limit the number of mimc input hash to 62

emulated/bn254/twistededwards/mimc7/mimc.go

@@ -1,11 +1,17 @@
 package mimc7
 
 import (
+	"fmt"
+
 	"github.com/consensys/gnark/frontend"
 	"github.com/consensys/gnark/std/algebra/emulated/sw_bn254"
 	"github.com/consensys/gnark/std/math/emulated"
 )
 
+// maxInputs constant is the maximum number of inputs that the MiMC hash
+// function can take. Experimentally, the maximum number of inputs is 62.
+const maxInputs = 62
+
 type MiMC struct {
 	params []emulated.Element[sw_bn254.ScalarField] // slice containing constants for the encryption rounds
 	h      emulated.Element[sw_bn254.ScalarField]   // current vector in the Miyaguchi–Preneel scheme
@@ -28,8 +34,12 @@ func NewMiMC(api frontend.API) (MiMC, error) {
 }
 
 // Write adds more data to the running hash.
-func (h *MiMC) Write(data ...emulated.Element[sw_bn254.ScalarField]) {
+func (h *MiMC) Write(data ...emulated.Element[sw_bn254.ScalarField]) error {
+	if len(h.data)+len(data) > maxInputs {
+		return fmt.Errorf("too many inputs. Max inputs is %d", maxInputs)
+	}
 	h.data = append(h.data, data...)
+	return nil
 }
 
 // Reset resets the Hash to its initial state.

emulated/bn254/twistededwards/mimc7/mimc_test.go

@@ -17,6 +17,8 @@ import (
 	"github.com/consensys/gnark/test"
 	qt "github.com/frankban/quicktest"
 	"github.com/iden3/go-iden3-crypto/mimc7"
+	"github.com/vocdoni/arbo"
+	"go.vocdoni.io/dvote/util"
 )
 
 type testMiMCCircuit struct {
@@ -29,7 +31,10 @@ func (circuit *testMiMCCircuit) Define(api frontend.API) error {
 	if err != nil {
 		return err
 	}
-	mimc.Write(circuit.Preimage)
+	if err := mimc.Write(circuit.Preimage); err != nil {
+		api.Println(err.Error())
+		api.AssertIsEqual(1, 0)
+	}
 	mimc.AssertSumIsEqual(circuit.Hash)
 	return nil
 }
@@ -53,6 +58,74 @@ func TestMiMC(t *testing.T) {
 	fmt.Println("solving tooks", time.Since(now))
 }
 
+type testMaxInputsMiMCCircuit struct {
+	Hash      emulated.Element[sw_bn254.ScalarField] `gnark:",public"`
+	Preimages [maxInputs]emulated.Element[sw_bn254.ScalarField]
+}
+
+func (circuit *testMaxInputsMiMCCircuit) Define(api frontend.API) error {
+	mimc, err := NewMiMC(api)
+	if err != nil {
+		return err
+	}
+	if err := mimc.Write(circuit.Preimages[:]...); err != nil {
+		api.Println(err.Error())
+		api.AssertIsEqual(1, 0)
+	}
+	mimc.AssertSumIsEqual(circuit.Hash)
+	return nil
+}
+
+type testLimitInputsMiMCCircuit struct {
+	Preimages [maxInputs + 1]emulated.Element[sw_bn254.ScalarField]
+}
+
+func (circuit *testLimitInputsMiMCCircuit) Define(api frontend.API) error {
+	mimc, err := NewMiMC(api)
+	if err != nil {
+		return err
+	}
+	if err := mimc.Write(circuit.Preimages[:]...); err == nil {
+		api.Println("too many inputs expected")
+		api.AssertIsEqual(1, 0)
+	}
+	return nil
+}
+
+func TestMaxAndLimitInputsMiMC(t *testing.T) {
+	c := qt.New(t)
+	// generate a random inputs
+	input := arbo.BigToFF(arbo.BN254BaseField, new(big.Int).SetBytes(util.RandomBytes(32)))
+	inputs := []*big.Int{}
+	emulatedInputs := [maxInputs]emulated.Element[sw_bn254.ScalarField]{}
+	limitEmulatedInputs := [maxInputs + 1]emulated.Element[sw_bn254.ScalarField]{}
+	for i := 0; i < maxInputs; i++ {
+		inputs = append(inputs, input)
+		emulatedInputs[i] = emulated.ValueOf[sw_bn254.ScalarField](input)
+		limitEmulatedInputs[i] = emulated.ValueOf[sw_bn254.ScalarField](input)
+	}
+	limitEmulatedInputs[maxInputs] = emulated.ValueOf[sw_bn254.ScalarField](input)
+
+	c.Run("max inputs", func(c *qt.C) {
+		// hash the max inputs
+		maxHash, err := mimc7.Hash(inputs, nil)
+		c.Assert(err, qt.IsNil)
+		// run the test
+		assert := test.NewAssert(t)
+		assert.SolvingSucceeded(&testMaxInputsMiMCCircuit{}, &testMaxInputsMiMCCircuit{
+			Preimages: emulatedInputs,
+			Hash:      emulated.ValueOf[sw_bn254.ScalarField](maxHash),
+		}, test.WithCurves(ecc.BLS12_377), test.WithBackends(backend.GROTH16))
+	})
+
+	c.Run("limit inputs", func(c *qt.C) {
+		assert := test.NewAssert(t)
+		assert.SolvingSucceeded(&testLimitInputsMiMCCircuit{}, &testLimitInputsMiMCCircuit{
+			Preimages: limitEmulatedInputs,
+		}, test.WithCurves(ecc.BLS12_377), test.WithBackends(backend.GROTH16))
+	})
+}
+
 func printConstrains(placeholder frontend.Circuit) error {
 	// compile circuit
 	p := profile.Start()