Abstract: In a communication network with cryptographically protected connections a unique key is used for each connection between two subscribers, so that when one or more keys fall into the hands of an unauthorized person it has no consequences for the protection of the remaining connections. In a network comprising N subscribers this implies that each participant must have N-1 keys for communication with the other participants. Such a set of N-1 keys is obtained by putting a column from a N.times.N key matrix (main diagonal empty) at the disposal of each participant. During the connection set-up the correct unique key is automatically selected by interchange of the column number. The key matrices used in the network can be regularly replaced without it being necessary to interrupt the operation of the network.