Evaluating Existing Audio CAPTCHAs and an Interface Optimized for Non-Visual Use



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INTRODUCTION AND MOTIVATION

The goal of a CAPTCHA1 is to dierentiate humans
from automated agents by requesting the solution to a
problem that is easy for humans but dicult for com-
puters. CAPTCHAs are used to guard access to web re-
sources and, therefore, prevent automated agents


Making CAPTCHAs Accessible

Audio CAPTCHAs were introduced soon after their vi-
sual alternatives [22, 9], and have been slowly adopted
by web sites using visual CAPTCHAs since that time.
Although the adoption of audio CAPTCHAs has been
slower than that of visual CAPTCHAs, many popular
sites now include audio alternatives, including services
oered by Google and Microsoft. Over 2600 web users
have signed a petition asking for Yahoo to provide an
accessible alternative [25]. The reCAPTCHA project,
a popular, centralized CAPTCHA service with the goal
of improving the automated OCR (Optical Character
Recognition) processing of books also provides an audio
alternative. Although audio CAPTCHAs exist, their
usability has not been adequately examined.


Other Alternatives

Because audio CAPTCHAs remain dicult to use and
are not oered on many web sites, several alternatives
have been developed supporting access for blind web
users. Many sites require blind web users to call or
email someone to gain access. This can be slow and de-
tracts from the instant gratication aorded to sighted
users. The WebVisum Firefox extension enables web
users to submit requests for CAPTCHAs to be solved,
which are then forwarded to their system to be solved
by a combination of automated and manual techniques
[24]. Because of the potential for abuse, the system is
currently oered by invitation only and questions re-
main about its long-term eectiveness. For many blind
web users the best solution continues to be asking a
sighted person for assistance when required to solve a
visual CAPTCHA.


Targeting Non-Visual Access

The interface that we developed for solving audio CAPT-
CHAs builds on work considering the development of
non-visual interfaces. Such interfaces are often very dif-
ferent than the interfaces developed for visual use even
though they enable equivalent interaction. For instance,
in the math domain, specialized interfaces have been
developed to make navigation of complex mathematics
feasible in the linear space exposed by non-visual inter-
faces [16]. Emacspeak explores the usability improve-
ment resulting from applications designed for non-visual
access instead of being adapted from visual interfaces
[17].


EVALUATION OF EXISTING CAPTCHAS

Many web services now oer audio CAPTCHAs because
they believe them to be an accessible alternative to vi-
sual CAPTCHAs. However, the accessibility and us-
ability of these audio CAPTCHAs has not been exten-
sively evaluated. Our initial study aims to evaluate the
accessibility of existing audio CAPTCHAs and search
for implications we could use to improve them. We
did this by gathering currently used CAPTCHAs from
the most popular web services and presented them to
study participants to solve. During the study, we col-
lected tracking data to investigate the means by which
both sighted and blind users solve CAPTCHAs. The
tracking data we collected allowed us to analyze the
timing (from page load to submit) of every key pressed
and button clicked, and search for problem areas and
possible improvements to existing CAPTCHAs.


Results
Of our 162 participants, 89 were blind and 73 were
sighted; 56 were female, 99 were male, and 7 chose not
to answer that question; and their ages ranged from 18
to 69 with an average age of 38.0 (SD = 13.2).
Before participating in our study, blind and sighted
participants showed diering levels of frustration to-
ward the audio and visual CAPTCHAs they had al-
ready come across. Participants were asked to rate
the following questions on a scale from Strongly Agree
(1) to Strongly Disagree (5) or opt out by answering \I
have never independently solved a visual[audio] CAPT-
CHA" for the following questions: \Audio CAPTCHAs
are frustrating to solve." and \Visual CAPTCHAs are
frustrating to solve."