A system, comprising: - a service platform adapted to provide at least one online service, accessible over a data network, said data network comprising a wireless LAN; - an authentication control function adapted to authenticate a user accessing said online service, by means of a communication device being at least connected to both a mobile phone network and the data network, wherein: - the authentication control function is in communication relationship with the service platform and is comprised in the mobile phone network, - the service platform comprises a provider-side authentication application configured to extract a digital token from at least one resource code received at the service platform, said at least one resource code being provided by the communication device to the service platform via the data network, the resource code identifying said online service, to generate at least one authentication request message comprising the received digital token, and to send the authentication request message to the authentication control function via a high-security communication link, - the authentication control function is configured to: - extract a digital token comprised in at least one message (250;251) sent from the communication device via the mobile phone network to the authentication control function, - extract the digital token from said at least one authentication request message, - compare the digital tokens received from the mobile communication device and from the service platform, - notify to the service platform of a successful user authentication in case the two digital tokens match by providing a successful authentication message to the provider-side authentication application, whereby the service platform grants to the user access to the online service. - a service platform adapted to provide at least one online service, accessible over a data network, said data network comprising a wireless LAN; - an authentication control function adapted to authenticate a user accessing said online service, by means of a communication device being at least connected to both a mobile phone network and the data network, wherein: - the authentication control function is in communication relationship with the service platform and is comprised in the mobile phone network, - the service platform comprises a provider-side authentication application configured to extract a digital token from at least one resource code received at the service platform, said at least one resource code being provided by the communication device to the service platform via the data network, the resource code identifying said online service, to generate at least one authentication request message comprising the received digital token, and to send the authentication request message to the authentication control function via a high-security communication link, - the authentication control function is configured to: - extract a digital token comprised in at least one message (250;251) sent from the communication device via the mobile phone network to the authentication control function, - extract the digital token from said at least one authentication request message, - compare the digital tokens received from the mobile communication device and from the service platform, - notify to the service platform of a successful user authentication in case the two digital tokens match by providing a successful authentication message to the provider-side authentication application, whereby the service platform grants to the user access to the online service. - extract a digital token comprised in at least one message (250;251) sent from the communication device via the mobile phone network to the authentication control function, - extract the digital token from said at least one authentication request message, - compare the digital tokens received from the mobile communication device and from the service platform, - notify to the service platform of a successful user authentication in case the two digital tokens match by providing a successful authentication message to the provider-side authentication application, whereby the service platform grants to the user access to the online service.