Let's say I have two databases on the same server (e.g. AWS RDS),
DB2. There are two users with some restrictions, and each user is assigned to a different database.
US1 have access to
US2 have access to
The users and their passwords are stored separately on different servers.
DB1 is used to store ciphertext, and
DB2 to store the encryption keys.
Can I be sure, in 100%, that without the passwords (ignoring bruteforcing and other attacks on the password itself), that while having access to only one of the users, an attacker won't be able to access the other database?