Method to register a user at a service

Method for the registration of a user (E) at a service (RA1) which is part of a subdomain (D1). The user asks registration at said service under identity Id1. The service requests a registration service (RA) outside the subdomain and/or another service (RA2) within the same subdomain to send a subdomain specific user code (IdKeyl) which is valid within the subdomain concerned. If the requested subdomain specific user code does not exist, the registration service asks the user to send a user code (Id) which is valid for that registration server, after which the registration service generates a subdomain specific user code (IdKeyl) being valid for the subdomain concerned, e.g. by means of a cryptographic operation, such as "secure hashing", of a secret, not subdomain specific user code (IdKey). If the requested subdomain specific user code (IdKey1) already exists or after being generated by the registration service, this code is sent to the service (RA1) where the user asks to be registered.