The Pivot Point: Tracing Public Inputs in a PSProve PoRep Bug Investigation

Introduction

In the course of a deep debugging session spanning dozens of messages, message [msg 1662] stands out as a pivotal moment of analytical clarity. At this point in the conversation, the assistant had been investigating a stubborn and narrow bug: PoRep (Proof of Replication) SNARK proofs generated through the CuZK GPU proving engine were failing verification when submitted through the PSProve (ProofShare/proof service) path, even though the exact same proofs worked correctly when generated through the standard Filecoin FFI path. The bug was intermittent, making it particularly maddening, and the investigation had already ruled out numerous potential causes — enum mappings, struct field definitions, prover ID derivation, and Rust dependency versions were all confirmed to be identical between the working and failing paths.

Message [msg 1662] represents the moment when the assistant pivots from broad structural comparisons to a focused examination of how public inputs flow through the system. It is a reasoning-first message: the assistant does not issue a definitive conclusion or implement a fix, but rather works through a critical logical comparison aloud, then issues a read tool call to examine the verification code that will confirm or refute its emerging hypothesis.

The Message in Full

The subject message reads:

Now the critical comparison. In the FFI path: - cgo.SealCommitPhase2(phase1Output, sectorId, proverId) — passes sectorId and proverId as separate arguments to the C function - The C function seal_commit_phase2 uses these to build the public inputs for the Groth16 circuit

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In the cuzk path: - seal::seal_commit_phase2(c1_output, prover_id, sector_id) — also passes them as separate arguments - Both derive prover_id from miner_id the same way

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But wait — let me look at what the Rust seal_commit_phase2 actually does with these arguments. The critical thing is that the SNARK public inputs include the prover_id and sector_id. If these differ between generation and verification, the proof fails.

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Let me check what ffi.VerifySeal does and compare: [read] /tmp/czk/extern/filecoin-ffi/proofs.go

The message then shows the beginning of the VerifySeal function in the Go FFI code, which reveals that verification uses SealProof, CommR (from SealedCID), CommD (from UnsealedCID), ProverID (from info.Miner via toProverID()), Randomness (ticket), InteractiveRandomness (seed), SectorID.Number, and the Proof bytes.

Why This Message Was Written

The message was written at a specific inflection point in a multi-threaded investigation. Prior to this message, the assistant had spent considerable effort ruling out structural causes:

  1. Enum mappings (msg [msg 1651]): All RegisteredSealProof values were traced across Go, C FFI, and Rust — they matched perfectly.
  2. JSON struct fields (msg [msg 1651]): Every field in Rust's SealCommitPhase1Output was modeled in Go's Commit1OutRaw, including PhantomData fields.
  3. Prover ID derivation (msg [msg 1661]): CuZK's make_prover_id() was confirmed to use the same LEB128/varint encoding as Go's toProverID().
  4. Rust dependency versions (msg [msg 1670]): filecoin-proofs-api 19.0.0 and related crates matched between FFI and CuZK.
  5. Data flow through proofshare (msg [msg 1654]): The serialization chain was analyzed and no corruption was found. With all these avenues exhausted, the assistant was left with a single remaining variable: the content of the Phase1Out JSON bytes. The working path used raw bytes directly from ffi.SealCommitPhase1() (Rust's native serde_json output), while the failing path used Go-re-serialized JSON after round-tripping through Go structs (json.Marshal(Commit1OutRaw{...})). Message [msg 1662] represents the assistant's attempt to reason about why a byte-level difference in the JSON payload would cause verification to fail. The key insight is that the SNARK public inputs — prover_id and sector_id — must be identical between generation and verification. If the JSON round-trip altered any value that feeds into the public input derivation, CuZK would generate a valid SNARK for the wrong public inputs, and verification would fail because it uses the correct values.

The Thinking Process Visible in the Message

The message reveals a clear chain of reasoning. The assistant begins by stating its intent: "Now the critical comparison." It then lays out the two paths side by side:

Assumptions and Potential Pitfalls

The assistant makes several assumptions in this message, most of which are well-justified:

  1. The SNARK public inputs are deterministic: The assistant assumes that if prover_id and sector_id are the same between generation and verification, the proof should verify. This is true for Groth16, but only if all other public inputs (comm_r, comm_d, etc.) are also identical. The assistant is aware of this and is about to check the full set of inputs used by VerifySeal.
  2. The C function and Rust function are semantically equivalent: The assistant assumes that the C FFI function seal_commit_phase2 and the Rust function seal::seal_commit_phase2 perform the same operations. While they should be (the C function is a thin wrapper over the Rust function), version mismatches or patching could introduce subtle differences.
  3. The prover ID derivation is the only source of public input difference: At this point, the assistant is focused on prover_id and sector_id as the potential source of mismatch. However, as the investigation later reveals, the actual culprit may be more subtle — the JSON round-trip could alter comm_r, comm_d, or other fields embedded in the C1 output that feed into the circuit's constraint system.
  4. The CuZK server correctly forwards parameters: The assistant implicitly trusts that the CuZK gRPC service correctly passes SectorNumber and MinerId from the request to prove_porep_c2. This is a reasonable assumption given that the normal CuZK PoRep path (which works) uses the same service. One potential mistake in the reasoning is the assistant's focus on prover_id and sector_id as separate arguments. While these are indeed separate parameters to seal_commit_phase2, the Rust function may also extract values from the deserialized c1_output struct (such as comm_r, comm_d, seed, ticket) and use them to derive public inputs. If the Go JSON round-trip altered any of these embedded values, CuZK would generate a SNARK for the altered values, but verification would use the original values — causing a mismatch even though prover_id and sector_id are correct.

Input Knowledge Required

To fully understand this message, the reader needs knowledge of:

  1. The Filecoin proof architecture: PoRep (Proof of Replication) is a two-phase proving system. Phase 1 (C1) generates a "vanilla proof" — a JSON blob containing commitments, seeds, and merkle proofs. Phase 2 (C2) compresses this into a Groth16 SNARK. The SNARK is bound to public inputs including prover_id, sector_id, comm_r, comm_d, and the randomness seeds.
  2. The CuZK GPU prover: CuZK is a GPU-accelerated proving engine that replaces the CPU-based FFI for C2 computation. It accepts the same C1 JSON output but processes it through a gRPC service. The service deserializes the JSON, runs the C2 circuit on the GPU, and returns the SNARK proof.
  3. The PSProve/ProofShare system: This is a marketplace for proof outsourcing. Miners can submit C1 outputs to the ProofShare service, which then assigns the C2 computation to worker nodes. The PSProve path involves additional JSON serialization/deserialization steps that the normal proving path does not.
  4. Groth16 SNARKs: The assistant's reasoning hinges on the property that Groth16 proofs are bound to specific public inputs. If the prover uses one set of public inputs and the verifier uses another, verification fails. This is a fundamental cryptographic property.
  5. Go and Rust JSON serialization: Go's json.Marshal and Rust's serde_json may produce different output for the same logical data — field ordering, numeric formatting, and whitespace can differ. While JSON parsers are generally field-order-independent, the assistant is investigating whether a subtle serialization difference causes Rust to interpret the data differently.

Output Knowledge Created

This message creates several important outputs:

  1. A clear articulation of the public input hypothesis: The assistant explicitly states that if prover_id or sector_id differ between generation and verification, the proof will fail. This frames the investigation in terms of public input consistency, which is a more precise framing than "the JSON is different."
  2. A structural comparison of the two paths: By laying out the FFI and CuZK paths side by side, the message creates a clear reference for understanding where the paths diverge. Both paths call the same underlying Rust function with the same arguments — the only difference is how the C1 JSON bytes are produced.
  3. A targeted read of VerifySeal: The message initiates a read of the verification code, which will reveal the exact set of public inputs used during verification. This is essential for comparing against the inputs used during generation.
  4. Narrowing the search space: By confirming that both paths call the same Rust function with the same argument structure, the message eliminates the possibility that the bug is in how CuZK calls the Rust library. The bug must be in the data fed to that call — specifically, the C1 JSON bytes.

The Broader Significance

Message [msg 1662] is significant not because it solves the bug, but because it represents a methodological shift in the investigation. Prior to this message, the assistant was systematically ruling out structural causes — checking that types, enums, and function signatures matched. This message marks the transition to a data-centric investigation: the focus shifts from "are the code paths structurally identical?" to "are the actual bytes identical?"

This is a common pattern in debugging distributed systems. Structural analysis (comparing types, interfaces, and call graphs) can eliminate many potential causes, but it cannot detect semantic differences in data. When two code paths are structurally identical but produce different results, the investigator must examine the actual data flowing through the system — often requiring instrumentation, logging, or byte-level comparison.

The message also demonstrates an important debugging technique: articulating the hypothesis before checking the evidence. The assistant states "if these differ between generation and verification, the proof fails" before reading the verification code. This creates a falsifiable prediction: if VerifySeal uses the same prover_id and sector_id that CuZK used during generation, then the public inputs are consistent and the bug must be elsewhere. If they differ, the root cause is found.

Conclusion

Message [msg 1662] captures a moment of analytical pivot in a complex debugging session. The assistant, having exhausted structural comparisons, turns its attention to the data flowing through the system. By comparing how the FFI and CuZK paths handle public inputs, and by initiating a read of the verification code, the assistant sets the stage for the next phase of the investigation: byte-level comparison of the JSON payloads.

The message is a testament to the value of explicit reasoning in debugging. The assistant does not simply issue a tool call and wait for results — it works through the logic aloud, articulating hypotheses, identifying assumptions, and framing the investigation in terms of falsifiable predictions. This approach not only helps the assistant itself (by clarifying its own thinking) but also provides a clear record of the investigation for the human collaborator.

In the broader context of the session, this message is the turning point. After this, the investigation moves from "what could be different?" to "what is different?" — a shift from structural analysis to data analysis that ultimately leads to the root cause.