Just a hash challenge (actually using Whirlpool) to resolve for masters in order to authenticate themselves, using a symmetric key, that needs to be sent to slaves.
Key storage/auth methods
Plugins on master side can be made to store and do the hash in an other way than plain raw key storage on the master's filesystem.
Key sending procedure
Log on the slave side :
Log of the master which stores and sends its key to the slave :
Log of the master which authorizes the first master to send the key, and chooses associated permissions :
Logs of initial key sending (accepting the first key without being authenticated) :