The Jargon File (version 4.4.7, 29 Dec 2003):

Orange Book
 n.

    The U.S. Government's (now obsolete) standards document Trusted Computer
    System Evaluation Criteria, DOD standard 5200.28-STD, December, 1985 which
    characterize secure computing architectures and defines levels A1 (most
    secure) through D (least). Modern Unixes are roughly C2. See also book
    titles.


The Free On-line Dictionary of Computing (30 December 2018):

Orange Book
A1 security
B1 security
B2 security
B3 security
C1 security
C2 security

    A standard from the US Government
   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.

   See also crayola books, book titles.

   [Jargon File]

   (1997-01-09)