A communication system includes a plurality of nodes, the communication system being arranged to assign each of the plurality of nodes a certificate by means of which it can authenticate itself to other nodes in the communication system and periodically distribute to the plurality of nodes an update formed by compressing a data set representing the validity of the certificates assigned to the plurality of nodes. The update is such that a node may not be able to unambiguously determine from the update whether or not a particular certificate is valid. The system further provides the plurality of nodes with a source of information about the validity of the plurality of certificates that is different from the update and by means of which a node may resolve an ambiguity in the update regarding a particular certificate's validity.