## 5 Attacks and Countermeasures
### Countermeasures in MP2P  
For both P2P systems and MANETs, countermeasures for the attacks presented in the previous sections were proposed in the recent years. (Some of) the approaches may be used to provide security and robustness in MP2P networks. Yet, questions regarding possible synergy effects as well as possible limitations when the countermeasures are deployed in MP2P networks are still open. We discuss the efficiency of countermeasures for P2P systems and MANETs when applied in MP2P networks in this section. MANETs and P2P systems are vulnerable to malicious behavior during routing or data transmission. Whereas countermeasures proposed for MANETs are mostly based on trust, encryption, and monitoring the behavior of other nodes, most P2P countermeasures provide robustness by increasing the redundancy of the lookup requests and the stored objects. These redundancy-based countermeasures such as _Iterative_, _Redundant_, or _Resistant Routing_ require an increased amount of bandwidth and, thus, may be not efficient regarding the bandwidth-constraint MANET underlay of an MP2P network. Further, also _Replicas_ require an increased amount of resources, since objects have to be stored and maintained at multiple nodes. Yet, as the availability of nodes can not be guaranteed in a MANET, using _Replicas_ might be unavoidable in MP2P networks. The efficiency of the _Routing Failure Test_ depends strongly on the implementation of the MP2P system. For example, this test can not be used in architectures as _MADPastry_ since the distribution of node-IDs also depends on the number of nodes in the particular physical area. On the other hand, countermeasures developed for MANET routing algorithms are still applicable in MP2P systems as no further limitations occur due to the combination with a P2P network. Therefore, MANET countermeasures are only limited by the same well known factors as when used in a pure MANET architecture. For instance, computational power required for asymmetric encryption or predefined trust relationships can still be problematic. On the other hand, some of the security mechanisms developed for MANETs such as _Watchdog_ can be improved by harnessing cross-layer information to increase the overall security of the MP2P network. We present an example for such a cross-layer approach in Section 6.1. Similar to the countermeasures for the routing attack, countermeasures for the _Sybil_ attack in P2P systems are also limited by the MANET underlay. Therefore, the countermeasures proposed by [PERSON] that are based on resource testing (e.g., computational puzzles) are limited due to the underlay resource restrictions. Further, approaches that are based on trust or social relations such as _SybilGuard_ can be compromised as the mobile devices can get stolen easier than fixed devices locked up at home or office in, e.g., an Internet-based P2P system. On the other hand, as CAs are most likely not available in an MP2P network, binding nodes to real-world identities or charging money for each node that joins the network is hardly realizable. Yet, providing robustness of the routing algorithm by using the bootstrapping graph to establish routes as proposed by [PERSON] _et al_. is applicable in an MP2P network. Other promising countermeasures for _Sybil_ attacks in MP2P networks were originally proposed for MANETs. The _PASID-GD_ system works similar to _Watchdog_ and is efficient in MP2P networks but can be improved by harnessing cross-layer information. Altogether, a single countermeasure may be sufficient to prevent _Sybil_ attacks in MP2P networks if an integrated or a cross-layered MP2P architecture is used. Also to prevent manipulations of routing tables in P2P networks, multiple countermeasures were proposed but have to be analyzed regarding their applicability in MP2P networks. Sending test messages to each entry added to the routing table to check whether a node is available can be reasonable when the MP2P routing table stores complete underlay routes.