International Journal of Advanced Research in Computer Science and Software Engineering (IJARCSSE)
CAPTCHAs are short for Completely Automated Public Turing test to tell Computer and Humans Apart. The purpose of a CAPTCHA is to block form submissions from spam bots - automated scripts that harvest email addresses from publicly available web forms. The term \"CAPTCHA\" was coined in 2000 by Luis Von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University, and John Langford (then of IBM). CAPTCHAs are used because of the fact that it is difficult for the computers to extract the text from such a distorted image, whereas it is relatively easy for a human to understand the text hidden behind the distortions.