Abstract: An authentication server according to an embodiment disclosed includes a number receiver that receives a number of a user terminal from a callee who has received a call originated from the user terminal, an device authenticator that determines whether to generate an authentication key based on the number of the user terminal received from the callee and a caller number received from the user terminal, and generates the authentication key when it is determined to generate the authentication key, and an authentication key transmitter that transmits the authentication key to the user terminal.

Abstract: A flexible display device is disclosed. The flexible display device includes a first body, a second body, a flexible display, a first rack, a second rack, a first gear, and a second gear. The first gear and the second gear rotate in conjunction with each other. The flexible display is divided into a first region and a second region, and the second rack forms a continuous loop together with the second region of the flexible display. Accordingly, when the second body slides with respect to the first body, tension can be stably applied to the flexible display, and the flexible display is prevented from being loosened.