Page 490 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

confidentiality and integrity of data by disallowing unauthorized
access by authorized or unauthorized subjects.

Trust and Assurance

Proper security concepts, controls, and mechanisms must be
integrated before and during the design and architectural period in
order to produce a reliably secure product. Security issues should not
be added on as an afterthought; this causes oversights, increased costs,

and less reliability. Once security is integrated into the design, it must
be engineered, implemented, tested, audited, evaluated, certified, and
finally accredited.

A trusted system is one in which all protection mechanisms work
together to process sensitive data for many types of users while
maintaining a stable and secure computing environment. Assurance is
simply defined as the degree of confidence in satisfaction of security

needs. Assurance must be continually maintained, updated, and
reverified. This is true if the trusted system experiences a known
change or if a significant amount of time has passed. In either case,
change has occurred at some level. Change is often the antithesis of
security; it often diminishes security. So, whenever change occurs, the
system needs to be reevaluated to verify that the level of security it
provided previously is still intact. Assurance varies from one system to

another and must be established on individual systems. However,
there are grades or levels of assurance that can be placed across
numerous systems of the same type, systems that support the same
services, or systems that are deployed in the same geographic location.
Thus, trust can be built into a system by implementing specific

security features, whereas assurance is an assessment of the reliability
and usability of those security features in a real-world situation.