Since TDE relies on a certificate stored in master (which is used to encrypt the database encryption key), then this would work only work if you could restore the master database to another server in such a way that the certificate could be decrypted.
This is the TDE encryption hierarchy:
- Service master key (protected by Windows; tied to the service account credentials, and a machine key)
- Database master key (in this case, the one for the master database)
- Certificate
- TDE encryption key
The first three items are stored in the master database, and can all be backed up. The fourth is stored (encrypted by the certificate from #3) in the header of the encrypted database.
So in a failure scenario, you would have to restore enough of the encryption hierarchy to allow you to read the TDE key. SQL Server creates the service master key at installation; thus while restoring the master database to a different instance will also restore items 2 and 3, the necessary key(s) to decrypt them will not be present. Result: unreadable data.
The two best options are to either restore the certificate (#3) from a backup (a good option if master cannot be restored for whatever reason), or restore your master database and its master key (#2) from a backup. Restoring the master key may be a better option if you have a lot of certificates/keys protected by this key, and need to make them all accessible at once. This comes with the same precautions normally associated with restoring the master database (collations, logins, database names and file paths, etc.)
Generally, I'd only recommend restoring master in a recovery scenario. For a migration/scale-out scenario (such as using Availability Groups/mirroring with a TDE-encrypted database), it's better to backup/restore the certificate (#3) so that it's encrypted using the master keys unique to each instance it's moving to. You will need to include the private key with the certificate backup.
In any case, you're going to have to make key/certificate backups, so guard them well, and store them in redundant, secure locations. Simply having a backup of master will not get you out of a TDE disaster; you're going to need a backup of at least one key or certificate.
Best Answer
This is from the encryption hierarchy and protection. When using TDE you need to create at least two keys:
The way this works is as follows:
This means, in order to OPEN the DEK to decrypt the database data you need to open the Server Certificate. To open the Server Certificate you'll need to open the Database Master Key (DMK). To open the DMK you'll need to open the SMK.
This is normally done automatically for you as part of automatic key decryption if all levels of the hierarchy are setup properly.
Thus, if we attempt to restore the database using TDE to another instance and that instance does not have the server certificate then it will not be able to read the database and cannot open the database. This is why the Server Certificate is required and we can open it properly.
Encryption Hierarchy: https://msdn.microsoft.com/en-us/library/ms189586.aspx