In a computer-implemented authentication method, a first authentication request from a first machine is received at an authentication server. The first authentication request includes an identification of a second machine that is to provide a requested service. An authentication token including client-specific and server-specific portions is generated at the authentication server, responsive to receiving the first authentication request from the first machine. An authentication identifier and the server-specific portion of the authentication token are transmitted from the authentication server to the second machine, responsive to receiving the first authentication request from the first machine. A second authentication request, including the authentication identifier and both the server-specific and the client-specific portions of the authentication token, is received at the authentication server from the second machine. An authentication status for the requested service is determined at the authentication server, responsive to receiving the second authentication request from the second machine.