# A Key Management Scheme for Mobile Wireless Sensor Networks
## 1 Introduction  
Wireless sensor networks (WSNs) are a widely employed type of ubiquitous technology. They are usually composed of low cost devices with limited communication and computational resources, and with a small memory. Although the majority of WSNs are composed of homogeneous devices, often the device that acts as a gateway (sink) has larger resources. WSNs are used for many applications, such as smart cities [1], health-care [2], industrial automation [3], smart transport systems [4] and food control [5]. Since WSNs have limited capabilities, they adopt specific solutions instead of standard network protocols (e.g., message authentication code (MAC) [6], routing [7, 8], and deployment strategies [9]). Although some WSNs use public-key cryptography [10], security is normally based on symmetric-key cryptography [11]. Therefore, two nodes within the reciprocal communication range must share a key in order to communicate. The establishment of symmetric keys is called key management. Although there are techniques for the detection of compromised nodes and for the recovery of security [12, 13], key management significantly affects the performance and the level of security of the network. Moreover, WSNs have drastically different characteristics, so there exist many valid key management schemes that can be applied to different kinds of networks. Two basic approaches [14] for the establishment of secret keys in WSNs are:  
* Plain global key (PGK), where a unique key is used in the network;
* Full pairwise keys (FPWK), where a specific key is shared by each possible couple of nodes. Since in these approaches all keys are predistributed before the deployment, the nodes do not execute any additional operation for the key establishment. Moreover, they do not require special information (e.g., deployment knowledge). The PGK approach involves a limited memory overhead,but it provides a poor level of security, since an opponent that steals a key is able to eavesdrop on all links of the network and produce fake nodes. The FPWK approach provides a better level of security, since a key can only be used to eavesdrop on a unique link. However, each node stores a number of keys equal to the quantity of nodes in the network, so a large memory area is required. Therefore, both schemes have relevant drawbacks: the PGK scheme has a poor level of security, while the FPWK approach is only compliant with small networks. WSNs have different characteristics that affect the security and allow or require specific key management solutions (e.g., heterogeneous nodes [15] and deployment knowledge [16]). However, many key management schemes have been proposed with the aim to increase the level of security of generic homogeneous WSNs, even exploiting public-key cryptography. The basic approach based on public-key cryptography has been proposed in [17]. It employs digital certificates, used for the authentication of the identification number and of the public key of a node. In a WSN, digital certificates correspond to a signature generated by the administrator of the network. This approach, like that of FPWK, provides a high protection against an adversary that has compromised some nodes. However, it requires more computational and communication resources than symmetric encryption. In order to reduce the overhead introduced by public-key cryptography without reducing the level of security, a new authentication mechanism is proposed. The novel scheme, called public-key cryptography with authentication table key management scheme (PCAT), is suitable for mobile and static WSNs. The privacy of messages used for the establishment of symmetric pairwise keys is guaranteed by public-key cryptography. Similarly to FPWK, in the proposed scheme each node stores a table with an entry for every other node in the WSN. An identification mechanism allows the authentication of valid nodes by cheeking their public keys and identification numbers. This mechanism avoids the introduction of fake nodes, since an opponent cannot have a valid secret key.