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dkg/gennaro/round4.go

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+//
+// Copyright Coinbase, Inc. All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package gennaro
+
+import (
+	"math/big"
+
+	"github.com/sonr-io/sonr/crypto/core"
+	"github.com/sonr-io/sonr/crypto/core/curves"
+	"github.com/sonr-io/sonr/crypto/internal"
+	v1 "github.com/sonr-io/sonr/crypto/sharing/v1"
+)
+
+// Round4 computes the public shares used by tECDSA during signing
+// that are converted to additive shares once the signing participants
+// are known. This function is idempotent
+func (dp *Participant) Round4() (map[uint32]*curves.EcPoint, error) {
+	// Check participant is not empty
+	if dp == nil || dp.curve == nil {
+		return nil, internal.ErrNilArguments
+	}
+
+	// Check participant has the correct dkg round number
+	if dp.round != 4 {
+		return nil, internal.ErrInvalidRound
+	}
+
+	n := len(dp.otherParticipantShares) + 1 //+1 to include self
+	// Wj's
+	publicShares := make(map[uint32]*curves.EcPoint, n)
+
+	// 1. R = {{R1,...,Rt},{Rij,...,Rit}i!=j}
+	r := make(map[uint32][]*v1.ShareVerifier, n)
+	r[dp.id] = dp.pedersenResult.Verifiers
+	for j := range dp.otherParticipantShares {
+		r[j] = dp.otherParticipantShares[j].Verifiers
+	}
+
+	// 2. for j in 1,...,n
+	for j, v := range r {
+		// 3. Wj = Pk
+		publicShares[j] = &curves.EcPoint{
+			Curve: dp.verificationKey.Curve,
+			X:     new(big.Int).Set(dp.verificationKey.X),
+			Y:     new(big.Int).Set(dp.verificationKey.Y),
+		}
+
+		// 4. for k in 1,...,t
+		for k := 0; k < len(dp.pedersenResult.Verifiers); k++ {
+			// 5. ck = pj * k mod q
+			pj := big.NewInt(int64(j))
+			ck, err := core.Mul(pj, big.NewInt(int64(k+1)), dp.curve.Params().N)
+			if err != nil {
+				return nil, err
+			}
+
+			// 6a. t = ck * Rj
+			t, err := v[k].ScalarMult(ck)
+			if err != nil {
+				return nil, err
+			}
+
+			// 6b. Wj = Wj + t
+			publicShares[j], err = publicShares[j].Add(t)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return publicShares, nil
+}