curve: Fix the RistrettoPoint.UnmarshalBinary method

Teaches me a lesson about copy-paste adding these as an afterthought.

curve/ristretto.go

@@ -56,8 +56,8 @@ func (p *CompressedRistretto) MarshalBinary() ([]byte, error) {
 func (p *CompressedRistretto) UnmarshalBinary(data []byte) error {
 	p.Identity() // Foot + gun avoidance.
 
-	var ep EdwardsPoint
-	if err := ep.UnmarshalBinary(data); err != nil {
+	var rp RistrettoPoint
+	if err := rp.UnmarshalBinary(data); err != nil {
 		return err
 	}
 
@@ -411,6 +411,13 @@ func (p *RistrettoPoint) MultiscalarMulVartime(scalars []*scalar.Scalar, points
 	return p
 }
 
+// IsIdentity returns true iff the point is equivalent to the identity element
+// of the curve.
+func (p *RistrettoPoint) IsIdentity() bool {
+	var id RistrettoPoint
+	return p.Equal(id.Identity()) == 1
+}
+
 func (p *RistrettoPoint) elligatorRistrettoFlavor(r_0 *field.FieldElement) {
 	c := constMINUS_ONE
 

curve/ristretto_test.go

@@ -32,6 +32,7 @@
 package curve
 
 import (
+	"bytes"
 	"testing"
 
 	"github.com/oasisprotocol/curve25519-voi/curve/scalar"
@@ -60,6 +61,7 @@ func TestRistretto(t *testing.T) {
 	t.Run("Ristretto/FourTorsion/Random", testRistrettoFourTorsionRandom)
 	t.Run("Ristretto/Elligator", testRistrettoElligator)
 	t.Run("Ristretto/TestVectors", testRistrettoVectors)
+	t.Run("Ristretto/Serialization", testRistrettoSerialization)
 }
 
 func testRistrettoSum(t *testing.T) {
@@ -270,3 +272,49 @@ func testRistrettoRandomRoundtrip(t *testing.T) {
 		}
 	}
 }
+
+func testRistrettoSerialization(t *testing.T) {
+	var p RistrettoPoint
+
+	if _, err := p.SetRandom(nil); err != nil {
+		t.Fatalf("p.SetRandom: %v", err)
+	}
+	if p.IsIdentity() {
+		t.Fatalf("random point is identity???")
+	}
+
+	b, err := p.MarshalBinary()
+	if err != nil {
+		t.Fatalf("RistrettoPoint.MarshalBinary: %v", err)
+	}
+
+	// Check that RistrettoPoints round-trip.
+	var pp RistrettoPoint
+	if err = pp.UnmarshalBinary(b); err != nil {
+		t.Fatalf("RistrettoPoint.UnmarshalBinary: %v", err)
+	}
+	if p.Equal(&pp) != 1 {
+		t.Fatalf("p != pp (Got %v, %v)", p, pp)
+	}
+
+	// Check that CompressedRistrettos round-trip.
+	var pc CompressedRistretto
+	pp.Identity()
+	if err = pc.UnmarshalBinary(b); err != nil {
+		t.Fatalf("CompressedRistretto.UnmarshalBinary: %v", err)
+	}
+	if _, err = pp.SetCompressed(&pc); err != nil {
+		t.Fatalf("RistrettoPoint.SetCompressed: %v", err)
+	}
+	if p.Equal(&pp) != 1 {
+		t.Fatalf("compressed p != pp (Got %v, %v)", p, pp)
+	}
+
+	bb, err := pc.MarshalBinary()
+	if err != nil {
+		t.Fatalf("CompressedRistretto.MarshalBinary: %v", err)
+	}
+	if !bytes.Equal(bb, b) {
+		t.Fatalf("b != bb (Got %v, %v)", b, bb)
+	}
+}