A secure system does not simply ask whether something exists.
It asks whether it should be trusted.
Digital signatures, certificates, cryptographic keys, approval records, trust anchors, and validation policies are all part of the structures used to make that decision.
But trust is not permanent.
Keys can be compromised.
Credentials can be abused.
A legitimate approval can be used outside the scope for which it was issued.
When the foundation of trust itself becomes uncertain, finding every bad file may no longer be enough.
Sometimes the only viable response is to revoke the old basis of trust and establish a new one.
This section examines trust anchors, revocation, key rotation, validation, compromise recovery, and the problem of rebuilding trust after the system that decides what is legitimate can no longer be fully trusted.