Authentication and encryption are very important factors in GSM system security. This book explains the authentication and encryption scheme used in GSM. It explains the A3, A5 and A8 protocols used in the security mechanism in GSM. It explains the different possible attacks on cellular systems, which include SIM cloning, eavesdropping, location tracking, SMS ping, SMS denial of service, authentication denial of service and SMS spam. It classifies these attacks on the basis of target of the attack, type of attack, motivation and severity. It points out the vulnerabilities that are exploited by attacks on cellular systems. The book also looks into how risk analysis, risk modeling and fuzzy logic can be applied to security in mobile systems. It proposes a fuzzy logic based risk model to secure Short Message Service in GSM based cellular networks. The proposed model uses a hybrid approach towards risk modeling and the model continues to evolve with constant feedback from the user. This approach puts the user in control and allows the model to be refined with time to better match the existing circumstances.