Communication system comprising a central data processing device, access stations and external stations, and incorporating a cryptographic check against falsification of an external station, and external stations for use in such a communication system

Claim 1

1 . A communication system comprising a central data processing device (30) and a number of access stations ( 2 0, 46) which are coupled thereto and which are situated in different locations, and also comprising a group of external, portable, pocket-size stations (26, 48, 50), it being possible to couple an external station to an access station via a bidirectional data channel (22, 24), the relevant access station comprising a message generator (32) for forming a message and an encoder (34) for receiving the message thus formed and for encoding said message by way of a first encoding key in order to apply a first code message thus formed to the data channel, the relevant external station comprising a first decoder (36) for receiving the first code message and for decoding this message by way of a first decoding key which is associated with the first encoding key in order to apply a first reconstructed message to a digital processing device (37) in the relevant external station, a locally present initiation station which is coupled to the external station in an initiation state thereof comprising a generator for applying the first decoding key to the external station, characterized in that the relevant external station comprises a second encoder (38) for receiving the first reconstructed message and for encoding this message by way of a second encoding key which is independent of the first encoding key in order to apply a second code message thus formed to the data channel, the relevant access station comprising a second decoder (40) for receiving the second code message and for decoding this message by way of a second decoding key which matches the second encoding key in order to form a second reconstructed message, the initiation station comprising a second generator for applying the second encoding key to the external station in the initiation state thereof, the access station comprising a third generator for generating said first unpredictable message, and also comprising a comparison element (42) for comparing the first message and the second reconstructed message and for generating a confirmation of identity in the case of correspondence, said first decoding key being mutually identical for a number of external stations and said second encoding key being mutually equal for a number of external stations, said first decoding key and second encoding key being not readable therein.