How to securely control access to a backend key server?

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http://serverfault.com – I need to securely encrypt data in my database so that if the database is dumped, hackers are unable to decrypt the data. I'm planning on creating a simple key server on a different machine, and allowing the DB server access to it (restricted by IP address on the key server to permit the DB server). The key server would contain the key required to encrypt/decrypt data. However, if a hacker were able to get a shell on the DB server, they could request the key from the key server and therefore decrypt the data in the database. How could I prevent this (assuming all firewalls are in place, DB (HowTos)