Encrypting an Unencrypted RDS Snapshot
RDS snapshots can be unencrypted or they can be encrypted at rest. Today, best practice is to use encryption-at-rest on your RDS instances and clusters, and to encrypt your RDS snapshots. When you create an RDS snapshot from an RDS instance or cluster, the resulting snapshot will be encrypted if the source instance or cluster is encrypted. But if the source is not encrypted, then your RDS snapshot is not encrypted. When you create an RDS snapshot, you are not given the option to encrypt it.