Encrypt Time Machine backup data sent over the network I have three relevant machines:


An iMac running 10.10 as my Time Machine server. 
A MacBook Pro running 10.10, which backs up to the server. Backup encryption is enabled.
A PowerBook G4 running 10.5, which also backs up to the server. Backup encryption is not supported by 10.5.


In the case of the 10.10 MacBook, I understand that setting the MacBook to encrypt backups means that the backups stored on the server will be in an encrypted sparse image. However, the data being sent to the server is sent in the clear, unless the client handles encryption (unlikely).

Could the data transfer be secured by SSH for both machines?
The easiest way to be sure that all your clients' backups are encrypted is to create an encrypted sparse (bundle) image using Leopard (10.5) on the server using hdutil. The newer versions of OSX should be able to read/write to the older encrypted volume without a problem. 

Here is an example using a Linux AFP server...
http://www.cognizo.com/2012/04/encrypted-network-backups-with-os-x-time-machine/

And another
http://bubba.org/wiki/Encrypted_Remote_Backups_with_Sparse_Bundles

This way all data encryption/decryption is done at the client end. Problem sovled.