Resource sharing is the most attractive feature of distributed computing. Information is also a kind of resource. The portable computing devices and wireless networks are playing a dominant role in enhancing the information sharing and thus in the advent of many new variants of distributed computing viz. ubiquitous, grid, cloud, pervasive and mobile. However, the open and distributed nature of Mobile Ad Hoc Networks (MANETs), Vehicular Ad Hoc Networks (VANETs) and cloud computing systems, pose a threat to information that may be coupled from one user (or program) to another. The chapter illustrates the general characteristics of ad hoc networks and computing models that make obligatory to design secure protocols in such environments. Further, we present a generic classification of various threats and attacks. In the end, we describe the security in MANETs, VANETs and cloud computing. The chapter concludes with a description of tools that are popularly used to analyze and access the performance of various security protocols.
