Secure Software Design

Key Concepts

Secure software design involves integrating security principles into the architecture and design phases of software development. The key concepts include:

• Principle of Least Privilege
• Defense in Depth
• Separation of Duties
• Fail-Safe Defaults
• Economy of Mechanism

Principle of Least Privilege

The Principle of Least Privilege (PoLP) states that a user or process should have the minimum level of access necessary to perform their tasks. This principle reduces the risk of unauthorized actions and limits the potential damage from security breaches.

For example, a user account in a banking system should only have access to the specific functions and data required for their role, such as viewing account balances or processing transactions. They should not have access to administrative functions or sensitive customer data.

Defense in Depth

Defense in Depth involves implementing multiple layers of security controls to protect the system. This approach ensures that if one layer is breached, other layers can still provide protection.

Consider a network where multiple security measures are in place: firewalls, intrusion detection systems, and encryption. If an attacker bypasses the firewall, the intrusion detection system can still detect and respond to the threat. If both are compromised, encryption can still protect data in transit.

Separation of Duties

Separation of Duties (SoD) is a principle that divides critical tasks among multiple users to prevent fraud and errors. No single user should have control over an entire critical process.

In a financial system, the tasks of approving transactions, processing payments, and reconciling accounts should be divided among different users. This ensures that no single user can authorize and execute a transaction without oversight.

Fail-Safe Defaults

Fail-Safe Defaults ensure that the system defaults to a secure state in case of failure or error. This principle minimizes the risk of unauthorized access or data exposure.

For instance, a file-sharing application should default to denying access to files unless explicitly granted. If the application fails to apply permissions correctly, files remain protected by default.

Economy of Mechanism

Economy of Mechanism (EoM) suggests that security mechanisms should be as simple and small as possible. Complexity can introduce vulnerabilities and make the system harder to secure.

A login system that uses a straightforward password authentication method is easier to secure than one that includes multiple layers of authentication, such as biometrics and security tokens. The simpler system is less likely to have overlooked vulnerabilities.