A deep dive into Axiom’s Halo2 circuits

Summary: Trail of Bits reviewed Axiom's Halo2 circuits over two audits, from February 10 to May 17, 2023 and September 11 to September 29, 2023, covering low-level primitives, SNARK verification logic, and Axiom's changes between assessments. The reviews identified 35 security issues, four of them high severity and tied to soundness or under-constrained bugs, including an assert_equal function that compared a value with itself and a range_check enforced only by debug_assert.
Key Takeaway: A single typo in assert_equal, used in proof verification, would have broken proof soundness, and an under-constrained scalar multiplication circuit could have enabled signature forgery - in a system where a malicious prover generating a fake proof that verifies as correct defeats the entire purpose of the ZKP.

Read the original at Trail of Bits Blog ↗