An arrangement and corresponding method for authentication synchronizing cryptographic key information between a server and a client device, via data signals, where the client device at least comprises one client. The server is at least configured to generate and send to the client device a current encryption key and a next encryption key. The client device is at least configured to encrypt information on the client device using the next encryption key and the client device is at least configured to return a correct One Time Password using the current encryption key. As a consequence of the received correct One Time Password the server then knows that the client has received the current encryption key, used it and stored the information with the next encryption key.