Method with which a mobile user can confirm a transaction with a service provider, in which: an offer of said service provider is reproduced with the mobile device of said mobile user, said mobile user selects said offer with input means of his mobile device, a transaction confirmation is automatically forwarded from said mobile device to an authentication server, a plurality of transactions between different mobile users and different service providers being stored in said authentication server, and said service provider retrieves said confirmation from said authentication server.