Backup files in the backup chain often need to be transformed, for example, when the earliest incremental backup file in the chain becomes obsolete and its data should be included into the full backup file. When Veeam Agent for Microsoft Windows transforms a full backup file, it writes data blocks from several restore points to the full backup file. As a result, the full backup file contains data blocks that are encrypted in different job sessions with different session keys.
To restore data from such “composed” backup file, Veeam Agent for Microsoft Windows would require a bunch of session keys. For example, if the backup chain contains restore points for 2 months, Veeam Agent for Microsoft Windows would have to keep session keys for a 2-month period.
In such situation, storing and handling session keys would be resource consuming and complicated. To facilitate the encryption process, Veeam Agent for Microsoft Windows uses another type of service key — a storage key.
For storage keys, Veeam Agent for Microsoft Windows uses the AES algorithm with a 256-bit key length in the CBC-mode. A storage key is directly associated with one restore point in the backup chain. The storage key is used to encrypt the following keys in the encryption hierarchy:
- All session keys for all data blocks in one restore point
- Metakey encrypting backup metadata
During the restore process, Veeam Agent for Microsoft Windows uses one storage key to decrypt all session keys for one restore point, no matter how many session keys were used to encrypt data blocks in this restore point. As a result, Veeam Agent for Microsoft Windows does not need to keep the session keys history in the Veeam Agent for Microsoft Windows database. Instead, it requires only one storage key to restore data from one file.
In the encryption process, storage keys are encrypted with a key of a higher layer — a user key. Cryptograms of storage keys are stored to the resulting file next to encrypted data blocks, and cryptograms of session keys and metakeys.
Storage keys are also kept in the Veeam Agent for Microsoft Windows database. To maintain a set of valid storage keys in the database, Veeam Agent for Microsoft Windows uses retention policy settings specified for the job. When some restore point is removed from the backup chain by retention, the storage key corresponding to this restore point is also removed from the Veeam Agent for Microsoft Windows database.