Biblio

Experience shows that even with a well-intentioned user at the keyboard, a motivated attacker can compromise a computer system at a layer below or adjacent to the shallow forms of authentication that are now accepted as commonplace[3]. Therefore, rather than asking "Can we trust the person behind the keyboard", a still better question might be: "Can we trust the computer system underneath?". An emerging technology for gaining trust in a remote computing system is remote attestation. Remote attestation is the activity of making a claim about properties of a target by supplying evidence to an appraiser over a network[2]. Although many existing approaches to remote attestation wisely adopt a layered architecture-where the bottom layers measure layers above-the dependencies between components remain static and measurement orderings fixed. For modern computing environments with diverse topologies, we can no longer fix a target architecture any more than we can fix a protocol to measure that architecture.Copland [1] is a domain-specific language and formal framework that provides a vocabulary for specifying the goals of layered attestation protocols. It also provides a reference semantics that characterizes system measurement events and evidence handling; a foundation for comparing protocol alternatives. The aim of this work is to refine the Copland semantics to a more fine-grained notion of attestation manager execution-a high-privilege thread of control responsible for invoking attestation services and bundling evidence results. This refinement consists of two cooperating components called the Copland Compiler and the Attestation Virtual Machine (AVM). The Copland Compiler translates a Copland protocol description into a sequence of primitive attestation instructions to be executed in the AVM. When considered in combination with advances in virtualization, trusted hardware, and high-assurance system software components-like compilers, file-systems, and OS kernels-a formally verified remote attestation infrastructure creates exciting opportunities for building system-level security arguments.

We present Copland, a language for specifying layered attestations. Layered attestations provide a remote appraiser with structured evidence of the integrity of a target system to support a trust decision. The language is designed to bridge the gap between formal analysis of attestation security guarantees and concrete implementations. We therefore provide two semantic interpretations of terms in our language. The first is a denotational semantics in terms of partially ordered sets of events. This directly connects Copland to prior work on layered attestation. The second is an operational semantics detailing how the data and control flow are executed. This gives explicit implementation guidance for attestation frameworks. We show a formal connection between the two semantics ensuring that any execution according to the operational semantics is consistent with the denotational event semantics. This ensures that formal guarantees resulting from analyzing the event semantics will hold for executions respecting the operational semantics. All results have been formally verified with the Coq proof assistant.