A Low-Cost Solution for Frequent Symmetric Key Exchange in Ad-hoc Networks

Next to authentication, secure key exchange is considered the most critical and complex issue regarding ad-hoc network security. We present a low-cost, (i.e. low hardware-complexity) solution for feasible frequent symmetric key exchange in adhoc networks, based on a Tree Parity Machine Rekeying Architecture. A key exchange can be performed within a few milliseconds, given practical wireless communication channels and their limited bandwidths. A flexible rekeying functionality enables the full exploitation of the achievable key exchange rates. Characteristics of a standardcell ASIC design realisation as IP-core in -technology are evaluated. 1 Key Exchange in Ad-hoc Networks Authentication and key exchange are of supreme importance with regard to security and secure routing in ad-hoc networks. Such networks typically lack infrastructure, which penalises approaches needing a central authority like a trust centre or another third trusted party as e.g. in ID-based cryptosystems. The dynamic and ephemeral network topology demands frequent key exchanges (cf. [KA02] for an unbiased survey). In this regard, cryptographic methods with appropriate computational efficiency, that also consider a certain message or protocol overhead, are inevitable. In ubiquitous and pervasive computing applications, such as sensor networks, RFID-systems or Near Field Communication (NFC), the devices in use as nodes of the network often impose severe size limitations and power consumption constraints. Consequently, the available size for additional cryptographic hardware components is limited as well [KA02, St03, WGP03]. Public key or threshold cryptography are in general computationally intensive and a distributed certificate authority does not address the resource limitations of devices in ad-hoc networks. Asymmetric algorithms for key exchange like RSA and El Gamal perform computationally intensive arithmetics, typically implemented in software on limited microcontrollers. The state-of-the-art is represented by (Hyper-)Elliptic Curve Cryptography (see e.g. [PWP03]). Retaining the security, these representations decrease the size of numbers in the necessary arithmetic operations, but increase the operations’ complexity. Still, a (frequent) key exchange in practice often remains of prohibitive cost.