  Providing security for webservers against unwanted and automated
registrations has become a big concern. To prevent these kinds of false
registrations many websites use CAPTCHAs. Among all kinds of CAPTCHAs OCR-Based
or visual CAPTCHAs are very common. Actually visual CAPTCHA is an image
containing a sequence of characters. So far most of visual CAPTCHAs, in order
to resist against OCR programs, use some common implementations such as
wrapping the characters, random placement and rotations of characters, etc. In
this paper we applied Gaussian Blur filter, which is an image transformation,
to visual CAPTCHAs to reduce their readability by OCR programs. We concluded
that this technique made CAPTCHAs almost unreadable for OCR programs but, their
readability by human users still remained high.
