Learn in the textbookSecure Development (By-Design Principles and Vulnerability Countermeasures)
DefinitionSecurity by Design (セキュリティバイデザイン) is the approach of building information security measures in from the earliest stages of development, such as planning and design, instead of adding them after the system is finished.
ExplanationTrying to add protection once the system is finished often means reworking the design, so the rework tends to be large. If threats are identified early and countermeasures are written into the requirements and design, vulnerabilities (security weaknesses) are less likely to creep in, and the cost of fixing them stays down.
AnalogyIt is like building a house: instead of buying locks and fences after it is finished, you settle the positions of doors and windows with burglary in mind while the plans are still on paper.
Memory hook"By design" means "on purpose": the system is safe because it was planned that way from the start, not by luck.