I came across your software and it looks great. However, I am missing one feature. That is, that client encrypts user data with user’s password or key. This is a feature DejaDup does have (DejaDup is default backup software in Ubuntu Linux, however, it has several problems with stability and reliability).
Anyway, this feature would be great, because sometimes user wants to have his own data to be private and protected even from administrator. Sometimes administrator is not allowed to see that data (for. instance medical records, etc.). If user would be able to encrypt his own backup data and send that to backup server, administrator of a backup server will not have the option to see the data. In that case you eliminate the problem of trust in administrator. There is at least one person less, which would need a clearance, for example.
BTW: I have set up a VPN network. Machines in VPN network have IP addresses in range 10.10.7.0/24. One of the machine is a backup server. Users do not know it’s real IP address and it’s real location. Their backups are done on an encrypted disk. But additionally to that, users encrypt their data with their own (“local”) key/passphrase, so I (as administrator) cannot see the contents of their backups. Backups are also incremental.
This means, users have a secure off-site backup, which cannot be destroyed in case of a disaster on user’s location. Backups are stored on encrypted disk (so at server’s location) no one can see anything about user’s or their backups), and me as administrator cannot access their data.
It would be great if something similar would be possible with urBackup.