Computer Dictionary/Orange Book
National Computer Security Council (an arm of the U.S. National Security Agency), "Trusted Computer System Evaluation Criteria, DOD standard 5200.28-STD, December 1985" which defines criteria for trusted computer products. There are four levels, A, B, C, and D. Each level adds more features and requirements.
D is a non-secure system.
C1 requires user log-on, but allows group ID.
C2 requires individual log-on with password and an audit mechanism. (Most Unix implementations are roughly C1, and can be upgraded to about C2 without excessive pain).
Levels B and A provide mandatory control. Access is based on standard Department of Defense clearances.
B1 requires DOD clearance levels.
B2 guarantees the path between the user and the security system and provides assurances that the system can be tested and clearances cannot be downgraded.
B3 requires that the system is characterised by a mathematical model that must be viable.
A1 requires a system characterized by a mathematical model that can be proven.