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HuMan: an accessible, polymorphic and personalized CAPTCHA interface with preemption feature tailored for persons with visual impairments


Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA) is one of the major security components in the provision of fair web access by differentiating human access from malicious, automated access by bots. Though the CAPTCHA strengthens the security aspect of web access, their accessibility to people with visual impairments has inherent unresolved challenges. This paper presents an accessible CAPTCHA model termed HuMan (human or machine?) which aims at providing an audio-based CAPTCHA for people with visual impairments. The HuMan model incorporates personalization into the CAPTCHA access. The polymorphic nature of resolving the HuMan CAPTCHA facilitates kaleidoscopic behavior in CAPTCHA rendering. The presence of ambient noise and requirement of common sense knowledge to answer the questions presented by HuMan CAPTCHA model makes it friendlier toward human users. The HuMan model has a CAPTCHA preemption feature which enables the user to stop the challenge audio as soon as the answer is identified. The results of experiments conducted on the prototype implementation of HuMan model project the mean success rate of 92.46 % and system usability scale score of 82.44 for persons with visual impairments and 82.63 for sighted users.

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Correspondence to K. S. Kuppusamy.

Appendix I: HuMan CAPTCHA Algorithm

Appendix I: HuMan CAPTCHA Algorithm


HuMan algorithm receives the url of a page as input. With detectTrace function, it checks for any trace of visit of the same page earlier from the same device. Based on this, it renders the preferences menu (renderUItoGetPref()) for the explicit scenario (E) and allows the user to choose the domain (fetchDomain()).

In the case of the implicit scenario (I), the domain is selected either from the cookies or IP address from TrackDB.

When both cookies and IP scenarios are not available, a thematic domain is chosen (MatchDomain()) by fetching title (getTitle()), meta-keywords (getMeta()) and keywords (FetchKeyTerms()) from the page.

Once the domain is chosen, a random challenge (challengeID) is selected. After the selection of a challenge, one question (qnID) is selected randomly from the list of available questions for that challenge.

Then HuMan CAPTCHA constructed with specified challengeID and qnID is rendered (RenderCAPTCHA(challengeIDqnID)).

Response from the user is gathered in uResponse through getUserResponse(). The correct answer for the challenge (cAns) is fetched through getCorrectAnswer().

As the HuMan model incorporates fuzzy answer matching to incorporate spelling errors, the Jaro–Winkler edit distance is computed (JW(uResponsecAns)) between the user response and the correct answer. If it less than the threshold value (\(\tau\)), then true is returned from the algorithm indicating successful CAPTCHA solving. Before this, the preemption point is recorded updated through addTrackingInfo(). The tracking information is used at the server side to check number of times this CAPTCHA is answered correctly.

If the edit distance is above the threshold value (\(\tau\)), then preemption point (getPreemptionpoint()), error margin (getErrorMargin()) is computed and the tracking information is updated. The algorithm returns false indicating failure of CAPTCHA solving attempt.

If the failure rate for a challenge audio or specific question belonging to that challenge crosses the specified threshold values (\(\nu , \xi\)), it is added to review (addToReview()) by the administrator.

If a response to a CAPTCHA receives a specific incorrect answer many times (above a threshold), then with the approval of administrator the answer is updated for that CAPTCHA.

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Kuppusamy, K.S., Aghila, G. HuMan: an accessible, polymorphic and personalized CAPTCHA interface with preemption feature tailored for persons with visual impairments. Univ Access Inf Soc 17, 841–864 (2018).

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  • Web accessibility
  • Accessible CAPTCHA
  • Non-visual access
  • CAPTCHA preemption