usability of image CAPTCHA design factors through eye-tracking, efficiency, effectiveness, and satisfaction measures, providing guidelines for more user-friendly CAPTCHA development.

Contribution

It offers the first comprehensive four-dimensional usability evaluation of image CAPTCHA design factors, including layout, size, quantity, and tilting angle.

Findings

Square and horizontal rectangle layouts are preferred.

Image quantities should not exceed 16 for optimal usability.

Larger images (>55x55 pixels) and tilting angles within -45 to 45 degrees are recommended.

Abstract

Image CAPTCHA, aiming at effectively distinguishing human users from malicious script attacks, has been an important mechanism to protect online systems from spams and abuses. Despite the increasing interests in developing and deploying image CAPTCHAs, the usability aspect of those CAPTCHAs has hardly been explored systematically. In this paper, the universal design factors of image CAPTCHAs, such as image layouts, quantities, sizes, tilting angles and colors were experimentally evaluated through the following four dimensions: eye-tracking, efficiency, effectiveness and satisfaction. The cognitive processes revealed by eye-tracking indicate that the distribution of eye gaze is equally assigned to each candidate image and irrelevant to the variation of image contents. In addition, the gazing plot suggests that more than 70% of the participants inspected CAPTCHA images row-by-row, which is more efficient than scanning randomly. Those four-dimensional evaluations essentially suggest that square and horizontal rectangle are the preferred layout; image quantities may not exceed 16 while the image color is insignificant. Meanwhile, the image size and tilting angle are suggested to be larger than 55 pixels x 55 pixels and within -45~45 degrees, respectively. Basing on those usability experiment results, we proposed a design guideline that is expected to be useful for developing more usable image CAPTCHAs.