Requirements For Root Of Trust Establishment

Root-of-Trust (RoT) establishment assures that either a state of an untrusted system contains all and only content chosen by an external verifier and the verifier's code begins execution in that state, or the verifier discovers the existence of unaccounted content. RoT establishment is sufficient to assure program booting in malware-free system states, and necessary for establishing secure initial states for any software system. In particular, it is necessary for software deployed in access control and cryptographic applications despite the presence of an adversary (e.g., persistent malware) that controls that system. In this paper, we define requirements for RoT establishment and their relationships. These requirements differ from those for software-based and cryptographic attestation protocols. We point out these differences and explain why these protocols cannot be expected to satisfy the defined RoT requirements. Then we argue that jointly satisfying all these requirements yields a secure solution for establishing malware-free states - a strictly weaker requirement than RoT establishment. However, to establish RoT, it is sufficient to load a family of almost universal hash functions in a malware-fee state and then verify their outputs when applied to state components.