Systems, methods and computer readable media for authenticating one or more client devices (1) to a server (5). A shared unpredictable secret (50) is generated. The shared unpredictable secret (50) is stored in the client device (1) and in the server device (5). The client device (1) proves possession of the correct shared unpredictable secret (50) to the server (5). The shared unpredictable secret (50) is replaced by a new shared unpredictable secret (54) each time the client device (1) logs in to the server device (5).