[BUG] Fix HyperKZG verify ignoring pairing check result

[jsemldonado]

coeff_form_uni_hyperkzg_verify was discarding the result of the pairing check and returning true unconditionally.

coeff_form_uni_kzg_verify(...);  // result discarded
true                              // always passed

This meant any proof would pass verification.

Fixed by returning the actual result. Added regression test that corrupts a proof and verifies it gets rejected.

[gemini-code-assist]

Summary of Changes

Hello @jsemldonado, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the HyperKZG polynomial commitment scheme's verification process, where the coeff_form_uni_hyperkzg_verify function was inadvertently ignoring the result of its core pairing check and always returning a successful verification. The fix ensures that the actual verification outcome is respected, restoring the cryptographic security of the system. Additionally, a robust regression test has been added to validate this fix and safeguard against similar issues in the future.

Highlights

• Critical Bug Fix in HyperKZG Verification: The coeff_form_uni_hyperkzg_verify function was incorrectly returning true unconditionally, effectively bypassing the actual pairing check and allowing any proof to pass verification.
• Corrected Verification Logic: The function now correctly returns the result of the pairing check, ensuring that only valid proofs are accepted.
• Added Regression Test: A new test case has been introduced to explicitly verify that the HyperKZG system rejects intentionally corrupted proofs, preventing future regressions of this critical bug.

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[gemini-code-assist]

Code Review

This pull request addresses a critical vulnerability in coeff_form_uni_hyperkzg_verify where proof verification was being skipped entirely. The fix correctly propagates the result of the pairing check, ensuring proofs are actually verified. The inclusion of a regression test that confirms a corrupted proof is rejected is a great addition. My review includes a minor suggestion to improve the readability of the new test code by reducing repetition.

I am having trouble creating individual review comments. Click here to see my feedback.

poly_commit/src/kzg/uni_kzg/hyper_kzg.rs (255-257)

This change fixes a critical bug where the result of the pairing check was discarded and true was always returned, effectively bypassing proof verification. Returning the result of coeff_form_uni_kzg_verify directly is the correct fix.

poly_commit/tests/test_uni_kzg.rs (108-145)

To improve readability and reduce repetition, you could introduce a type alias for HyperUniKZGPCS<Bn256>. This would make the repeated calls to the PolynomialCommitmentScheme trait methods cleaner and easier to read.

    type PCS = HyperUniKZGPCS<Bn256>;
    let mut rng = test_rng();
    let num_vars = 4;

    let (srs, _) =
        <PCS as PolynomialCommitmentScheme<Fr>>::gen_srs_for_testing(
            &num_vars, &mut rng,
        );
    let (proving_key, verification_key) = srs.into_keys();

    let poly = MultiLinearPoly::<Fr>::random(num_vars, &mut rng);
    let x: Vec<Fr> = (0..num_vars).map(|_| Fr::random_unsafe(&mut rng)).collect();

    let mut scratch_pad = ();
    let commitment =
        <PCS as PolynomialCommitmentScheme<Fr>>::commit(
            &num_vars, &proving_key, &poly, &mut scratch_pad,
        );

    let mut transcript = BytesHashTranscript::<Keccak256hasher>::new();
    let (eval, opening) = <PCS as PolynomialCommitmentScheme<Fr>>::open(
        &num_vars, &proving_key, &poly, &x, &scratch_pad, &mut transcript,
    );

    // Valid proof should pass
    let mut transcript_v = BytesHashTranscript::<Keccak256hasher>::new();
    assert!(<PCS as PolynomialCommitmentScheme<Fr>>::verify(
        &num_vars, &verification_key, &commitment, &x, eval, &opening, &mut transcript_v,
    ));

    // Corrupted proof should fail
    let mut corrupted = opening.clone();
    corrupted.quotient_delta_x_commitment =
        (corrupted.quotient_delta_x_commitment.to_curve() * Fr::from(2u64)).to_affine();

    let mut transcript_c = BytesHashTranscript::<Keccak256hasher>::new();
    assert!(!<PCS as PolynomialCommitmentScheme<Fr>>::verify(
        &num_vars, &verification_key, &commitment, &x, eval, &corrupted, &mut transcript_c,
    ));