Abstract: A distributed client/server computer network wherein the identity of at least one complex image, selected from a plurality of complex images stored by a client, is transmitted to a remote server. The remote server then determines, from the identity of the or each image selected, whether the client is authorised to gain access, via the server, to a particular network resource. The network is thus efficient, as the complex images themselves do not need to be transmitted from the client to the server, and secure, as no authorization information is stored by the client.