A Survey on Security in Mobile Peer-to-Peer Architectures—Overlay-Based vs. Underlay-Based Approaches <s> Countermeasures in MP2P <s> We propose a straightforward technique to provide peer-to-peer security in mobile networks. We show that far from being a hurdle, mobility can be exploited to set up security associations among users. We leverage on the temporary vicinity of users, during which appropriate cryptographic protocols are run. We illustrate the operation of the solution in two scenarios, both in the framework of mobile ad hoc networks. In the first scenario, we assume the presence of an offline certification authority and we show how mobility helps to set up security associations for secure routing; in this case, the security protocol runs over one-hop radio links. We further show that mobility can be used for the periodic renewal of vital security information (e.g., the distribution of hash chain/Merkle tree roots). In the second scenario, we consider fully self-organized security: Users authenticate each other by visual contact and by the activation of an appropriate secure side channel of their personal device; we show that the process can be fuelled by taking advantage of trusted acquaintances. We then show that the proposed solution is generic: It can be deployed on any mobile network and it can be implemented either with symmetric or with asymmetric cryptography. We provide a performance analysis by studying the behavior of the solution in various scenarios. <s> BIB001 </s> A Survey on Security in Mobile Peer-to-Peer Architectures—Overlay-Based vs. Underlay-Based Approaches <s> Countermeasures in MP2P <s> The Scalable Source Routing (SSR) protocol combines overlay-like routing in a virtual network structure with source routing in the physical network to a single cross-layer architecture. Thereby, it can provide indirect routing in networks that lack a well-crafted structure. SSR is well suited for mobile ad hoc networks, sensor-actuator networks, and especially for mesh networks. Moreover, SSR directly provides the routing semantics of a structured routing overlay, making it an efficient basis for the scalable implementation of fully decentralized applications. In this paper we analyze SSR with regard to security: We show where SSR is prone to attacks, and we describe protocol modifications that make SSR robust in the presence of malicious nodes. The core idea is to introduce cryptographic certificates that allow nodes to discover forged protocol messages. We evaluate our proposed modifications by means of simulations, and thus demonstrate that they are both effective and efficient. <s> BIB002 </s> A Survey on Security in Mobile Peer-to-Peer Architectures—Overlay-Based vs. Underlay-Based Approaches <s> Countermeasures in MP2P <s> The growth of mobile devices with near PC equivalent capabilities has brought with it the possibility of mobile P2P systems. However, the nature of mobile devices brings additional issues that need to be considered, especially for when considering system security. This paper presents our initial work on the EU funded PEPERS project that seeks to support the development of secure mobile P2P applications. A core part of this support is with the design and development of a mobile P2P security platform. <s> BIB003
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 Douceur 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 Danezis 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. Otherwise, a routing approach would be required that is based on flooding the network when the requested node does not exist. On the other hand, generating and maintaining a second routing table in parallel to the main routing table as proposed by Castro et al. could consume large amounts of the available bandwidth. Still, this approach could be reasonable in MP2P implementations that maintain routing tables by harnessing overheard messages. To sum it up, the resource restriction of MP2P networks affects most security measures that were proposed for P2P systems operating on top of a less resource-constraint underlay such as the Internet. Most security mechanisms proposed for MANETs are still applicable in the context of MP2P but can be improved by harnessing cross-layer information. To the best of our knowledge, only sparse related work on security in MP2P networks exists.Čapkun et al. proposed a face-to-face authentication system for MP2P networks BIB001 . As nodes are mobile, nodes can be carried during bootstrapping to the bootstrapping node for a direct authentication. The authentication process is based on social interaction as the owner of the bootstrapping node has to decide whether a node may join the network. Kutzner et al. propose a cryptographic approach to secure the integrated Scalable Source Routing (SSR) protocol BIB002 . They authenticate routing messages and certify links to prevent malicious nodes from forging messages based on a PKI. In the PEPERS project BIB003 , a basic framework for MP2P security mechanisms was developed. By now, only the framework itself is introduced and no further MP2P security mechanisms were proposed.