## 5 M2M Security  
The emergence of the M2M ecosystem has led to the revision of the conventional network security paradigm [127]. The M2M communication systems, by essence, interconnect heterogeneous network segments, where the heterogeneity is expressed in terms of functional capabilities and capacities. This encompasses, for instance, the interconnection of a WAN with low-power network segments like WSNs in multihop communication schemes. The security of communications involving several hops is usually addressed using hop by hop security where each segment of the communication path from source to destination is secured using distinct credentials possibly managed by distinct parties involving rekeying operations at each transmission node. Credential management with this model may be complex. Furthermore, in order to provide reasonable security, the various parties involved in credentials distribution and management needs to trust each other. This requirement is often difficult to achieve in the case of M2M communications and it is generally accepted that end to end security involving the use of a single set of credential from source to destination is a better model. However the deployment of end to end security is a challenging task as it requires solving the problem of credential distribution at a global level. A typical network security service (e.g., authentication, access control, data confidentiality, _etc._) is made up of two phases: bootstrapping and enforcement. Providing an end-to-end security service in M2M requires thus exploring methods for both phases, where the said methods should take into account both the interoperability and trust issues. Security service bootstrapping comprises authentication and key agreement methods needed for the security enforcement phase. Therefore, given the interoperability and trust challenges in M2M systems, a flexible and adaptive bootstrapping phase is needed in order to guarantee the enforcement of an efficient end-to-end security service. M2M communications are commonly classified into three main communication domains: M2M device, network and application domains. Those three domains are associated to three distinct security domains involving generally distinct business actors to perform credential management. Thus, the device domain corresponds to M2M capillary communications occurring in a LAN or PAN proximity network. Generally the owner of this network has the responsibility to manage the credentials used to secure those communications occurring between the devices and the gateway. In some cases there is a need to secure the connection between the Gateway/device and the point of entry to the Internet (WAN). This is the case when using 3 GPP communications to protect the radio transmission. In this case, the SIM card holding credentials used to secure the access to the wide area network is managed by the mobile network operator to achieve \"network access security\". Table 5 lists the major security technologies used in the LAN (device) domains to offer, e.g., authentication, integrity and confidentiality. Table 6 lists the major network technologies, or security technologies used to secure the network access. These technologies provide, e.g., interoperability for secure communication in WANs but in some cases also secure and interoperable communication between devices in WANs and devices in LANs. Table 7 lists security technologies that have been or can be used at application layer, e.g., in M2M services, and also in M2M architectures.