These principles guide the design, implementation, and management of secure architectures and controls in real-world environments.

1. Categorization of NIST Principles

The NIST principles are organized into three broad families;

1.1 Security Architecture and Design

These principles focus on the organization, structure, and interfaces of systems. They help define how security is embedded into the architecture from the ground up.

1.2 Security Capability and Intrinsic Behaviors

This category addresses what protections the system provides, including specific security controls and their functions.

1.3 Life Cycle Security

These principles cover process and management aspects, ensuring that security is maintained throughout the system's life cycle.

Together, these principles help system designers ensure a comprehensive approach to security, encompassing everything from foundational structures to operational practices.

2. Evolution from Saltzer and Schroeder

Several NIST principles map directly to the classic principles by Saltzer and Schroeder, including;

• Least Common Mechanism
• Efficiency Mediated Access
• Minimized Security Elements
• Reduced Complexity
• Least Privilege
• Secure Defaults and Predicate Permission
• Acceptable Security

These traditional principles remain relevant and provide the basis for many modern security controls.

3. Modern Enhancements for Contemporary Systems

As computing systems have grown more complex, NIST introduces new principles that address these modern challenges;

3.1 Clear Abstraction, Modularity, and Layering

Encourages clean modular design to simplify system understanding and maintenance.

3.2 Partially Ordered Dependencies

Ensures system components interact in a defined and secure sequence.

3.3 Secure Evolvability

Promotes the ability to evolve a system secure over time, without introducing vulnerabilities.

3.4 Hierarchical Trust Structure

Recognizes that not all components require the same level of assurance. Components with lower assurance levels should not compromise those with higher assurance.

3.5 Inverse Modification Threshold

The most critical components must be the hardest to modify, ensuring protection from unauthorized tampering.

3.6 Hierarchical Protection

Less Critical components do not need protection from more critical ones, streamlining system protection layers.

4. Security in Interconnected Systems

In today's networked environments, NIST acknowledges the need for interconnected systems to maintain security;

• Trusted Communication Channels: Networks must use secure channels to prevent eavesdropping or data manipulation.
• Secured Distributed Composition: When secure systems are combined, their integration must maintain or exceed the original security standards.
• Self-Reliant Trustworthiness: A secure system should maintain its integrity even when isolated from other systems or remote components.

5. Balancing Security and Usability

Recognizing the practical limitations in the real world, NIST introduces principles like;

• Economic Security: Security measures should be cost-effective.
• Performance Security: Controls should not significantly degrade system performance.
• Human Factored Security: Systems must be user-friendly and avoid complex mechanisms that users may bypass.
• Acceptable Security: Controls must align with user expectations and not hinder productivity or usage.

These principles emphasize that security is not a goal in itself, but a supportive aspect of system functionality.

6. NIST Security Architecture Strategies

Beyond principles, NIST also outlines three key strategies for implementing secure system architectures:

1. Reference Monitor Concept: An abstract control that can enforce all system security properties effectively. It's central to mandatory access control models.
2. Defense in Depth: Multiple overlapping controls are used to ensure that if one fails, others remain effective.
3. Isolation: Components are physically or logically separated to prevent interference and reduce information leakage.

7. Application and Adaptation

Both NIST and Saltzer & Schroeder stress that security principles are guidelines, not rigid rules. While they provide essential direction, they must be applied skillfully to specific contexts. If deviations are made from any principle, it is important to:

• Identify the deviation
• Assess potential risks
• Mitigate any issues appropriately

Only through thoughtful application can these principles lead to robust and resilient systems.