ADAMAS: Interweaving unicode and color to enhance CAPTCHA security

• A new interactive CAPTCHA is proposed that offers resistance against various forms of attacks.
• The large Unicode character set is used as the input space of this CAPTCHA.
• The use of homoglyphs makes segmentation and recognition more difficult for attackers.
• A large amount of randomness is incorporated in this CAPTCHA in order to minimize patterns and prevent attacks.
• Use of color in the foreground and background of this CAPTCHA aims to balance security and usability.

We propose, implement and test a new CAPTCHA, called Adamas, which offers resistance against pre-processing and various forms of segmentation and recognition attacks. The multi-layered security approach employed in this CAPTCHA mainly comes from its use of Unicode as an input space, a virtual keyboard as the input device, homoglyphs and correlated usage of color in foreground and background as well as several layers of randomization that aim to minimize the formation of detectable patterns that can be exploited by machines. A user study conducted to measure the usability of Adamas indicates that its solving accuracy is comparable to major CAPTCHAs in use today and offers insights into factors that affect CAPTCHA usability.