utils: add MuSig2Sign function

utils/musig.go

@@ -0,0 +1,82 @@
+package utils
+
+import (
+	"fmt"
+
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
+	"github.com/lightningnetwork/lnd/input"
+)
+
+// MuSig2Sign will create a MuSig2 signature for the passed message using the
+// passed private keys.
+func MuSig2Sign(version input.MuSig2Version, privKeys []*btcec.PrivateKey,
+	pubKeys []*btcec.PublicKey, tweaks *input.MuSig2Tweaks,
+	msg [32]byte) ([]byte, error) {
+
+	// Next we'll create MuSig2 sessions for each individual private
+	// signing key.
+	sessions := make([]input.MuSig2Session, len(privKeys))
+	for i, signingKey := range privKeys {
+		_, muSigSession, err := input.MuSig2CreateContext(
+			version, signingKey, pubKeys, tweaks, nil,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("error creating "+
+				"signing context: %v", err)
+		}
+
+		sessions[i] = muSigSession
+	}
+
+	// Next we'll pass around all public nonces to all MuSig2 sessions so
+	// that they become usable for creating the partial signatures.
+	for i := 0; i < len(privKeys); i++ {
+		nonce := sessions[i].PublicNonce()
+
+		for j := 0; j < len(privKeys); j++ {
+			if i == j {
+				// Step over if it's the same session.
+				continue
+			}
+
+			_, err := sessions[j].RegisterPubNonce(nonce)
+			if err != nil {
+				return nil, fmt.Errorf("error sharing "+
+					"MuSig2 nonces: %v", err)
+			}
+		}
+	}
+
+	// Now that the sessions are properly set up, we can generate
+	// each partial signature.
+	signatures := make([]*musig2.PartialSignature, len(privKeys))
+	for i, session := range sessions {
+		sig, err := input.MuSig2Sign(session, msg, true)
+		if err != nil {
+			return nil, err
+		}
+
+		signatures[i] = sig
+	}
+
+	// Now that we have all partial sigs we can just combine them to
+	// get the final signature.
+	var haveAllSigs bool
+	for i := 1; i < len(signatures); i++ {
+		var err error
+		haveAllSigs, err = input.MuSig2CombineSig(
+			sessions[0], signatures[i],
+		)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if !haveAllSigs {
+		return nil, fmt.Errorf("combinging MuSig2 signatures " +
+			"failed")
+	}
+
+	return sessions[0].FinalSig().Serialize(), nil
+}