From 5da29cf495d337b0cb5ce6157d345f9142ddebe6 Mon Sep 17 00:00:00 2001
From: Marius van der Wijden <m.vanderwijden@live.de>
Date: Thu, 15 Aug 2024 09:26:27 +0200
Subject: [PATCH] tests/fuzzers/bls12381: add blst fuzzer for multiexp

---
 tests/fuzzers/bls12381/bls12381_fuzz.go | 36 +++++++++++++++++++------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/tests/fuzzers/bls12381/bls12381_fuzz.go b/tests/fuzzers/bls12381/bls12381_fuzz.go
index 7058f6f84e..1959189163 100644
--- a/tests/fuzzers/bls12381/bls12381_fuzz.go
+++ b/tests/fuzzers/bls12381/bls12381_fuzz.go
@@ -179,6 +179,8 @@ func fuzzCrossG1MultiExp(data []byte) int {
 		input        = bytes.NewReader(data)
 		gnarkScalars []fr.Element
 		gnarkPoints  []bls12381.G1Affine
+		blstScalars  []*blst.Scalar
+		blstPoints   []*blst.P1Affine
 	)
 	// n random scalars (max 17)
 	for i := 0; i < 17; i++ {
@@ -188,25 +190,33 @@ func fuzzCrossG1MultiExp(data []byte) int {
 			break
 		}
 		// get a random G1 point as basis
-		cp1, _, err := getG1Points(input)
+		cp1, bl1, err := getG1Points(input)
 		if err != nil {
 			break
 		}
-		var gnarkScalar = &fr.Element{}
-		gnarkScalar = gnarkScalar.SetBigInt(s)
-		gnarkScalars = append(gnarkScalars, *gnarkScalar)
 
+		gnarkScalar := new(fr.Element).SetBigInt(s)
+		gnarkScalars = append(gnarkScalars, *gnarkScalar)
 		gnarkPoints = append(gnarkPoints, *cp1)
+
+		blstScalar := new(blst.Scalar).FromBEndian(common.LeftPadBytes(s.Bytes(), 32))
+		blstScalars = append(blstScalars, blstScalar)
+		blstPoints = append(blstPoints, bl1)
 	}
 
 	// gnark multi exp
 	cp := new(bls12381.G1Affine)
 	cp.MultiExp(gnarkPoints, gnarkScalars, ecc.MultiExpConfig{})
 
-	expected := multiExpG1(gnarkPoints, gnarkScalars)
-	if !bytes.Equal(cp.Marshal(), expected.Marshal()) {
+	expectedGnark := multiExpG1Gnark(gnarkPoints, gnarkScalars)
+	if !bytes.Equal(cp.Marshal(), expectedGnark.Marshal()) {
 		panic("g1 multi exponentiation mismatch")
 	}
+
+	expectedBlst := multiExpG1Blst(blstPoints, blstScalars)
+	if !bytes.Equal(expectedGnark.Marshal(), expectedBlst.Serialize()) {
+		panic("g1 multi exponentiation mismatch, gnark/blst")
+	}
 	return 1
 }
 
@@ -266,8 +276,8 @@ func randomScalar(r io.Reader, max *big.Int) (k *big.Int, err error) {
 	}
 }
 
-// multiExpG1 is a naive implementation of G1 multi-exponentiation
-func multiExpG1(gs []bls12381.G1Affine, scalars []fr.Element) bls12381.G1Affine {
+// multiExpG1Gnark is a naive implementation of G1 multi-exponentiation
+func multiExpG1Gnark(gs []bls12381.G1Affine, scalars []fr.Element) bls12381.G1Affine {
 	_, _, res, _ := bls12381.Generators()
 	for i := 0; i < len(gs); i++ {
 		tmp := new(bls12381.G1Affine)
@@ -278,3 +288,13 @@ func multiExpG1(gs []bls12381.G1Affine, scalars []fr.Element) bls12381.G1Affine
 	}
 	return res
 }
+
+// multiExpG1Blst is a naive implementation of G1 multi-exponentiation
+func multiExpG1Blst(gs []*blst.P1Affine, scalars []*blst.Scalar) *blst.P1Affine {
+	gen := blst.P1Generator()
+	for i := 0; i < len(gs); i++ {
+		p2 := new(blst.P1Affine).From(scalars[i])
+		gen.Add(p2)
+	}
+	return gen.ToAffine()
+}