A significant feature of smart door lock systems is the Temporary Key Function (TKF) for visitors. However, existing TKFs in smart door lock systems in use are either vulnerable to attacks or inconvenient for use. This paper thereby proposes elliptic curve cryptography (ECC)-based innovative authentication module for smart door lock systems which enables highly secure and convenient TKF. Based on the authentication module, two protocols are designed for the host and visitor respectively to unlock the door lock system. Security features for the two protocols are verified via Gong Needham Yahalom (GNY) logic. Besides, the proposed prototypes are realized, and the simulation experiments are carried out to study the performance of the protocols. The results show that our scheme is secure and efficient.