Abstract: A server downloads a program to a client for secure services such as secure email messaging. A one-time session key is generated by both the server and the client. There are a number of options for the client to obtain private keys, and where it is generated by the client it may be sent to the server for an escrow service. Time-varying authentication is achieved by the server transmitting a random value to the client and the client uses this value to transmit a password to the server. The server establishes a pre-registered state for the client to allow a message to be sent to a non-registered recipient. The server also downloads a library of components which are subsequently used by applications to generate user displays.