High Availability encryption adds security to the communication between appliances in a HA pair. HA control messages between active and standby firewalls, such as heartbeats, configuration sync and HA state information, are encrypted to ensure security for inter-node communication.
This option is available in Active-Standby HA mode only and does not apply to messages exchanged for stateful synchronization even in Active-Standby mode. Discovery messages (find-peer and found-peer) are transmitted without encryption. After the discovery stage, however, all control messages are encrypted between the firewalls:
Messages used for incremental config updates
Various messages for sending HA commands between the firewall pair