When copying to an RSA-encrypted storage, we relied on the RSA encryption
version to determine if a chunk is a snapshot chunk or a file chunk. This is
wrong when the source storage is not encrypted or not RSA-encrypted. There
is a more reliable to determine if a chunk is a snapshot chunk or not.
The field encryptionVersion in the Chunk struct is supposed to pass the status
of RSA encrytpion from a source chunk to a destination chunk in a copy command.
This field needs to be a 3-state boolean in order to pass the status correctly.
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.
