Question about Hash

[kysee]

I wonder why the cc.H1 is not same as h2 in the Define() method.
How can I get the same hash value outside and inside of circuit ?
And I wonder how to append a frontend.Variable's value ([]byte type) to the end of another frontend.Variable's value ([]byte type too)?
Please let me know these....!!!

P.S
My english is not good. Sorry...

type MyCircuit struct {
	ByteSliceA   frontend.Variable
	ByteSliceB   frontend.Variable
	H1                 frontend.Variable `gnark:",public"`
}

func (cc *MyCircuit) Define(curveID ecc.ID, cs *frontend.ConstraintSystem) error {
        hFunc, err := mimc.NewMiMC("seed", curveID)
	if err != nil {
		return err
	}
	h2 := hFunc.Hash(cs, cc.ByteSliceA, cc.ByteSliceB)

        // 
        // THESE ARE NOT EQUAL !!!
        // 

        cs.AssertIsEqual(cc.H1, h2) 
}


func TestCircuit(t *testing.T) {

        ...

        var myCirCuit MyCircuit
	r1cs, err := frontend.Compile(ecc.BN254, backend.GROTH16, &myCirCuit)
	assert.NoError(err)

	pk, vk, err := groth16.Setup(r1cs)
	assert.NoError(err)

        ...

        hash := hash.MIMC_BN254.New("seed")
        hash.Write(byte_slice_A)
        hash.Write(byte_slice_B)
        h1 := hash.Sum(nil)

        var witness MyCircuit
        witness.ByteSliceA.Assign(byte_slice_A)
        witness.ByteSliceB.Assign(byte_slice_B)
        witness.H1.Assign(h1)
        ...
}

[gbotrel]

Hi! Thanks for raising this, at first sight it seems indeed there is an issue with the hash inside a circuit with several parameters. @ThomasPiellard will have a look and get back to you.

[ThomasPiellard]

Hi @kysee , can you post the exact code which makes the test fail? I suspect that the error is linked to the fact that in a snark circuit, when you write hFunc.Hash(cs, cc.ByteSliceA, cc.ByteSliceB), cc.ByteSliceA and cc.ByteSliceA are interpreted as 254bits variables (in case of BN254), on the other hand when you write

hash.Write(byte_slice_A)
hash.Write(byte_slice_B)

then byte_slice_A and byte_slice_B are not interpreted as 254bits variables but as a raw byte slice. So it works when byte_slice_A and byte_slice_Bare of size 254bits, e.g. this works:

type MyCircuit struct {
	ByteSliceA frontend.Variable
	ByteSliceB frontend.Variable
	H1         frontend.Variable `gnark:",public"`
}

func (cc *MyCircuit) Define(curveID ecc.ID, cs *frontend.ConstraintSystem) error {

	hFunc, err := NewMiMC("seed", curveID)
	if err != nil {
		return err
	}

	h := hFunc.Hash(cs, cc.ByteSliceA, cc.ByteSliceB)

	cs.AssertIsEqual(cc.H1, h)

	return nil
}

func TestCircuit(t *testing.T) {

	assert := groth16.NewAssert(t)

	var myCirCuit MyCircuit
	r1cs, err := frontend.Compile(ecc.BN254, backend.GROTH16, &myCirCuit)
	assert.NoError(err)

	_, _, err = groth16.Setup(r1cs)
	assert.NoError(err)

	// This works
	{
		var b1, b2 fr.Element
		b1.SetRandom() // b1 is of size 254 bits
		b2.SetRandom() // b2 is of size 254 bits
		hash := hash.MIMC_BN254.New("seed")
		hash.Write(b1.Marshal())
		hash.Write(b2.Marshal())
		h := hash.Sum(nil)

		// var witness MyCircuit
		var witness MyCircuit
		witness.ByteSliceA.Assign(b1.Marshal())
		witness.ByteSliceB.Assign(b2.Marshal())
		witness.H1.Assign(h)

		// check the circuit...
		assert.ProverSucceeded(r1cs, &witness)
	}

	// This fails
	{
		var b1 fr.Element
		b1.SetRandom()
		b := b1.Marshal()
		//b2.SetRandom()
		hash := hash.MIMC_BN254.New("seed")
		hash.Write(b[:10])   // take the first 10 bytes
		hash.Write(b[10:20]) // take the next 10 bytes
		h := hash.Sum(nil)

		// var witness MyCircuit
		var witness MyCircuit
		witness.ByteSliceA.Assign(b[:10])
		witness.ByteSliceB.Assign(b[10:20])
		witness.H1.Assign(h)

		// check the circuit...
		assert.ProverSucceeded(r1cs, &witness)
	}
}

[kysee]

Thank you so much!!! @ThomasPiellard