A CAPTCHA based on the human visual systems masking characteristics

In this paper, a CAPTCHA is presented based on the masking characteristics of the Human Visual System (HVS). Knowing that noise can be masked by high activity regions and showing that edges can be masked by noise for a human observer while still being detected by machines, the suggested CAPTCHA is composed of English alphabets that are picked randomly and written with a combination of texture and edges with added noise such as to deceive the machine by randomly changing the visibility of characters for humans. The proposed CAPTCHA is highly legible and robust to brute-force attacks and sophisticated Object Character Recognition (OCR) segmentation algorithms.