chapter, the machine learning module is intended to be used to help decide whether the challenge/response test is needed, or even drop the intercepted call. 23.4.1.5 Challenge/Response Module Challenge/response (C/R) is a common authentication/registration technique whereby an entity is prompted to provide a valid answer or some informa- tion in order to have access to computational resources. A well-known chal- lenge/response protocol is user authentication via pass phrase, where a service provider is asking for a pass phrase (challenge) and the valid answer (response) is the correct password. Since C/R is widely used to prevent spam, it can also be used to prevent SPIT. The idea behind a C/R spam preventing system is that the spammer is not willing to consume time or resources to respond to a challenge, while people who are interested in contacting with the callee will be willing to spend a minute or part of their resources. There are two main C/R categories that can be considered for the anti-SPIT scenario. The first one is about economic solutions, SO called computational puzzles. These puzzles prompt the SIP client to perform a calculation that limits the rate of incoming SPIT calls and increases their establishment cost. The second one is about puzzles, which can tell humans and computers apart. © 2009 by Taylor & Francis Group, LLC A Conceptual Architecture for SPIT Mitigation These are puzzles, which are considered easy for a human and difficult for an automated process to solve. The main advantage of the C/R mechanism is that it can utilize any Turing test implementation. Everyone who wishes to implement such a mechanism can use visual, audio or commonsense challenges. Moreover, a single imple- mentation can support multiple options according to the capabilities of the devices and the preferences of the caller. So, if a caller holds a device without a screen, the challenge could be audio. On the other hand, if the caller is hear- ing impaired or is a non-native speaker then this caller must be submitted 