Blind and Human: Exploring More Usable Audio CAPTCHA Designs

For people with visual impairments (PVIs), audio CAPTCHAs are accessible alternatives to standard visual CAPTCHAs. However, current audio CAPTCHA designs are slower to complete and less accurate than their visual counterparts. We designed and evaluated four novel audio CAPTCHAs that we hypothesized would increase accuracy and speed. To evaluate our designs along these measures, we ran a three-session, within-subjects experiment with 67 PVIs from around the world --- the majority being from the U.S. and India. Thirty three participants completed all three sessions, each separated by one week. These participants completed a total of 39 distinct audio CAPTCHA challenges across our prototype designs and the control, all presented in random order. Most importantly, all four of our new designs were significantly more accurate and faster than the control condition, and were rated as preferable over the control. A post-hoc security evaluation suggested that our designs had different strengths and weaknesses vis-a-vis two adversaries: a random guessing adversary and a NLP adversary. Ultimately, our results suggest that the best design to use is dependent on use-context.