homomorphic: big refactor, renamed homomorphic -> elgamal

elgamal/add.go

@@ -0,0 +1,35 @@
+package elgamal
+
+import (
+	ecc_tweds "github.com/consensys/gnark-crypto/ecc/twistededwards"
+	"github.com/consensys/gnark/frontend"
+	"github.com/consensys/gnark/std/algebra/native/twistededwards"
+)
+
+type Ciphertext struct {
+	C1, C2 twistededwards.Point
+}
+
+// Add sets z to the sum x+y and returns z.
+//
+// Panics if twistededwards curve init fails.
+func (z *Ciphertext) Add(api frontend.API, x, y *Ciphertext) *Ciphertext {
+	curve, err := twistededwards.NewEdCurve(api, ecc_tweds.BN254)
+	if err != nil {
+		panic(err)
+	}
+	for _, p := range []twistededwards.Point{x.C1, x.C2, y.C1, y.C2} {
+		curve.AssertIsOnCurve(p)
+	}
+	z.C1 = curve.Add(x.C1, y.C1)
+	z.C2 = curve.Add(x.C2, y.C2)
+	return z
+}
+
+// AssertIsEqual fails if any of the fields differ between z and x
+func (z *Ciphertext) AssertIsEqual(api frontend.API, x *Ciphertext) {
+	api.AssertIsEqual(z.C1.X, z.C1.X)
+	api.AssertIsEqual(z.C1.Y, x.C1.Y)
+	api.AssertIsEqual(z.C2.X, x.C2.X)
+	api.AssertIsEqual(z.C2.Y, x.C2.Y)
+}

homomorphic/add_test.go → elgamal/add_test.go

@@ -1,4 +1,4 @@
-package hadd
+package elgamal
 
 import (
 	"crypto/rand"
@@ -18,34 +18,21 @@ import (
 	"github.com/vocdoni/vocdoni-z-sandbox/ecc/format"
 )
 
-type testHomomorphicAddCircuit struct {
-	A1 twistededwards.Point `gnark:"a1,public"`
-	A2 twistededwards.Point `gnark:"a2,public"`
-	B1 twistededwards.Point `gnark:"b1,public"`
-	B2 twistededwards.Point `gnark:"b2,public"`
-	C1 twistededwards.Point `gnark:"c1,public"`
-	C2 twistededwards.Point `gnark:"c2,public"`
+type testElGamalAddCircuit struct {
+	A   Ciphertext `gnark:",public"`
+	B   Ciphertext `gnark:",public"`
+	Sum Ciphertext `gnark:",public"`
 }
 
-func (c *testHomomorphicAddCircuit) Define(api frontend.API) error {
-	// calculate and check c1
-	c1, err := Add(api, c.A1, c.B1)
-	if err != nil {
-		return err
-	}
-	api.AssertIsEqual(c.C1.X, c1.X)
-	api.AssertIsEqual(c.C1.Y, c1.Y)
-	// calculate and check c2
-	c2, err := Add(api, c.A2, c.B2)
-	if err != nil {
-		return err
-	}
-	api.AssertIsEqual(c.C2.X, c2.X)
-	api.AssertIsEqual(c.C2.Y, c2.Y)
+func (c *testElGamalAddCircuit) Define(api frontend.API) error {
+	// calculate and check sum
+	sum := &Ciphertext{}
+	sum.Add(api, &c.A, &c.B)
+	sum.AssertIsEqual(api, &c.Sum)
 	return nil
 }
 
-func TestHomomorphicAdd(t *testing.T) {
+func TestElGamalAdd(t *testing.T) {
 	// generate a public mocked key and a random k to encrypt first message
 	_, pubKey := generateKeyPair()
 	k1, err := randomK()
@@ -80,40 +67,46 @@ func TestHomomorphicAdd(t *testing.T) {
 	// profiling the circuit compilation
 	p := profile.Start()
 	now := time.Now()
-	_, _ = frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &testHomomorphicAddCircuit{})
+	_, _ = frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &testElGamalAddCircuit{})
 	fmt.Println("elapsed", time.Since(now))
 	p.Stop()
 	fmt.Println("constrains", p.NbConstraints())
 	// run the test to prove the homomorphic property
 	assert := test.NewAssert(t)
-	inputs := &testHomomorphicAddCircuit{
-		A1: twistededwards.Point{
-			X: xA1RTE,
-			Y: yA1RTE,
-		},
-		A2: twistededwards.Point{
-			X: xA2RTE,
-			Y: yA2RTE,
-		},
-		B1: twistededwards.Point{
-			X: xB1RTE,
-			Y: yB1RTE,
-		},
-		B2: twistededwards.Point{
-			X: xB2RTE,
-			Y: yB2RTE,
+	inputs := &testElGamalAddCircuit{
+		A: Ciphertext{
+			C1: twistededwards.Point{
+				X: xA1RTE,
+				Y: yA1RTE,
+			},
+			C2: twistededwards.Point{
+				X: xA2RTE,
+				Y: yA2RTE,
+			},
 		},
-		C1: twistededwards.Point{
-			X: xC1RTE,
-			Y: yC1RTE,
+		B: Ciphertext{
+			C1: twistededwards.Point{
+				X: xB1RTE,
+				Y: yB1RTE,
+			},
+			C2: twistededwards.Point{
+				X: xB2RTE,
+				Y: yB2RTE,
+			},
 		},
-		C2: twistededwards.Point{
-			X: xC2RTE,
-			Y: yC2RTE,
+		Sum: Ciphertext{
+			C1: twistededwards.Point{
+				X: xC1RTE,
+				Y: yC1RTE,
+			},
+			C2: twistededwards.Point{
+				X: xC2RTE,
+				Y: yC2RTE,
+			},
 		},
 	}
 	now = time.Now()
-	assert.SolvingSucceeded(&testHomomorphicAddCircuit{}, inputs, test.WithCurves(ecc.BN254), test.WithBackends(backend.GROTH16))
+	assert.SolvingSucceeded(&testElGamalAddCircuit{}, inputs, test.WithCurves(ecc.BN254), test.WithBackends(backend.GROTH16))
 	fmt.Println("elapsed", time.Since(now))
 }