Root of Trust and Chain of Trust

In a chain of trust, the trustworthiness of each layer of software that composes the chain is guaranteed by the previous layer, until reaching the root of the chain, or root of trust. Immutability and formal verification provide the foundation for a root of trust. An example is the code present in read-only memory (ROM). The root of trust initiates the chain of trust.

A chain of trust, for example, can include: bootROM to boot loader to TrustZone operating system.