A portable storage device, for example a secure smart card, contains network identification information for a processing unit that is connectable to a data communications network, which processing unit includes a device reader for reading the portable storage device. The portable storage device includes storage and an access controller. The storage holds a network identity for the processing unit and at least one encryption key. The access controller is operable to control access to the storage by implementing key-key encryption. An embodiment of the invention thus provides a medium not only for storing a network identity for processing unit, but also for other secure information such as an encryption key associated therewith. The processing unit is operable to access a secure portion of the storage of the portable storage device by supplying a request key to the access controller of the portable storage device, and, in response to receipt of an access key from the access controller, to send an encrypted command to access the content of the storage of the portable storage device. In response to the return of an access key, the processing unit can be operable to use the access key to encrypt a command for access to a secure storage in the portable storage device.