How does MariaDB Galera Cluster authenticate between each node?

I am new to the MariaDB Galera cluster. I have successfully set up my first Galera cluster, but I have a question regarding how the node communicate each other? I am using SST=mariabackup. According to the documentation, the donor needs to stream the backup to the joiner using socat, but how does the donor authenticate to the joiner? I Googled a lot, but can't find the answer. Could you please advise? Same as SST=rsync, how does the authentication among the cluster nodes work? Why username and password is not required? Thanks.