Methods and apparatus are provided for authenticating communications between a user computer and a server via a data communications network. A security device has memory containing security data, and security logic to use the security data to generate an authentication response to an authentication message received from the server in use. An interface device communicates with the security device. The interface device has a receiver for receiving from the user computer an authentication output containing the authentication message sent by the server to the user computer in use, and interface logic adapted to extract the authentication message from the authentication output and to send the authentication message to the security device. Includes a communications interface for connecting to the server via a communications channel bypassing the user computer. Either the security device or interface device sends the authentication response to the server via the communications channel bypassing the user computer.