Recovering from Lost Keys
If you followed the instructions on the website or the client configuration page when setting up your clients, then you will have a copy of the raw encryption key stored in a safe place. This key is used to encrypt and decrypt your data, and nothing else. It has no connection with your SSL private key, which is used to authenticate you to the backup server, allowing you to log in.
The raw encryption key is the only file which cannot be replaced. If you have a disaster and lose all the other Box Backup configuration files, you can generate a new SSL private key and certificate signing request, which your server operator or service provider should sign for you.
The easiest way to do this is to follow the instructions for client configuration again, to generate a complete new set of keys, including a new raw key. However, the new raw key is not useful if you want to access your existing backups, so you should replace it with the copy of the old key that you kept safe.