Secure On-line Key Generation for MANETs by Fragment Assembly

Traditional key-management requires either the use of public key cryptography or that an on-line trusted third party arbitrate the selection and distribution of symmetric keys to communicating nodes. Recently much work has been published on random key predistribution methods, in which keys are predistributed between a limited set of node pairs and are used to bootstrap other required node pairs. We propose an alternate scheme, in which key fragments rather than entire cryptographic keys are predistributed to nodes. This modification leads to a different security analysis model and yields substantially better security guarantees than basic random key predistribution for a fixed amount of per node storage. I. I NTRODUCTION We propose a key-management scheme that allows nodes to communicate securely without requiring access to an on-line trusted key-management system or the use of public key cryptography. Such a scheme is suitable for deployment in wireless ad-hoc networks, which exhibit low reliability and are composed of resource constrained devices. We focus on defending against adversaries capable of capturing a bounded number of network nodes and using the information present in these nodes to subvert system operations. Resource requirements increase with the number of colluding adversaries. A relatively small symmetric key (on the order of 128 bits) is sufficient to secure a communications channel with a high level of security. It is, however, impractical in a large network for each pair of nodes to share this amount of information. We seek to maximize security given a constraint on the amount of storage available at a node for the storage of such cryptographic information. A trusted key-management service could be used to distribute pairwise keys, however, this service must be constantly available throughout the lifetime of the network. State of the art solutions use public key cryptography and signed key certificates to provide high resilience against colluding adversaries but require a significant amount of storage at each node. Unfortunately, assymetric cryptographic systems are very expensive, so that, despite continued improvements in computational power, lowand mediumend devices will continue to experience difficulty implementing assymetric cryptosystems secure against high-end devices for the foreseable future. In [10] the authors present a randomized key pre-distribution method for Distributed Sensor Networks (DSNs) that enables nodes to be pre-configured with a set of pre-chosen cryptographic keys. This scheme was designed to protect only against adversaries without knowledge of secret information at other nodes. Our scheme is related, but allows nodes to communicate securely in the presence of colluding adversaries. Our scheme functions by a dealer choosing a master secret, and distributing to nodes in the network a small fraction of this master secret. Any pair of initialized nodes can then construct a symmetric shared key from the intersection of their fractions of the master secret. We determine rules for choosing the size of the master secret and the size of the fragments distributed to nodes to maximize the security achieved against an adversary with a given number of colluding partners.