A key management unit of a service provider system generates an individual key corresponding to a portable module connected to a service client making an access request and informs an authenticating unit of this individual key. The individual key is stored beforehand also in the portable module. A random-number generator generates a random number, transmits this random number to the portable module and informs the authenticating unit of it. The portable module encrypts the random number with the individual key and sends it back to the service provider system. An encrypting portion of the authenticating unit encrypts the random number with the individual key. A comparator of the authenticating unit compares data encrypted by the encrypting portion with encrypted data transmitted from the portable module and, if the two items of data are coincident with each other, confirms an access request from the portable module.