Abstract: A method for authenticating a user of certain service provided by a system through a first communication channel, in one aspect including receiving an access request from a first terminal of the user through the first communication channel; receiving an address or number of a second terminal of the user through the first communication channel; transferring data including an identification code, to the second terminal of the user through a second communication channel; receiving a user confirmation response, including the user identification code, from the second terminal of the user through the second communication channel; determining whether the identification code transferred to the second terminal is identical to the user identification code received from the second terminal; generating an authentication code if it is determined that both the user identification codes are identical to each other; transferring the user authentication code to the first terminal of the user through the first communication ch