1. A method performed by an authentication server for authenticating a subscriber, the method comprising: receiving, at the authentication server, a request message including a plurality of attributes having respective attribute names and respective attribute values; retrieving, from a profile storage, an authentication profile object; identifying a plurality of authentication attributes configured to be used for authentication from said authentication profile object; constructing, by the authentication server, a subscription identifier from the authentication profile object by retrieving a subscription identifier definition; extracting, by the authentication server, said plurality of authentication attribute values from said message based on the subscription identifier; pre-processing, by the authentication server, one or more of said extracted attribute values using the subscription identifier definition; and authenticating, by the authentication server, said request message based on said pre-processed extracted attribute values.