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A device or function designed to provide one or more security services usually rated in terms of strength of service and assurance of the design.
A method, tool, or procedure that is the realization of security requirements. Note 1: A security mechanism exists in machine, technology, human, and physical forms. Note 2: A security mechanism reflects security and trust principles. Note 3: A security mechanism may enforce security policy and therefore must have capabilities consistent with the intent of the security policy.
NIST SP 800-160 Vol. 1