CAPTCHAs
The Internet has established firm deep roots in our day to day life. It has brought many revolutionary changes in the way we do things. One important consequence has been the way it has replaced human to human contact. This has also presented us with a new issue which is the requirement for differentiating between real humans and automated programs on the Internet. Such automated programs are usually written with a malicious intent. CAPTCHAs play an important role in solving this problem by presenting users with tests which only humans can solve. This chapter looks into the need, the history, and the different kinds of CAPTCHAs that researchers have come up with to deal with the security implications of automated bots pretending to be humans. Various schemes are compared and contrasted with each other, the impact of CAPTCHAs on Internet users is discussed, and to conclude, the various possible attacks are discussed. The author hopes that the chapter will not only introduce this interesting field to the reader in its entirety, but also simulate thought on new schemes.