A server for identity authentication includes a variable keypad generating unit for generating a variable keypad including encryption keys and a signature input part which receives an input of the signature of a user, wherein the position of each of the encryption keys is changed every time the encryption keys are generated; an authentication information saving unit for saving authentication information of a user of a mobile terminal; and an authentication unit for receiving, from the mobile terminal, position information of the encryption keys according to the order inputted by the user, and signature information inputted by the user on the signature input part, and using same to authenticate identity.