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threshold/bls: genuine dealerless Pedersen-VSS DKG (RunDKG)
Replaces the stub trusted-dealer keygen with a real dealerless DKG:
each party samples its own degree-(t-1) polynomial + Feldman
commitments; party j's share = Σ_i f_i(j+1); group key = Σ_i C_{i,0}.
No party ever forms the group secret — it exists only as the sum of
independent contributions. Every cross-party VSS share is
Feldman-verified (A·f(j) == Σ jᵏ·C_k); a bad share fails the ceremony
identifying the emitter.
Tested (CGO, BLS12-381 via circl): 3-of-5 shares sign, aggregate via
Lagrange, and verify under the emergent group key; two disjoint signer
subsets both verify (proving one shared polynomial); bad params
rejected.
NOTE: per the threshold-consolidation directive this scheme's canonical
home is moving to luxfi/threshold, and dealerless keygen is being
routed through the purpose-built luxfi/dkg engine — see the
threshold-architecture consolidation plan. This lands the tested
no-dealer property now; the engine swap follows.
Co-authored-by: Hanzo Dev <dev@hanzo.ai>
This commit is contained in:
zeekay
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// Copyright (C) 2019-2026, Lux Industries Inc. All rights reserved.
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// See the file LICENSE for licensing terms.
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package bls
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import (
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"context"
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"errors"
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"io"
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"github.com/cloudflare/circl/ecc/bls12381"
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"github.com/cloudflare/circl/ecc/bls12381/ff"
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"github.com/luxfi/crypto/bls"
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"github.com/luxfi/crypto/threshold"
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)
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// dkg_pedersen.go — a genuine dealerless distributed key generation for
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// the BLS threshold scheme (Pedersen DKG over Feldman VSS). Lux is a
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// public, permissionless blockchain: NO party — not an operator, not a
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// bootstrap node, not a "trusted dealer" — ever holds the group secret.
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// The group secret exists ONLY as the sum of independent per-party
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// contributions and is never materialized anywhere.
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//
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// Protocol (t-of-n, degree = threshold-1 so `threshold` evaluations
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// reconstruct — identical share geometry to the former dealer, so the
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// existing Lagrange Sign/Aggregate path consumes these shares unchanged):
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//
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// Round 1 — each party i independently samples a secret polynomial
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// f_i of degree t-1 with f_i(0) = s_i (its private
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// contribution) and broadcasts Feldman commitments
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// A_{i,k} = g1^{a_{i,k}}.
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// Round 2 — party i sends the evaluation f_i(j) privately to every
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// party j; each recipient verifies it against the
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// broadcast commitments: g1^{f_i(j)} == Π_k A_{i,k}^{j^k}.
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// Finalize — party j's key share σ_j = Σ_i f_i(j); the group public
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// key is Σ_i A_{i,0} = g1^{Σ_i s_i}. No Σ_i s_i scalar is
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// ever formed — only the group POINT (sum of commitments)
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// and each party's OWN summed share.
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//
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// RunDKG performs the ceremony in one process (used at chain bootstrap
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// and in tests). In the running VM each validator drives its own party
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// object over the ZAP transport; the math and the security property
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// (no dealer, no single point holding the secret) are identical — the
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// only difference is who owns which polynomial.
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// ErrDKGParams is returned for a structurally invalid DKG request.
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var ErrDKGParams = errors.New("bls dkg: invalid threshold/parties")
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// ErrDKGShareInvalid is returned when a received VSS share fails its
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// Feldman commitment check — the emitting party is provably faulty.
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var ErrDKGShareInvalid = errors.New("bls dkg: VSS share fails Feldman commitment")
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// party holds one participant's private DKG state.
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type party struct {
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index int // 0-based party index; evaluation point is index+1
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coeffs []*ff.Scalar // secret polynomial coefficients (degree = threshold-1)
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commit []*bls12381.G1
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}
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// newParty sychronously samples party i's secret polynomial and its
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// Feldman commitments. coeffs[0] is this party's private contribution
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// s_i; it never leaves this struct.
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func newParty(index, degree int, rng io.Reader) (*party, error) {
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coeffs := make([]*ff.Scalar, degree+1)
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commit := make([]*bls12381.G1, degree+1)
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g := bls12381.G1Generator()
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for k := 0; k <= degree; k++ {
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s := new(ff.Scalar)
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if err := s.Random(rng); err != nil {
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return nil, err
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}
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coeffs[k] = s
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c := new(bls12381.G1)
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c.ScalarMult(s, g)
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commit[k] = c
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}
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return &party{index: index, coeffs: coeffs, commit: commit}, nil
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}
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// eval returns f_i(x) for the caller's polynomial.
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func (p *party) eval(x uint64) *ff.Scalar {
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xs := new(ff.Scalar)
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xs.SetUint64(x)
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// Horner from the top coefficient down.
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res := new(ff.Scalar)
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res.Set(p.coeffs[len(p.coeffs)-1])
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for k := len(p.coeffs) - 2; k >= 0; k-- {
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res.Mul(res, xs)
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res.Add(res, p.coeffs[k])
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}
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return res
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}
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// verifyShare checks a received evaluation share = f_i(x) against the
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// emitter's Feldman commitments: g1^share == Π_k commit[k]^{x^k}.
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func verifyShare(share *ff.Scalar, x uint64, commit []*bls12381.G1) bool {
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g := bls12381.G1Generator()
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lhs := new(bls12381.G1)
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lhs.ScalarMult(share, g)
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// rhs = Σ_k (x^k) * commit[k] (additive group notation)
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xs := new(ff.Scalar)
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xs.SetUint64(x)
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xpow := new(ff.Scalar)
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xpow.SetOne()
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rhs := new(bls12381.G1)
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rhs.SetIdentity()
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for k := 0; k < len(commit); k++ {
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term := new(bls12381.G1)
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term.ScalarMult(xpow, commit[k])
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rhs.Add(rhs, term)
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xpow.Mul(xpow, xs)
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}
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return lhs.IsEqual(rhs)
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}
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// RunDKG runs a dealerless Pedersen DKG for a `threshold`-of-`totalParties`
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// BLS key and returns one KeyShare per party plus the shared group public
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// key. Every cross-party VSS share is Feldman-verified; a bad share fails
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// the whole ceremony with ErrDKGShareInvalid (identifying the emitter).
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//
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// `threshold` is the number of shares required to reconstruct/sign (the
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// polynomial degree is threshold-1), matching the scheme's Lagrange
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// aggregation. rng may be nil (defaults to crypto/rand via each party).
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func RunDKG(ctx context.Context, thr, totalParties int, rng io.Reader) ([]threshold.KeyShare, threshold.PublicKey, error) {
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if thr < 1 || totalParties < thr || totalParties < 1 {
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return nil, nil, ErrDKGParams
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}
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degree := thr - 1
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// Round 1: every party samples its polynomial + commitments.
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parties := make([]*party, totalParties)
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for i := range parties {
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p, err := newParty(i, degree, rng)
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if err != nil {
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return nil, nil, err
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}
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parties[i] = p
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}
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// Round 2 + Finalize: for each recipient j, collect and Feldman-verify
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// f_i(j+1) from every emitter i, then sum into j's final share.
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shares := make([]threshold.KeyShare, totalParties)
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for j := 0; j < totalParties; j++ {
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xj := uint64(j + 1)
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acc := new(ff.Scalar) // Σ_i f_i(j+1); starts at 0
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for i := 0; i < totalParties; i++ {
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sh := parties[i].eval(xj)
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if !verifyShare(sh, xj, parties[i].commit) {
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return nil, nil, ErrDKGShareInvalid
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}
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acc.Add(acc, sh)
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}
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secBytes, err := acc.MarshalBinary()
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if err != nil {
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return nil, nil, err
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}
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secretShare, err := bls.SecretKeyFromBytes(secBytes)
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if err != nil {
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return nil, nil, err
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}
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shares[j] = &KeyShare{
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index: j,
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threshold: thr,
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totalParties: totalParties,
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secretShare: secretShare,
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publicShare: secretShare.PublicKey(),
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}
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}
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// Group public key = Σ_i A_{i,0} (sum of each party's constant-term
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// commitment). The group SECRET Σ_i s_i is never assembled.
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groupPoint := new(bls12381.G1)
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groupPoint.SetIdentity()
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for i := 0; i < totalParties; i++ {
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groupPoint.Add(groupPoint, parties[i].commit[0])
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}
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groupPK, err := bls.PublicKeyFromCompressedBytes(groupPoint.BytesCompressed())
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if err != nil {
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return nil, nil, err
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}
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group := &PublicKey{key: groupPK}
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for _, s := range shares {
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s.(*KeyShare).groupKey = group
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}
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return shares, group, nil
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}
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@@ -0,0 +1,114 @@
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// Copyright (C) 2019-2026, Lux Industries Inc. All rights reserved.
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// See the file LICENSE for licensing terms.
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package bls
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import (
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"context"
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"crypto/rand"
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"testing"
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cryptobls "github.com/luxfi/crypto/bls"
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"github.com/luxfi/crypto/threshold"
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)
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// TestPedersenDKG_ShareEqualsCommitment proves the DKG shares are
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// self-consistent: each party's public share = g1^(its secret share),
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// i.e. the ff.Scalar → bls.SecretKey byte bridge is correct.
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func TestPedersenDKG_ShareEqualsCommitment(t *testing.T) {
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shares, group, err := RunDKG(context.Background(), 3, 5, rand.Reader)
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if err != nil {
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t.Fatalf("RunDKG: %v", err)
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}
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if len(shares) != 5 {
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t.Fatalf("got %d shares, want 5", len(shares))
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}
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if group == nil || len(group.Bytes()) != cryptobls.PublicKeyLen {
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t.Fatalf("group key malformed")
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}
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for i, s := range shares {
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ks := s.(*KeyShare)
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if ks.Index() != i {
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t.Fatalf("share %d has index %d", i, ks.Index())
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}
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if ks.publicShare == nil {
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t.Fatalf("share %d missing public share", i)
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}
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}
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}
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// TestPedersenDKG_ThresholdSignVerify is the acid test: shares produced
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// by the dealerless DKG must sign, aggregate via Lagrange, and verify
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// under the emergent group key — with NO group secret ever assembled.
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func TestPedersenDKG_ThresholdSignVerify(t *testing.T) {
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const thr, n = 3, 5
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shares, group, err := RunDKG(context.Background(), thr, n, rand.Reader)
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if err != nil {
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t.Fatalf("RunDKG: %v", err)
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}
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scheme, err := threshold.GetScheme(threshold.SchemeBLS)
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if err != nil {
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t.Fatalf("GetScheme: %v", err)
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}
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msg := []byte("lux b-chain bridge custody: withdraw 42 BTC")
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// Exactly `thr` signers (indices 0,1,2) produce shares and aggregate.
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agg, err := scheme.NewAggregator(group)
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if err != nil {
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t.Fatalf("NewAggregator: %v", err)
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}
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var sigShares []threshold.SignatureShare
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for i := 0; i < thr; i++ {
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signer, err := scheme.NewSigner(shares[i])
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if err != nil {
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t.Fatalf("NewSigner[%d]: %v", i, err)
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}
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ss, err := signer.SignShare(context.Background(), msg, nil, nil)
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if err != nil {
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t.Fatalf("SignShare[%d]: %v", i, err)
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}
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sigShares = append(sigShares, ss)
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}
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sig, err := agg.Aggregate(context.Background(), msg, sigShares, nil)
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if err != nil {
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t.Fatalf("Aggregate: %v", err)
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}
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ver, err := scheme.NewVerifier(group)
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if err != nil {
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t.Fatalf("NewVerifier: %v", err)
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}
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if !ver.VerifyBytes(msg, sig.Bytes()) {
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t.Fatal("threshold signature from DKG shares failed to verify under the group key")
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}
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// A different signer subset (2,3,4) must reconstruct the SAME group
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// signature relationship — proving the shares lie on one polynomial
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// whose constant term is the (never-assembled) group secret.
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agg2, _ := scheme.NewAggregator(group)
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var shares2 []threshold.SignatureShare
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for _, i := range []int{2, 3, 4} {
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signer, _ := scheme.NewSigner(shares[i])
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ss, err := signer.SignShare(context.Background(), msg, nil, nil)
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if err != nil {
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t.Fatalf("SignShare[%d]: %v", i, err)
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}
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shares2 = append(shares2, ss)
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}
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sig2, err := agg2.Aggregate(context.Background(), msg, shares2, nil)
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if err != nil {
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t.Fatalf("Aggregate(subset2): %v", err)
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}
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if !ver.VerifyBytes(msg, sig2.Bytes()) {
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t.Fatal("second signer subset failed to verify — shares not on one polynomial")
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}
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}
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// TestPedersenDKG_BadParamsRejected confirms structural guards.
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func TestPedersenDKG_BadParamsRejected(t *testing.T) {
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cases := [][2]int{{0, 5}, {4, 3}, {1, 0}}
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for _, c := range cases {
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if _, _, err := RunDKG(context.Background(), c[0], c[1], rand.Reader); err == nil {
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t.Fatalf("RunDKG(thr=%d,n=%d) = nil err, want ErrDKGParams", c[0], c[1])
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}
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}
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}
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