use backup storage on serene, but encrypt the contents stored there

[Fred T. Hamster]

can do this with a simple trick:
+ do the backup to a local backups folder.
+ archive that with like squish directories.
+ encrypt that file using our private key for fred of some sort.
+ go over to the backups mount (with new location that's obviously not supposed to be used for clear archives).
+ move any 'current' backups into a 'history' folder.
+ clean out the oldest backup in 'history', if count of files is over a threshold.
+ copy over the newest backup into 'current'.
+ done.