A client device for use in a system having a server, the client device including: a receiving device configured to receive information material from the server, the information material containing a reversible modification providing a visually perceptible disturbing effect having been applied to the information material by using an invertible algorithm which uses values generated from one or more keys, and to receive identifying information that distinguishes the information material from other information material available from the server and condition information which defines a predetermined condition that should be met in order for the reversible modification to be reversed.