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Due to the dynamic nature of wireless ad-hoc networks (WAHNs) and the multi-node involvement in most WAHN applications, group key management has been proposed for efficient support of secure communications. Exclusion basis systems (EBS) provide a framework for scalable and efficient group key management where the number of keys per node and the number of re-key messages can be relatively adjusted. EBS-based solutions, however, may suffer from collusion attacks, where a number of nodes may collaborate to reveal all system keys and consequently capture the network. In this paper we investigate the collusion problem in EBS and demonstrate that a careful assignment of keys to nodes reduces collusion. Since an optimal assignment is NP hard, we propose a location-based heuristic where keys are assigned to neighboring nodes depending on the Hamming distance between the strings of bits representing the used subset of the keys.