The document discusses several security design principles for software including: - Least privilege - Give users and processes only the minimum permissions necessary to perform their tasks - Defense in depth - Use multiple layers of security instead of just one - Secure the weakest link - Focus on securing common weak points like passwords, firewalls, and buffer overflows - Fail-safe stance - Design systems to fail securely and deny access by default if failures occur - Secure by default - Only enable necessary features and harden systems by disabling unnecessary services - Simplicity - Reduce complexity to minimize security vulnerabilities - Usability - Design security that works with, not against, how users actually behave