Secure by design
Secure by design (SBD), in software engineering, means that the product has been designed from the foundation to be secure. In such an approach, the alternate security tactics and patterns are first thought; among these, the best are selected and enforced by the architecture design, and then, they are used as guiding principles for developers. It is also encouraged to use design patterns that have beneficial effects on security, even though those design patterns were not originally devised with security in mind.  Secure by Design is increasingly becoming the mainstream development approach to ensure security and privacy of software systems. In this approach, security is built into the system from the ground up and starts with a robust architecture design. Security architectural design decisions are often based on well-known security tactics, and patterns defined as reusable techniques for achieving specific quality concerns. Security tactics/patterns provide solutions for enforcing the necessary authentication, authorization, confidentiality, data integrity, privacy, accountability, availability, safety and non-repudiation requirements, even when the system is under attack. In order to ensure the security of a software system, not only it is important to design a robust security architecture (intended) but also it is necessary to preserve the (implemented) architecture during software evolution. Malicious practices are taken for granted and care is taken to minimize impact in anticipation of security vulnerabilities, when a security vulnerability is discovered or on invalid user input. Closely related is the practice of using "good" software design, such as domain-driven design or cloud native, as a way to increase security by reducing risk of vulnerability-opening mistakes—even though the design principles used were not originally conceived for security purposes.
Generally, designs that work well do not rely on being secret. Often, secrecy reduces the number of attackers by demotivating a subset of the threat population. The logic is that if there is an increase in complexity for the attacker, the increased attacker effort to compromise the target. While this technique implies reduced inherent risks, a virtually infinite set of threat actors and techniques applied over time will cause most secrecy methods to fail. While not mandatory, proper security usually means that everyone is allowed to know and understand the design because it is secure. This has the advantage that many people are looking at the computer code, which improves the odds that any flaws will be found sooner (see Linus's law). Attackers can also obtain the code, which makes it easier for them to find vulnerabilities as well.
Also, it is important that everything works with the fewest privileges possible (see the principle of least privilege). For example, a Web server that runs as the administrative user ("root" or admin) can have the privilege to remove files and users that do not belong. A flaw in such a program could put the entire system at risk, whereas a Web server that runs inside an isolated environment and only has the privileges for required network and filesystem functions, cannot compromise the system it runs on unless the security around it is in itself also flawed.
Secure Design should be a consideration at all points in the development lifecycle (whichever development methodology is chosen).
Some pre-built Secure By Design development methodologies exist (e.g. Microsoft Security Development Lifecycle).
Microsoft Security Development Lifecycle
Standards and Legislation
Standards and Legislation exist to aide secure design by controlling the definition of "Secure", and providing concrete steps to testing and integrating secure systems.
Some examples of standards which cover or touch on Secure By Design principles:
- ETSI TS 103 645  which is included in part in the UK Government "Proposals for regulating consumer smart product cyber security" 
- ISO/IEC 27000-series covers many aspects of secure design.
In server/client architectures, the program at the other side may not be an authorised client and the client's server may not be an authorised server. Even when they are, a man-in-the-middle attack could compromise communications.
Often the easiest way to break the security of a client/server system is not to go head on to the security mechanisms, but instead to go around them. A man in the middle attack is a simple example of this, because you can use it to collect details to impersonate a user. Which is why it is important to consider encryption, hashing, and other security mechanisms in your design to ensure that information collected from a potential attacker won't allow access.
Another key feature to client-server security design is good coding practices. For example, following a known software design structure, such as client and broker, can help in designing a well-built structure with a solid foundation. Furthermore, if the software is to be modified in the future, it is even more important that it follows a logical foundation of separation between the client and server. This is because if a programmer comes in and cannot clearly understand the dynamics of the program, they may end up adding or changing something that can add a security flaw. Even with the best design, this is always a possibility, but the better the standardization of the design, the less chance there is of this occurring.
- "A Catalog of Security Architecture Weaknesses". 2017 IEEE International Conference on Software Architecture (ICSA). doi:10.1109/ICSAW.2017.25.
- Manning (2019). Secure By Design. ISBN 9781617294358.
- "Growing a pattern language (for security)". Onward! 2012: Proceedings of the ACM International Symposium on New Ideas, New Paradigms, and Reflections on Programming and Software: 139–158. October 2012. doi:10.1145/2384592.2384607.
- Dougherty, Chad; Sayre, Kirk; Seacord, Robert C.; Svoboda, David; Togashi, Kazuya (October 2009). "Secure Design Patterns". doi:10.1184/R1/6583640.v1. Cite journal requires
- "ETSI TS 103 645" (PDF).
- "Policy paper: Proposals for regulating consumer smart product cyber security - call for views".
- Computer security
- Cyber security standards
- Multiple Independent Levels of Security
- Secure by default
- Security through obscurity
- Software Security Assurance
- Secure Programming for Linux and Unix HOWTO
- Secure UNIX Programming FAQ
- Top 10 Secure Coding Practices
- Security by Design Principles