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.

Abstract: Method of coding and decoding messages wherein a basic key and a random message key, which is transmitted prior to the message, are used. Using a plurality of basic keys entails the problem that the proper basic key must be set in the receiving station. In accordance with the present method, the sequence of a portion (II) of the message key and of a number of check characters derived therefrom are coded, prior to the actual message transmission, using the basic key and a different portion (I) of the message key. Decoding in the receiving station is done by using a basic key which is chosen from a group of basic keys, and the different portion (I) of the message key. A test circuit 21 examines whether the decoded check characters correspond to the check characters derived in the receiving station from the decoded portion (II) of the message key. A positive test result indicates that the desired basic key has been found.