In an interactive broadcast system signals transmitted by a service provider (30) are passed via a gateway (12) to a subscriber terminal (2). In order to gain access to services, the subscriber terminal (2) transmits an authorization request message via the gateway (12) to the RADIUS server (18). This RADIUS server (18) transmits in response to the authorization request message an authorization challenge message to the subscriber terminal (2). In return the subscriber terminal responds to the challenge message with a challenge response. This challenge response is checked by the RADIUS server (18) and if correct, the RADIUS server (18) transmits a message to the gateway to give the subscriber terminal access to the service.