Formal methods for guaranteeing that a protocol satisfies a cryptographic security definition have advanced substantially, but such methods are still labor intensive and the need remains for an automated tool that can positively identify an insecure protocol. In this work, we demonstrate that property-based testing, "run it a bunch of times and see if it breaks", is effective for detecting security bugs in secure protocols. We specifically target Secure Multi-Party Computation (MPC), because formal methods targeting this security definition for bit-model implementations are particularly difficult. Using results from the literature for Probabilistic Programming Languages and statistical inference, we devise a test that can detect various flaws in a bit-level implementation of an MPC protocol. The test is grey-box; it requires only transcripts of randomness consumed by the protocol and of the inputs, outputs, and messages. It successfully detects several different mistakes and biases introduced into two different implementations of the classic GMW protocol. Applied to hundreds of randomly generated protocols, it identifies nearly all of them as insecure. We also include an analysis of the parameters of the test, and discussion of what makes detection of MPC (in)security difficult.

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