A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't:The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. At the time, they developed the first CAPTCHA to be used by Yahoo.
Applications of CAPTCHAsCAPTCHAs have several applications for practical security, including (but not limited to):
- Preventing Comment Spam in Blogs. Most bloggers are
familiar with programs that submit bogus comments, usually for the
purpose of raising search engine ranks of some website (e.g., "buy penny
stocks here"). This is called comment spam. By using a CAPTCHA, only
humans can enter comments on a blog. There is no need to make users sign
up before they enter a comment, and no legitimate comments are ever