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An important principle of security engineering is defense-in-depth. There should never be a single point of failure that would allow for the whole system to be compromised.
Redundancy is vital because no matter how much attention a team puts into secure engineering, perfection is beyond reach. Although we limit our use of third party software significantly, we do use a combination of both proprietary and third party software. Vulnerabilities and bugs are regularly discovered even in fundamental software like operating systems that are developed and supported with multibillion-dollar engineering teams. Our system’s security should never rely on every single component being bug-free.