A security system for a hierarchical network including L hierarchical levels each corresponding to one or more security domains, wherein a first hierarchical level corresponds to a single highest level security domain and an Lth hierarchical level corresponds to two or more lowest level security domains, the security system comprising: a plurality of local network nodes (A, B,..., Z); a keying material generator which is arranged to generate a plurality of correlated sets of keying material, wherein each keying material set includes L keying material sub-sets, each keying material sub-set corresponding to an associated security domain; and a set up server which is arranged to distribute a respective generated keying material set to each network node (A, B,..., Z), said generated keying material set belonging to the plurality of the correlated sets, to thereby associate each network node with L security domains, wherein each network node shares at least one of the associated L security domains with each other network node, so that each network node is able to communicate with any other network node using one or more of its associated L security domains; wherein the network nodes (A, B,..., Z) are arranged to communicate with one another via at least a normal communication mode and an unusual communication mode: said normal communication mode taking place if it is possible for the network nodes to communicate using a security domain corresponding to the Lth hierarchical level, the normal communication mode comprising communicating using a security domain corresponding to the Lth hierarchical level; and said unusual communication mode taking place if it is not possible for the network nodes to communicate using a security domain corresponding to the Lth hierarchical level, the unusual communication mode comprising communicating using a higher level security domain than any of the lower level security domains associated with the Lth hierarchical level. a plurality of local network nodes (A, B,..., Z); a keying material generator which is arranged to generate a plurality of correlated sets of keying material, wherein each keying material set includes L keying material sub-sets, each keying material sub-set corresponding to an associated security domain; and a set up server which is arranged to distribute a respective generated keying material set to each network node (A, B,..., Z), said generated keying material set belonging to the plurality of the correlated sets, to thereby associate each network node with L security domains, wherein each network node shares at least one of the associated L security domains with each other network node, so that each network node is able to communicate with any other network node using one or more of its associated L security domains; wherein the network nodes (A, B,..., Z) are arranged to communicate with one another via at least a normal communication mode and an unusual communication mode: said normal communication mode taking place if it is possible for the network nodes to communicate using a security domain corresponding to the Lth hierarchical level, the normal communication mode comprising communicating using a security domain corresponding to the Lth hierarchical level; and said unusual communication mode taking place if it is not possible for the network nodes to communicate using a security domain corresponding to the Lth hierarchical level, the unusual communication mode comprising communicating using a higher level security domain than any of the lower level security domains associated with the Lth hierarchical level. said normal communication mode taking place if it is possible for the network nodes to communicate using a security domain corresponding to the Lth hierarchical level, the normal communication mode comprising communicating using a security domain corresponding to the Lth hierarchical level; and said unusual communication mode taking place if it is not possible for the network nodes to communicate using a security domain corresponding to the Lth hierarchical level, the unusual communication mode comprising communicating using a higher level security domain than any of the lower level security domains associated with the Lth hierarchical level.