Exodus Round6 proof cap sized by threshold count, production-scale sparse validator index silently fails

Bug Bounty
1

Hi team

Finding: the Round6 proof size estimate is sized by participant count while the production BindingFactorsProof cap is 8, and the authority set is a sparse bitmap. With validators at indices 0 through 200 plus 299, the generated proof length is 9 against the cap of 8, so the honest Round6 package is rejected by the real dispatch with Round6PackageInvalidProof before signature verification. The full 300-authority production drive was not executed; the sparse bitmap shape was confirmed line for line in source and the rejection was executed through the real dispatch path.

generated proof length 9 vs production cap 8 -> Round6PackageInvalidProof

Impact: silent DKG liveness death at production scale. The honest-input constraint that triggers it is a sparse authority set, which rotation produces naturally.

Suggested fix: size the Round6 estimate by highest index and size the share proof cap by MaxAuthorities, not by threshold.

Shahmeer

2

Thank you for your report!

Shahmeer, that was a good take - it really made me think. But then I realized that BindingFactorsProof is associated with signature shares only. It is used in the BoundedVec to limit the number of entries for binding_factors_proof, as you can see here.

You can verify that it is used only there:

$ grep -rnw BindingFactorsProof pallets/
pallets/exodus/src/types/packages.rs:228:    pub binding_factors_proof: BoundedVec<ExodusHash, crate::BindingFactorsProof<T>>,
pallets/exodus/src/types/packages.rs:346:            BoundedVec::<ExodusHash, crate::BindingFactorsProof<T>>::try_from(binding_factors_proof_vec)
pallets/exodus/src/lib.rs:313:pub struct BindingFactorsProof<T>(sp_std::marker::PhantomData<T>);
pallets/exodus/src/lib.rs:314:impl<T: Config> Get<u32> for BindingFactorsProof<T> {

So BindingFactorsProof caps the binding-factors proof in the signature-share flow - not the Round6 verifying-share package.

Could you clarify whether I misunderstood you, or whether you are referring to something else?