Cobweb: Practical Remote Attestation Using Contextual Graphs

In theory, remote attestation is a powerful primitive for building distributed systems atop untrusting peers. Unfortunately, the canonical attestation framework defined by the Trusted Computing Group is insufficient to express rich contextual relationships between client-side software components. Thus, attestors and verifiers must rely on ad-hoc mechanisms to handle real-world attestation challenges like attestors that load executables in nondeterministic orders, or verifiers that require attestors to track dynamic information flows between attestor-side components. In this paper, we survey these practical attestation challenges. We then describe a new attestation framework, named Cobweb, which handles these challenges. The key insight is that real-world attestation is a graph problem. An attestation message is a graph in which each vertex is a software component, and has one or more labels, e.g., the hash value of the component, or the raw file data, or a signature over that data. Each edge in an attestation graph is a contextual relationship, like the passage of time, or a parent/child fork() relationship, or a sender/receiver IPC relationship. Cobweb's verifier-side policies are graph predicates which analyze contextual relationships. Experiments with real, complex software stacks demonstrate that Cobweb's abstractions are generic and can support a variety of real-world policies.