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Just to note, your backups will be unencrypted. You actually have to encrypt your time machine volume as well for the backups to be encrypted.
 
is that easy to do? do i need to go to disk utility?

Sadly I have not found an easy way of doing it. If you set a new time machine volume up there is a checkbox, but on an existing one it seems that the checkbox is greyed out. Hopefully someone can chime in with an idea.
 
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