A method performed by a computing device is described. The method includes (a) receiving an authentication request from an application server seeking to authenticate a user for access to a service provided by the application server, (b) communicating with a first authentication server to obtain a first authentication of the user, (c) communicating with a second authentication server to obtain a second authentication of the user, the second authentication server being distinct from the first authentication server and the second authentication being of a type distinct from the first authentication, (d) rejecting the authentication request if and only if one or both of the first authentication and the second authentication is negative, and (e) upon rejecting the authentication request, sending a rejection message to the application server without informing the application server whether the first authentication or the second authentication was negative.