This option is useful only when erasure coding is enabled. It will
download and re-upload chunks that contain corruption but are
generally recoverable. It can also be used to fix chunks that
are created by 3.0.1 on arm64 machines with wrong hashes.
This is to support public key encryption in the backup operation. You can use
the -key option to supply the public key to the backup command, and then the
same option to supply the private key when restoring a previous revision.
The storage must be encrypted for this to work.