Bootstrapping Mutual Attestation with Kleene's Second Recursion Theorem

Abstract

Mutual attestation among nodes with no central trusted operator requires each node to hold reference values (expected code measurements) for its peers. The naïve approach of mutually embedding these reference values in the nodes' code leads to an infinite regress. We call the problem of resolving this infinite regress the reference-value bootstrapping problem for mutual attestation. Existing solutions avoid this regress by relying on a trusted third party (TTP), externally supplied reference values, or architecture-specific measurement mechanisms. We instead express the bootstrapping problem as a system of mutual fixed-point equations and solve it by Kleene's second recursion theorem. The construction produces nodes that mutually reference one another's code and reconstruct every peer's exact source from built-in data alone. When a deployed source file is measured directly, as with a Python script, a node obtains the peer's reference value by applying the measurement function directly to the reconstructed source. When a built image is measured, as with AWS Nitro Enclaves, a node instead reproducibly rebuilds the peer's image from the reconstructed source and derives its reference measurement. For the first case, we develop PyReflect, a Python transpiler, and use it to implement a TPM mutual-attestation PoC. For the second, we develop NixReflect, a Nix transpiler, and use it in a PoC in which two Nitro Enclaves reproduce each other's reference PCRs from built-in data alone. Our solution is architecture-independent, requires neither a TTP nor externally supplied reference values, and works with existing attestation stacks unchanged.