internal/radix51: add a conditional swap

internal/radix51/fe.go

@@ -323,6 +323,26 @@ func (v *FieldElement) Select(a, b *FieldElement, cond int) *FieldElement {
 	return v
 }
 
+// CondSwap swaps a and b if cond == 1 or leaves them unchanged if cond == 0.
+func CondSwap(a, b *FieldElement, cond int) {
+	m := uint64(cond) * mask64Bits
+	t := m & (a[0] ^ b[0])
+	a[0] ^= t
+	b[0] ^= t
+	t = m & (a[1] ^ b[1])
+	a[1] ^= t
+	b[1] ^= t
+	t = m & (a[2] ^ b[2])
+	a[2] ^= t
+	b[2] ^= t
+	t = m & (a[3] ^ b[3])
+	a[3] ^= t
+	b[3] ^= t
+	t = m & (a[4] ^ b[4])
+	a[4] ^= t
+	b[4] ^= t
+}
+
 // CondNeg sets v to -u if cond == 1, and to u if cond == 0.
 func (v *FieldElement) CondNeg(u *FieldElement, cond int) *FieldElement {
 	tmp := new(FieldElement).Neg(u)

internal/radix51/fe_test.go

@@ -335,3 +335,29 @@ func TestInvert(t *testing.T) {
 		t.Errorf("random inversion identity failed, got: %x for field element %x", r, x)
 	}
 }
+
+func TestSelectSwap(t *testing.T) {
+	a := FieldElement([5]uint64{358744748052810, 1691584618240980, 977650209285361, 1429865912637724, 560044844278676})
+	b := FieldElement([5]uint64{84926274344903, 473620666599931, 365590438845504, 1028470286882429, 2146499180330972})
+
+	var c, d FieldElement
+
+	c.Select(&a, &b, 1)
+	d.Select(&a, &b, 0)
+
+	if (c.Equal(&a) != 1) || (d.Equal(&b) != 1) {
+		t.Errorf("Select failed")
+	}
+
+	CondSwap(&c, &d, 0)
+
+	if (c.Equal(&a) != 1) || (d.Equal(&b) != 1) {
+		t.Errorf("Swap failed")
+	}
+
+	CondSwap(&c, &d, 1)
+
+	if (c.Equal(&b) != 1) || (d.Equal(&a) != 1) {
+		t.Errorf("Swap failed")
+	}
+}