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With the growing and prevalent usage of Internet, threat for the service providers has arisen. One of the most important aspects of modern security concerns is to deal with users that are computer programs or bots. These programs pretend to be human users and exploit servers by submitting data automatically, thereby, hindering the services of the server. Security mechanism dealing with such attacks is called Completely Automated Turing test to tell Computers and Humans Apart (CAPTCHA). This paper discusses and analyzes broad CAPTCHA mechanisms and recent researches that have either reformed or flunked, previous techniques and methods. Also, a technique is proposed that counteracts the shortcomings or inabilities of such techniques.

Mobile Ad hoc NETwork (MANET) is one of the important and unique applications. MANET does not require a fixednetwork infrastructure, every single node works as both a transmitter and a receiver. Nodes communicate directly witheach other when they are both within the same communication range else they can propagate message to neighbor nodesto pass the message. A new intrusion detection system named Enhanced Adaptive Acknowledgement (EAACK) speciallydesigned for MANETs. By the adoption of MRA scheme, EAACK is capable of detecting malicious nodes despite theexistence of false misbehavior report. The results will demonstrate positive performances against Watchdog, TWOACKand AACK in the cases of receiver collision, limited transmission power and false misbehavior report. EAACKdemonstrates higher malicious behavior detection rates in certain circumstances while does not greatly affect the networkperformances. EAACK is designed based on the Digital signature Algorithm (DSA) and RSA. Those techniques havedrawback due to network overhead. We need techniques for security like encryptions, hybrid encryption or sign encryptionto protect our message.