A. Open Design
The principle of Open Design states that the security of a mechanism should not depend on the secrecy of its design or implementation.

The "Security by Obscurity" Fallacy: It is the opposite of "security by obscurity." For example, the inner workings of the AES encryption algorithm are public knowledge. This allows researchers to find and fix flaws.

Attack Surface: While Open Design makes the system more robust through peer review, it does not necessarily make the system "smaller" or reduce the number of features an attacker can target.

B. Economy of Mechanism
Economy of Mechanism is the philosophy that "Simple is Secure." It dictates that security mechanisms should be as small and simple as possible.

Why it's the Answer: Complex systems contain more code, more configuration options, and more potential for logic errors. By keeping a system lean and removing unnecessary "bells and whistles," you effectively minimize the attack surface. If a feature doesn't exist, it cannot be exploited.

Example: A "Minimal Install" of a Linux server has a much smaller attack surface than a full desktop installation because it lacks the extra software, drivers, and open ports that an attacker could use.

C. Least Privilege
Least Privilege ensures that a subject (user, process, or system) is granted only the minimum permissions necessary to perform its job function.

The Focus: This is primarily about damage control and preventing lateral movement. If a service account is compromised, the attacker can only do what that account was allowed to do.

Distinction: While it limits the capabilities of an attacker, the "attack surface" (the vulnerabilities they could exploit to get in) might still be large. Least Privilege is more about restricting access than simplifying the design.

D. Defense in Depth
Defense in Depth (also known as layered security) is the practice of implementing multiple, redundant security controls.

The Goal: If one control fails (e.g., a firewall), another control (e.g., an Intrusion Detection System or Host-based Antivirus) is there to catch the threat.

Attack Surface: Ironically, adding more security layers can sometimes increase the attack surface because each new security tool is a piece of software that could itself have a vulnerability. Defense in Depth focuses on resilience, not minimization.

Key Takeaway for the CISSP Exam

• Design Philosophy: > * Economy of Mechanism: Keep it simple to reduce vulnerabilities (Minimizes Attack Surface).
• Least Privilege: Limit user rights to reduce impact (Domain 5).
• Defense in Depth: Use multiple layers to reduce the chance of a total breach.
• Fail-Safe Defaults: If a system fails, it should fail in a secure state (e.g., a door that locks when power is lost).