According to the current implementation, the argument for --storage will be stored in backup meta and will be retrieved and sent to TiKV during restore.
In this case, for backup & restore with cloud storage, e.g. AWS S3, keys will also be included in the backup meta, which is insecure and should be avoided.

• Is it necessary to store the argument in the backup meta? The --storage option is required in each backup, checksum and restore command.
• It's also insecure to specify the keys in the argument of --storage, we may set the keys or other sensitive data in environment variables and retrieve them and send them to TiKV in BR.

@kennytm @overvenus @tennix What do you think?