add equality for field elements

internal/radix51/const.go

@@ -7,3 +7,8 @@ const (
 	// The vaule 2^51-1, used in carry propagation
 	maskLow51Bits = uint64(1)<<51 - 1
 )
+
+var (
+	FieldZero FieldElement = [5]uint64{0, 0, 0, 0, 0}
+	FieldOne  FieldElement = [5]uint64{1, 0, 0, 0, 0}
+)

internal/radix51/fe.go

@@ -210,6 +210,14 @@ func FeCSwap(a, b *FieldElement, c uint64) {
 	b[4] ^= t[4]
 }
 
+func FeEqual(a, b *FieldElement) uint64 {
+	var result uint64
+	for i := 0; i < 5; i++ {
+		result |= a[i] ^ b[i]
+	}
+	return (result ^ 0)
+}
+
 func FeFromBytes(v *FieldElement, x *[32]byte) {
 	v[0] = uint64(x[0])
 	v[0] |= uint64(x[1]) << 8

internal/radix51/fe_test.go

@@ -144,6 +144,21 @@ func vartimeEqual(x, y FieldElement) bool {
 	return true
 }
 
+func TestFeEqual(t *testing.T) {
+	var x FieldElement = [5]uint64{1, 1, 1, 1, 1}
+	var y FieldElement = [5]uint64{5, 4, 3, 2, 1}
+
+	eq := FeEqual(&x, &x)
+	if eq != 0 {
+		t.Errorf("wrong about equality")
+	}
+
+	eq = FeEqual(&x, &y)
+	if eq == 0 {
+		t.Errorf("wrong about inequality")
+	}
+}
+
 func TestFeInvert(t *testing.T) {
 	var x FieldElement = [5]uint64{1, 1, 1, 1, 1}
 	var one FieldElement = [5]uint64{1, 0, 0, 0, 0}