Backup of encrypted private key on uncontrolled disks

[Robert J. Hansen]

> My private key is encrypted with a very strong passphrase (10 word
> diceware [1], not written down, 129 bits of entropy). Given that, is it
> safe to back it up on disks I don't control, such as a private S3 bucket
> or a VPS? My intuition says yes, but I've learned to never trust my
> intuition when it comes to security.

If you are completely confident that no one will ever get your 
passphrase from you, this is safe.  Otherwise, it's not.

It may be appropriate to have a little caution with respect to whether 
you believe anyone will ever get your passphrase from you.