A system is described comprising a service provider and an identity provider. A user requests access to the service provider and the service provider seeks user credentials from the identity provider. In use, the service provider issues an authentication request, which request specifies details of a plurality of acceptable authentication formats. The identity provider responds to the request either by providing authentication details for said user in one of the formats specified in the request, or by returning an error message indicating that it cannot support any of the specified authentication formats.