how to convert the Variable which is calculated by Keccak256Circuit to bn254's fp.Element to calculate bn254 msm

[xiangjianmeng]

func VerifyBN254ScalarMul(
	api frontend.API, point [2]frontend.Variable, scalar frontend.Variable, expected [2]frontend.Variable,
) error {
	cr, err := sw_emulated.New[emparams.BN254Fp, emparams.BN254Fr](api, sw_emulated.GetCurveParams[emparams.BN254Fp]())
	if err != nil {
		return err
	}
	x, err := new(fp.Element).SetInterface(point[0])
	if err != nil {
		return err
	}
	y, err := new(fp.Element).SetInterface(point[1])
	if err != nil {
		return err
	}
	ps := &sw_emulated.AffinePoint[emulated.BN254Fp]{
		X: emulated.ValueOf[emparams.BN254Fp](x),
		Y: emulated.ValueOf[emparams.BN254Fp](y),
	}
	scalarEle, err := new(fp.Element).SetInterface(scalar)
	if err != nil {
		return err
	}

	s := emulated.ValueOf[emparams.BN254Fr](*scalarEle)
	res := cr.ScalarMul(ps, &s)

	expectedX, err := new(fp.Element).SetInterface(expected[0])
	if err != nil {
		return err
	}
	expectedY, err := new(fp.Element).SetInterface(expected[1])
	if err != nil {
		return err
	}
	expectedRes := sw_emulated.AffinePoint[emparams.BN254Fp]{
		X: emulated.ValueOf[emparams.BN254Fp](expectedX),
		Y: emulated.ValueOf[emparams.BN254Fp](expectedY),
	}
	cr.AssertIsEqual(res, &expectedRes)
	return nil
}

I encounter the error which is define circuit: can't set fp.Element from type expr.Term when compile my Circuit for this line
s := emulated.ValueOf[emparams.BN254Fr](*scalarEle)

[ivokub]

Hi, I'm not able to run the example without the full circuit definition and witness assignment. It would be better to add full example which causes the error.

But with seeing your current implementation, it seems that there is a mix of witness assignment and in-circuit computation. For reference, see the example for implementing scalar multiplication with assertion check as a doctest: https://pkg.go.dev/github.com/consensys/gnark@v0.10.0/std/algebra/emulated/sw_emulated#example-Curve. In the example ExampleCurveCircuit is the circuit definition where the struct fields represent what the inputs the circuit takes as initial witness and in main we perform the witness assignment (together with compilation and proof generation). The circuit operations are defined in the function ExampleCurveCircuit.Define method.

Let us know if you are able to adapt the example for your use case or there are some further issues.

[xiangjianmeng]

func ToElement[T emulated.FieldParams](api frontend.API, input frontend.Variable) (emulated.Element[T], error) {
	inputFiled, err := emulated.NewField[T](api)
	if err != nil {
		return emulated.Element[T]{}, err
	}

	results, err := api.Compiler().NewHint(Div128Hint, 2, input)
	if err != nil {
		return emulated.Element[T]{}, err
	}

	//log.Println("res:", results[0], results[1])
	// Decompose each part into binary representation
	upperBits := api.ToBinary(results[0], 128)
	lowerBits := api.ToBinary(results[1], 128)
	//log.Println(upperBits)
	//log.Println(lowerBits)

	// Concatenate the binary representations
	inputBits := append(lowerBits, upperBits...)

	//inputBits = api.ToBinary(input, 256)
	//log.Println(inputBits)
	inputEle := inputFiled.FromBits(inputBits...)
	return *inputEle, nil
}

Now I have fixed my problem through above code. thanks