Merge pull request #129 from ConsenSys/fix/reduce_constraints_eddsa

Fix/reduce constraints eddsa

examples/rollup/operator.go

@@ -175,8 +175,7 @@ func (o *Operator) updateState(t Transfer, numTransfer int) error {
 	o.witnesses.Transfers[numTransfer].Amount.Assign(t.amount)
 	o.witnesses.Transfers[numTransfer].Signature.R.X.Assign(t.signature.R.X)
 	o.witnesses.Transfers[numTransfer].Signature.R.Y.Assign(t.signature.R.Y)
-	o.witnesses.Transfers[numTransfer].Signature.S1.Assign(t.signature.S[:16])
-	o.witnesses.Transfers[numTransfer].Signature.S2.Assign(t.signature.S[16:])
+	o.witnesses.Transfers[numTransfer].Signature.S.Assign(t.signature.S[:])
 
 	// verifying the signature. The msg is the hash (o.h) of the transfer
 	// nonce || amount || senderpubKey(x&y) || receiverPubkey(x&y)

std/signature/eddsa/eddsa.go

@@ -18,7 +18,6 @@ limitations under the License.
 package eddsa
 
 import (
-	"github.com/consensys/gnark-crypto/ecc"
 	"github.com/consensys/gnark/frontend"
 	"github.com/consensys/gnark/std/algebra/twistededwards"
 	"github.com/consensys/gnark/std/hash/mimc"
@@ -32,12 +31,13 @@ type PublicKey struct {
 
 // Signature stores a signature  (to be used in gnark circuit)
 // An EdDSA signature is a tuple (R,S) where R is a point on the twisted Edwards curve
-// and S a scalar. S can be greater than r, the size of the zk snark field, and must
-// not be reduced modulo r. Therefore it is split in S1 and S2, such that if r is n-bits long,
-// S = 2^(n/2)*S1 + S2. In other words, S is written S1S2 in basis 2^(n/2).
+// and S a scalar. Since the base field of the twisted Edwards is Fr, the number of points
+// N on the Edwards is < r+1+2sqrt(r)+2 (since the curve has 2 points of multiplicity 2).
+// The subgroup l used in eddsa is <1/2N, so the reduction
+// mod l ensures S < r, therefore there is no risk of overflow.
 type Signature struct {
-	R      twistededwards.Point
-	S1, S2 frontend.Variable
+	R twistededwards.Point
+	S frontend.Variable
 }
 
 // Verify verifies an eddsa signature
@@ -61,48 +61,34 @@ func Verify(cs *frontend.ConstraintSystem, sig Signature, msg frontend.Variable,
 	//hramConstant := hash.Sum(data...)
 	hramConstant := hash.Sum()
 
-	// lhs = cofactor*SB
-	cofactorConstant := cs.Constant(pubKey.Curve.Cofactor)
+	// lhs = [S]G
+	cofactor := pubKey.Curve.Cofactor.Uint64()
 	lhs := twistededwards.Point{}
-
-	var basis frontend.Variable
-	switch pubKey.Curve.ID {
-	case ecc.BN254:
-		basis = cs.Constant("340282366920938463463374607431768211456") // 2**128
-	case ecc.BLS12_381:
-		basis = cs.Constant("340282366920938463463374607431768211456")
-	case ecc.BLS12_377:
-		basis = cs.Constant("340282366920938463463374607431768211456")
-	case ecc.BW6_761:
-		basis = cs.Constant("6277101735386680763835789423207666416102355444464034512896") // 2**192
-	case ecc.BLS24_315:
-		basis = cs.Constant("340282366920938463463374607431768211456")
-	default:
-		panic("curve is not supported")
-	}
-
-	// [cofactor*(2^basis*S1 +  S2)]G
-	lhs.ScalarMulFixedBase(cs, pubKey.Curve.BaseX, pubKey.Curve.BaseY, sig.S1, pubKey.Curve).
-		ScalarMulNonFixedBase(cs, &lhs, basis, pubKey.Curve)
-
-	tmp := twistededwards.Point{}
-	tmp.ScalarMulFixedBase(cs, pubKey.Curve.BaseX, pubKey.Curve.BaseY, sig.S2, pubKey.Curve)
-
-	lhs.AddGeneric(cs, &lhs, &tmp, pubKey.Curve)
-
-	lhs.ScalarMulNonFixedBase(cs, &lhs, cofactorConstant, pubKey.Curve)
-
+	lhs.ScalarMulFixedBase(cs, pubKey.Curve.BaseX, pubKey.Curve.BaseY, sig.S, pubKey.Curve)
 	lhs.MustBeOnCurve(cs, pubKey.Curve)
 
-	//rhs = cofactor*(R+H(R,A,M)*A)
+	// rhs = R+[H(R,A,M)]*A
 	rhs := twistededwards.Point{}
 	rhs.ScalarMulNonFixedBase(cs, &pubKey.A, hramConstant, pubKey.Curve).
-		AddGeneric(cs, &rhs, &sig.R, pubKey.Curve).
-		ScalarMulNonFixedBase(cs, &rhs, cofactorConstant, pubKey.Curve)
+		AddGeneric(cs, &rhs, &sig.R, pubKey.Curve)
 	rhs.MustBeOnCurve(cs, pubKey.Curve)
 
-	cs.AssertIsEqual(lhs.X, rhs.X)
-	cs.AssertIsEqual(lhs.Y, rhs.Y)
+	// lhs-rhs
+	rhs.Neg(cs, &rhs).AddGeneric(cs, &lhs, &rhs, pubKey.Curve)
+
+	// [cofactor](lhs-rhs)
+	switch cofactor {
+	case 4:
+		rhs.Double(cs, &rhs, pubKey.Curve).
+			Double(cs, &rhs, pubKey.Curve)
+	case 8:
+		rhs.Double(cs, &rhs, pubKey.Curve).
+			Double(cs, &rhs, pubKey.Curve).Double(cs, &rhs, pubKey.Curve)
+	}
+
+	//rhs.MustBeOnCurve(cs, pubKey.Curve)
+	cs.AssertIsEqual(rhs.X, 0)
+	cs.AssertIsEqual(rhs.Y, 1)
 
 	return nil
 }

std/signature/eddsa/eddsa_test.go

@@ -46,7 +46,8 @@ type eddsaCircuit struct {
 	Message   frontend.Variable `gnark:",public"`
 }
 
-func parseSignature(id ecc.ID, buf []byte) ([]byte, []byte, []byte, []byte) {
+//func parseSignature(id ecc.ID, buf []byte) ([]byte, []byte, []byte) {
+func parseSignature(id ecc.ID, buf []byte) ([]byte, []byte, []byte) {
 
 	var pointbn254 edwardsbn254.PointAffine
 	var pointbls12381 edwardsbls12381.PointAffine
@@ -58,35 +59,30 @@ func parseSignature(id ecc.ID, buf []byte) ([]byte, []byte, []byte, []byte) {
 	case ecc.BN254:
 		pointbn254.SetBytes(buf[:32])
 		a, b := parsePoint(id, buf)
-		s1 := buf[32:48] // r is 256 bits, so s = 2^128*s1 + s2
-		s2 := buf[48:]
-		return a[:], b[:], s1, s2
+		s := buf[32:]
+		return a[:], b[:], s
 	case ecc.BLS12_381:
 		pointbls12381.SetBytes(buf[:32])
 		a, b := parsePoint(id, buf)
-		s1 := buf[32:48]
-		s2 := buf[48:]
-		return a[:], b[:], s1, s2
+		s := buf[32:]
+		return a[:], b[:], s
 	case ecc.BLS12_377:
 		pointbls12377.SetBytes(buf[:32])
 		a, b := parsePoint(id, buf)
-		s1 := buf[32:48]
-		s2 := buf[48:]
-		return a[:], b[:], s1, s2
+		s := buf[32:]
+		return a[:], b[:], s
 	case ecc.BW6_761:
-		pointbw6761.SetBytes(buf[:48]) // r is 384 bits, so s = 2^192*s1 + s2
+		pointbw6761.SetBytes(buf[:48])
 		a, b := parsePoint(id, buf)
-		s1 := buf[48:72]
-		s2 := buf[72:]
-		return a[:], b[:], s1, s2
+		s := buf[48:]
+		return a[:], b[:], s
 	case ecc.BLS24_315:
 		pointbls24315.SetBytes(buf[:32])
 		a, b := parsePoint(id, buf)
-		s1 := buf[32:48]
-		s2 := buf[48:]
-		return a[:], b[:], s1, s2
+		s := buf[32:]
+		return a[:], b[:], s
 	default:
-		return buf, buf, buf, buf
+		return buf, buf, buf
 	}
 }
 
@@ -214,11 +210,13 @@ func TestEddsa(t *testing.T) {
 			witness.PublicKey.A.X.Assign(pubkeyAx)
 			witness.PublicKey.A.Y.Assign(pubkeyAy)
 
-			sigRx, sigRy, sigS1, sigS2 := parseSignature(id, signature)
+			// sigRx, sigRy, sigS1, sigS2 := parseSignature(id, signature)
+			sigRx, sigRy, sigS := parseSignature(id, signature)
 			witness.Signature.R.X.Assign(sigRx)
 			witness.Signature.R.Y.Assign(sigRy)
-			witness.Signature.S1.Assign(sigS1)
-			witness.Signature.S2.Assign(sigS2)
+			// witness.Signature.S1.Assign(sigS1)
+			// witness.Signature.S2.Assign(sigS2)
+			witness.Signature.S.Assign(sigS)
 
 			assert.SolvingSucceeded(r1cs, &witness)
 		}
@@ -232,11 +230,11 @@ func TestEddsa(t *testing.T) {
 			witness.PublicKey.A.X.Assign(pubkeyAx)
 			witness.PublicKey.A.Y.Assign(pubkeyAy)
 
-			sigRx, sigRy, sigS1, sigS2 := parseSignature(id, signature)
+			// sigRx, sigRy, sigS1, sigS2 := parseSignature(id, signature)
+			sigRx, sigRy, sigS := parseSignature(id, signature)
 			witness.Signature.R.X.Assign(sigRx)
 			witness.Signature.R.Y.Assign(sigRy)
-			witness.Signature.S1.Assign(sigS1)
-			witness.Signature.S2.Assign(sigS2)
+			witness.Signature.S.Assign(sigS)
 
 			assert.SolvingFailed(r1cs, &witness)
 		}