Bundling Evidence for Layered Attestation

Abstract

Systems designed with measurement and attestation in mind are often layered, with the lower layers measuring the layers above them. Attestations of such systems, which we call layered attestations, must bundle together the results of a diverse set of application-specific measurements of various parts of the system. Some methods of layered attestation are more trustworthy than others, so it is important for system designers to understand the trust consequences of different system configurations. This paper presents a formal framework for reasoning about layered attestations, and provides generic reusable principles for achieving trustworthy results.

User-Centered Attestation for Layered and Decentralised Systems

“…The semantics of trust, trustworthiness, and trust establishment in peer-to-peer systems are discussed further in [4], [18], [19]. Alternative approaches towards trust and trust management based on social notions and reputation systems are presented in [12], [16], [20].…”

Orchestrating Layered Attestations

“…By measuring the kernel at p before the userspace measurement occurs, the appraiser can learn that the kernel was uncompromised at the time of the userspace measurement. This bottom-up strategy is common in approaches to layered attestation [17,22]. Additionally, an appraiser may wish each piece of evidence to be signed as a rudimentary chain of evidence.…”

“…A layered attestation whose structure mimics the layered dependency structure of a target system can provide strong trust guarantees. In prior work, we have formally proved that "bottom-up" strategies for layered attestation force an adversary to either corrupt well-protected components or work within small time-of-check-time-of-use windows [17,18].…”

“…For this purpose we provide a denotational semantics relating phrases to a partially ordered set of events. This semantics is explicitly designed to connect with our prior work on analytic principles of layered attestation [17,18]. By applying those principles in static analysis, both target and appraiser can write policies determining which phrases may be used in which situations based on the trust guarantees they provide.…”

“…This ensures that any trust conclusions made from the event partial order are guaranteed to hold over the concrete execution. In particular, our previous work [17,18] characterizes what an adversary must do to avoid detection given a specific partial order of events, identifying strategies to force an adversary to work quickly in short time-of-check-timeof-use windows, or dig deeper into more protected layers of the system. This connection is particularly important in light of the flexibility of the language.…”