Five fundamental security principles of Defense against attacks
Diversity
Closely related to layering, if you are using layers of security you must use different types of security for each layer. Attacker must use different techniques to attack each layer.
Simplicity
Making a security system harder to use may make the users lazy and create bypasses. Also if security is complex to set up then it creates more work for the security professional and they may miss a vulnerability.
Obscurity
Making it more difficult for an outsider to recognize what is going on inside.
Limiting
People should only be authorized to the information they need for doing a task. Access must be restricted to a minimum.
Layering
Provides the most comprehensive protection. Instead of one security defense you have multiple defenses. If one of the defenses is broke the attacker must then penetrate the next layer of defense. More layers add more security, but is limited to the vulnerability of each defense.