When you back up into the Fog, all private files are ALWAYS encrypted. (It’s possible to back-up public files to other people’s Foggies, but privacy & security aren’t considerations with public files.)
The only way to decrypt your private files is with your encryption key. Your encryption key is never stored on the blockchain. It’s never stored on your Foggie Max. It’s up to you to maintain the security of your encryption key.
Back-ups the the Fog aren’t just secure — they’re completely private and anonymous as well.