Security in visible-light communication (VLC) has seen increasing importance in recent years. Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is recognized as one of the most powerful and efficient OFDM techniques. Therefore, it is well suited for use in both uplink and downlink connections. A security protocol based on this technique can facilitate secure uplink and downlink connections. In the present study, a low-complexity physical-layer key-generation encryption method is designed using the ACO-OFDM technique for indoor VLC networks. The security method is contingent on the generation of secret keys from the cyclic prefix OFDM samples positioned in the low-channel impact area to encrypt all signal frames before transmission, throughout the session. Numerical results indicate that the key-generation mechanism should be implemented during downlink data transmission throughout a session period to provide keys for both downlink and uplink connections. In this setup, the handset of the user employs the secret keys generated during downlink data transmission to encrypt its uplink transmission. This setup conserves the battery life of the handset. Additionally, the results indicate that the proposed security method can achieve a zero key mismatch rate with on-the-fly key creation.