There is provided a method for authenticating a mobile device user against an authenticating system connected to a client computer accessible to said user. The authenticating system uses a communication channel to send to the client computer a logon screen. This logon screen contains a 2D-code embedding, a URL of the authenticating system and a challenge generated by the authenticating system. With a 2D-code reader in the user's mobile device the URL and the challenge are decoded. The user then inputs a password and a response to the challenge is computed. The response is sent together with the user ID to the authenticating system. The authenticating system is able to ascertain that the response to the challenge necessarily comes from the user thereby verifying his identity. Once the user is authenticated, the authenticating system pushes to the client computer (identified by the challenge) a welcome screen.