When a deployer of a web application puts that application on a server (on-prem or cloud), how can they be sure that the application is executing as intended? This paper studies how the deployer can efficiently check that the execution is faithful. We seek mechanisms that: (i) work with web applications that are built with modern event-driven web frameworks, (ii) impose tolerable computation and communication overheads on the web server, and (iii) are complete and sound. We exhibit such a mechanism, based on a new record-replay algorithm. We have implemented our algorithm in Karousos, a system that audits Node.js web applications.

We present a zero-knowledge argument for NP with low communication complexity, low concrete cost for both the prover and the verifier, and no trusted setup, based on standard cryptographic assumptions. Communication is proportional to d log G (for d the depth and G the width of the verifying circuit) plus the square root of the witness size. When applied to batched or data-parallel statements, the prover's runtime is linear and the verifier's is sub-linear in the verifying circuit size, both with good constants. In addition, witness-related communication can be reduced, at the cost of increased verifier runtime, by leveraging a new commitment scheme for multilinear polynomials, which may be of independent interest. These properties represent a new point in the tradeoffs among setup, complexity assumptions, proof size, and computational cost. We apply the Fiat-Shamir heuristic to this argument to produce a zero-knowledge succinct non-interactive argument of knowledge (zkSNARK) in the random oracle model, based on the discrete log assumption, which we call Hyrax. We implement Hyrax and evaluate it against five state-of-the-art baseline systems. Our evaluation shows that, even for modest problem sizes, Hyrax gives smaller proofs than all but the most computationally costly baseline, and that its prover and verifier are each faster than three of the five baselines.