Method is provided for controlling access by a client computer to an on-line user group hosted by a second computer, wherein the first computer stores personal data of a user. A first set of projection images is acquired based on the emitted first radiation and digital tomosynthesis is performed on the first set of projection images to generate a first cross-sectional image. The payer receives the authentication code on the payer’s communications device and transmits it, together with a secret identification code, back to the authentication agency.