Port BNB cryptographic hardening follow-ups#6
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Record PR #6's changes: per-proof-system Fiat-Shamir domain tags (further transcript change), the PrepareForSigning error-return source break, and stricter NewParameters/SortPartyIDs validation as breaking changes; plus shared input validators, VSS reconstruction checks, idempotent message redelivery, and review-follow-up correctness fixes as non-breaking hardening. Add PR #6 to the composing-PRs list and update the source-compatibility note for PrepareForSigning.
Doc + small defensive-code follow-ups to the PR #2/#4/#5 review thread. None of these are security-blocking; they close out the cosmetic and pre-existing items that did not warrant their own PR earlier. - common/hash_utils.go: strengthen RejectionSample doc so the name does not mislead future readers — the function is modular reduction, not true rejection sampling, and the bias bound depends on q vs the hash width. - crypto/paillier/factor_proof.go: document that the tagged and legacy FactorChallenge paths emit different challenge distributions (positive-only vs signed), and note that FactorVerify's CmpAbs bounds exist to accommodate the legacy signed encoding. - tss/params.go: expand SetSessionNonceBytes docstring with the collision/uniqueness/entropy guidance reviewers asked for. - ecdsa/{keygen,signing}/rounds.go: document the round-1-capture invariant for getSSID so future refactors do not silently drift the hashed round.number domain separator. - crypto/ecpoint.go: flag SetCurve's in-place mutation in the doc comment; the chained-call style is a footgun on shared points. - crypto/vss/feldman_vss.go: reject nil shares, nil/zero share IDs, and duplicate IDs in ReConstruct before the Lagrange loop, so malformed inputs return an error instead of panicking through ModInverse(0). Add focused test coverage for each rejection path. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The previous text stated the bias bound as `q / 2^eHash.BitLen()`, which
evaluates to ≈ 1 for secp256k1 and cannot support the docstring's own
≈ 2^-128 conclusion. The "q significantly smaller than 2^eHash.BitLen()
(e.g., q = 2^256)" example was also internally inconsistent (q = 2^256
is not smaller than 2^256).
Rewrite the paragraph to:
- State the safe regime as a property of q ("close to 2^k from below")
rather than via a loose formula or call-site enumeration.
- Drop the unused curve25519 reference whose conclusion is correct but
not derivable from the simple `r/M` bound.
- Cross-reference HashToN / HashToNTagged for the large-modulus regime
that they were introduced to address.
Documentation-only change. RejectionSample's behavior is unchanged
(LiterallyJustMod under the hood), so no computed challenge value moves.
Record PR #6's changes: per-proof-system Fiat-Shamir domain tags (further transcript change), the PrepareForSigning error-return source break, and stricter NewParameters/SortPartyIDs validation as breaking changes; plus shared input validators, VSS reconstruction checks, idempotent message redelivery, and review-follow-up correctness fixes as non-breaking hardening. Add PR #6 to the composing-PRs list and update the source-compatibility note for PrepareForSigning.
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Summary
Stacked on #5. Ports the remaining BNB cryptographic hardening follow-ups into Threshold's secp256k1-only TSS stack.
GetRandomInt's zero-inclusive range, deterministic message wire bytes, and large-modulussampleYModNblock indexing.Compatibility
BREAKING: tagged Fiat-Shamir challenge inputs change proof transcripts. All operational nodes must upgrade in lockstep; mixed old/new proof verification is expected to fail.
Additional API/behavior changes:
ecdsa/signing.PrepareForSigningnow returns an error as its third return value.tss.NewParametersrejects party counts below 2, thresholds outside[1, partyCount), zero PartyID residues, and mod-q PartyID collisions.tss.SortPartyIDspanics on duplicate raw party keys instead of silently sorting invalid input.Downstream Check
threshold-network/keep-coreuses ofPrepareForSigning.threshold-network/keep-coreNewParameterscall sites use group-derived party sets; the visible test call site uses five parties.Validation
go test ./...go test ./common ./crypto/schnorr ./crypto/vss ./crypto/paillier ./ecdsa/keygen ./ecdsa/signing -run 'Test(GetRandomIntPreservesZeroInclusiveRange|Schnorr.*UnregisteredCurve|VerifyAllowsUnregisteredCurve|SampleYModNDeterministicAndSupportsManyBlocks|StoreMessage)'git diff --check