Kubernetes etcd must have a peer-key-file set for secure communication.
Kubernetes stores configuration and state information in a distributed key-value store called etcd. Anyone who can write to etcd can effectively control a Kubernetes cluster. Even just reading the contents of etcd could easily provide helpful hints to a would-be attacker. Using authenticity protection, the communication can be protected against man-in-the-middle attacks/session hijacking and the insertion of false information into sessions.
The communication session is protected by utilizing transport encryption protocols, such as TLS. TLS provides the Kubernetes API Server and etcd with a means to be able to authenticate sessions and encrypt traffic.
To enable encrypted communication for etcd, the parameter peer-key-file must be set. This parameter gives the location of the SSL certification file used to secure etcd communication.