btcec/schnorr/musig2: Allow infinity nonces

This commit updates the musig2 module to allow
infinity nonces, as per Musig2 0.4.0.

btcec/curve.go

@@ -4,13 +4,18 @@
 package btcec
 
 import (
+	"fmt"
+
 	secp "github.com/decred/dcrd/dcrec/secp256k1/v4"
 )
 
 // JacobianPoint is an element of the group formed by the secp256k1 curve in
 // Jacobian projective coordinates and thus represents a point on the curve.
 type JacobianPoint = secp.JacobianPoint
 
+// infinityPoint is the jacobian representation of the point at infinity.
+var infinityPoint JacobianPoint
+
 // MakeJacobianPoint returns a Jacobian point with the provided X, Y, and Z
 // coordinates.
 func MakeJacobianPoint(x, y, z *FieldVal) JacobianPoint {
@@ -61,3 +66,50 @@ func ScalarBaseMultNonConst(k *ModNScalar, result *JacobianPoint) {
 func ScalarMultNonConst(k *ModNScalar, point, result *JacobianPoint) {
 	secp.ScalarMultNonConst(k, point, result)
 }
+
+// ParseJacobian parses a byte slice point as a secp.Publickey and returns the
+// pubkey as a JacobianPoint. If the nonce is a zero slice, the infinityPoint
+// is returned.
+func ParseJacobian(point []byte) (JacobianPoint, error) {
+	var result JacobianPoint
+
+	if len(point) != 33 {
+		str := fmt.Sprintf("invalid nonce: invalid length: %v",
+			len(point))
+		return JacobianPoint{}, makeError(secp.ErrPubKeyInvalidLen, str)
+	}
+
+	if point[0] == 0x00 {
+		return infinityPoint, nil
+	}
+
+	noncePk, err := secp.ParsePubKey(point)
+	if err != nil {
+		return JacobianPoint{}, err
+	}
+	noncePk.AsJacobian(&result)
+
+	return result, nil
+}
+
+// JacobianToByteSlice converts the passed JacobianPoint to a Pubkey
+// and serializes that to a byte slice. If the JacobianPoint is the infinity
+// point, a zero slice is returned.
+func JacobianToByteSlice(point JacobianPoint) []byte {
+	if point.X == infinityPoint.X && point.Y == infinityPoint.Y {
+		return make([]byte, 33)
+	}
+
+	point.ToAffine()
+
+	return NewPublicKey(
+		&point.X, &point.Y,
+	).SerializeCompressed()
+}
+
+// GeneratorJacobian sets the passed JacobianPoint to the Generator Point.
+func GeneratorJacobian(jacobian *JacobianPoint) {
+	var k ModNScalar
+	k.SetInt(1)
+	ScalarBaseMultNonConst(&k, jacobian)
+}

btcec/error.go

@@ -17,3 +17,8 @@ type Error = secp.Error
 // errors.As, so the caller can directly check against an error kind when
 // determining the reason for an error.
 type ErrorKind = secp.ErrorKind
+
+// makeError creates an secp.Error given a set of arguments.
+func makeError(kind ErrorKind, desc string) Error {
+	return Error{Err: kind, Description: desc}
+}

btcec/schnorr/musig2/musig2_test.go

@@ -1461,7 +1461,7 @@ func TestMusig2AggregateNoncesTestVectors(t *testing.T) {
 			},
 			expectedNonce: append(
 				append([]byte{}, expectedNonce[0:33]...),
-				getGBytes()...,
+				getInfinityBytes()...,
 			),
 		},
 	}
@@ -1829,6 +1829,10 @@ func getNegGBytes() []byte {
 	return pk
 }
 
+func getInfinityBytes() []byte {
+	return make([]byte, 33)
+}
+
 func mustParseHex32(str string) [32]byte {
 	b, err := hex.DecodeString(str)
 	if err != nil {

btcec/schnorr/musig2/nonces.go

@@ -331,22 +331,20 @@ func AggregateNonces(pubNonces [][PubNonceSize]byte) ([PubNonceSize]byte, error)
 	// function to extra 33 bytes at a time from the packed 2x public
 	// nonces.
 	type nonceSlicer func([PubNonceSize]byte) []byte
-	combineNonces := func(slicer nonceSlicer) (*btcec.PublicKey, error) {
+	combineNonces := func(slicer nonceSlicer) (btcec.JacobianPoint, error) {
 		// Convert the set of nonces into jacobian coordinates we can
 		// use to accumulate them all into each other.
 		pubNonceJs := make([]*btcec.JacobianPoint, len(pubNonces))
 		for i, pubNonceBytes := range pubNonces {
 			// Using the slicer, extract just the bytes we need to
 			// decode.
 			var nonceJ btcec.JacobianPoint
-			pubNonce, err := btcec.ParsePubKey(
-				slicer(pubNonceBytes),
-			)
+
+			nonceJ, err := btcec.ParseJacobian(slicer(pubNonceBytes))
 			if err != nil {
-				return nil, err
+				return btcec.JacobianPoint{}, err
 			}
 
-			pubNonce.AsJacobian(&nonceJ)
 			pubNonceJs[i] = &nonceJ
 		}
 
@@ -359,27 +357,8 @@ func AggregateNonces(pubNonces [][PubNonceSize]byte) ([PubNonceSize]byte, error)
 			)
 		}
 
-		// Now that we've aggregated all the points, we need to check
-		// if this point is the point at infinity, if so, then we'll
-		// just return the generator. At a later step, the malicious
-		// party will be detected.
-		if aggregateNonce == infinityPoint {
-			// TODO(roasbeef): better way to get the generator w/
-			// the new API? -- via old curve params instead?
-			var generator btcec.JacobianPoint
-			one := new(btcec.ModNScalar).SetInt(1)
-			btcec.ScalarBaseMultNonConst(one, &generator)
-
-			generator.ToAffine()
-			return btcec.NewPublicKey(
-				&generator.X, &generator.Y,
-			), nil
-		}
-
 		aggregateNonce.ToAffine()
-		return btcec.NewPublicKey(
-			&aggregateNonce.X, &aggregateNonce.Y,
-		), nil
+		return aggregateNonce, nil
 	}
 
 	// The final nonce public nonce is actually two nonces, one that
@@ -392,17 +371,18 @@ func AggregateNonces(pubNonces [][PubNonceSize]byte) ([PubNonceSize]byte, error)
 	if err != nil {
 		return finalNonce, err
 	}
+
 	combinedNonce2, err := combineNonces(func(n [PubNonceSize]byte) []byte {
 		return n[btcec.PubKeyBytesLenCompressed:]
 	})
 	if err != nil {
 		return finalNonce, err
 	}
 
-	copy(finalNonce[:], combinedNonce1.SerializeCompressed())
+	copy(finalNonce[:], btcec.JacobianToByteSlice(combinedNonce1))
 	copy(
 		finalNonce[btcec.PubKeyBytesLenCompressed:],
-		combinedNonce2.SerializeCompressed(),
+		btcec.JacobianToByteSlice(combinedNonce2),
 	)
 
 	return finalNonce, nil

btcec/schnorr/musig2/sign.go

@@ -200,20 +200,6 @@ func Sign(secNonce [SecNonceSize]byte, privKey *btcec.PrivateKey,
 		option(opts)
 	}
 
-	// Next, we'll parse the public nonces into R1 and R2.
-	r1, err := btcec.ParsePubKey(
-		combinedNonce[:btcec.PubKeyBytesLenCompressed],
-	)
-	if err != nil {
-		return nil, err
-	}
-	r2, err := btcec.ParsePubKey(
-		combinedNonce[btcec.PubKeyBytesLenCompressed:],
-	)
-	if err != nil {
-		return nil, err
-	}
-
 	// Compute the hash of all the keys here as we'll need it do aggregate
 	// the keys and also at the final step of signing.
 	keysHash := keyHashFingerprint(pubKeys, opts.sortKeys)
@@ -259,19 +245,31 @@ func Sign(secNonce [SecNonceSize]byte, privKey *btcec.PrivateKey,
 	)
 	nonceBlinder.SetByteSlice(nonceBlindHash[:])
 
-	var nonce, r1J, r2J btcec.JacobianPoint
-	r1.AsJacobian(&r1J)
-	r2.AsJacobian(&r2J)
+	// Next, we'll parse the public nonces into R1 and R2.
+	r1J, err := btcec.ParseJacobian(
+		combinedNonce[:btcec.PubKeyBytesLenCompressed],
+	)
+	if err != nil {
+		return nil, err
+	}
+	r2J, err := btcec.ParseJacobian(
+		combinedNonce[btcec.PubKeyBytesLenCompressed:],
+	)
+	if err != nil {
+		return nil, err
+	}
 
 	// With our nonce blinding value, we'll now combine both the public
 	// nonces, using the blinding factor to tweak the second nonce:
 	//  * R = R_1 + b*R_2
+	var nonce btcec.JacobianPoint
 	btcec.ScalarMultNonConst(&nonceBlinder, &r2J, &r2J)
 	btcec.AddNonConst(&r1J, &r2J, &nonce)
 
 	// If the combined nonce it eh point at infinity, then we'll bail out.
 	if nonce == infinityPoint {
-		return nil, ErrNoncePointAtInfinity
+		G := btcec.Generator()
+		G.AsJacobian(&nonce)
 	}
 
 	// Next we'll parse out our two secret nonces, which we'll be using in
@@ -375,6 +373,7 @@ func (p *PartialSignature) Verify(pubNonce [PubNonceSize]byte,
 	signingKey *btcec.PublicKey, msg [32]byte, signOpts ...SignOption) bool {
 
 	pubKey := schnorr.SerializePubKey(signingKey)
+
 	return verifyPartialSig(
 		p, pubNonce, combinedNonce, keySet, pubKey, msg, signOpts...,
 	) == nil
@@ -399,19 +398,6 @@ func verifyPartialSig(partialSig *PartialSignature, pubNonce [PubNonceSize]byte,
 	// Next we'll parse out the two public nonces into something we can
 	// use.
 	//
-	// TODO(roasbeef): consolidate, new method
-	r1, err := btcec.ParsePubKey(
-		combinedNonce[:btcec.PubKeyBytesLenCompressed],
-	)
-	if err != nil {
-		return err
-	}
-	r2, err := btcec.ParsePubKey(
-		combinedNonce[btcec.PubKeyBytesLenCompressed:],
-	)
-	if err != nil {
-		return err
-	}
 
 	// Compute the hash of all the keys here as we'll need it do aggregate
 	// the keys and also at the final step of verification.
@@ -456,45 +442,61 @@ func verifyPartialSig(partialSig *PartialSignature, pubNonce [PubNonceSize]byte,
 	nonceBlindHash := chainhash.TaggedHash(NonceBlindTag, nonceMsgBuf.Bytes())
 	nonceBlinder.SetByteSlice(nonceBlindHash[:])
 
-	var nonce, r1J, r2J btcec.JacobianPoint
-	r1.AsJacobian(&r1J)
-	r2.AsJacobian(&r2J)
+	r1J, err := btcec.ParseJacobian(
+		combinedNonce[:btcec.PubKeyBytesLenCompressed],
+	)
+	if err != nil {
+		return err
+	}
+	r2J, err := btcec.ParseJacobian(
+		combinedNonce[btcec.PubKeyBytesLenCompressed:],
+	)
+	if err != nil {
+		return err
+	}
 
 	// With our nonce blinding value, we'll now combine both the public
 	// nonces, using the blinding factor to tweak the second nonce:
 	//  * R = R_1 + b*R_2
+
+	var nonce btcec.JacobianPoint
 	btcec.ScalarMultNonConst(&nonceBlinder, &r2J, &r2J)
 	btcec.AddNonConst(&r1J, &r2J, &nonce)
 
 	// Next, we'll parse out the set of public nonces this signer used to
 	// generate the signature.
-	pubNonce1, err := btcec.ParsePubKey(
+	pubNonce1J, err := btcec.ParseJacobian(
 		pubNonce[:btcec.PubKeyBytesLenCompressed],
 	)
 	if err != nil {
 		return err
 	}
-	pubNonce2, err := btcec.ParsePubKey(
+	pubNonce2J, err := btcec.ParseJacobian(
 		pubNonce[btcec.PubKeyBytesLenCompressed:],
 	)
 	if err != nil {
 		return err
 	}
 
+	// If the nonce is the infinity point we set it to the Generator.
+	if nonce == infinityPoint {
+		btcec.GeneratorJacobian(&nonce)
+	} else {
+		nonce.ToAffine()
+	}
+
 	// We'll perform a similar aggregation and blinding operator as we did
 	// above for the combined nonces: R' = R_1' + b*R_2'.
-	var pubNonceJ, pubNonce1J, pubNonce2J btcec.JacobianPoint
-	pubNonce1.AsJacobian(&pubNonce1J)
-	pubNonce2.AsJacobian(&pubNonce2J)
+	var pubNonceJ btcec.JacobianPoint
+
 	btcec.ScalarMultNonConst(&nonceBlinder, &pubNonce2J, &pubNonce2J)
 	btcec.AddNonConst(&pubNonce1J, &pubNonce2J, &pubNonceJ)
 
-	nonce.ToAffine()
+	pubNonceJ.ToAffine()
 
 	// If the combined nonce used in the challenge hash has an odd y
 	// coordinate, then we'll negate our final public nonce.
 	if nonce.Y.IsOdd() {
-		pubNonceJ.ToAffine()
 		pubNonceJ.Y.Negate(1)
 		pubNonceJ.Y.Normalize()
 	}