Telling humans and computers apart automatically
Communications of the ACM - Information cities
Recognizing objects in adversarial clutter: breaking a visual captcha
CVPR'03 Proceedings of the 2003 IEEE computer society conference on Computer vision and pattern recognition
Usability of CAPTCHAs or usability issues in CAPTCHA design
Proceedings of the 4th symposium on Usable privacy and security
A survey and analysis of current CAPTCHA approaches
Journal of Web Engineering
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We present an implementation of CAPTCHA image generation as a REST-style web service, currently available at http://captchaservice.org. We argue that CAPTCHA generation is well suited to a web services approach, particularly one powered by open-source code, and discuss techniques for using such a service to protect weblogs from comment spam attacks. We describe the captchservice.org API by example, and detail the workings of the two image-distortion techniques that the service offers. We also discuss accessibility objections to visual CAPTCHAs, describe our early attempts at non-visual alternatives, and summarize future development directions.