Merge pull request dedis#536 from dedis/kilian-missing-drand-commit

avoid race condition in MakeAffine/ValidatePairing (drand#59)

pairing/bn254/suite.go

@@ -132,10 +132,13 @@ func (s *Suite) Pair(p1 kyber.Point, p2 kyber.Point) kyber.Point {
 	return s.GT().Point().(*pointGT).Pair(p1, p2)
 }
 
+// NB: Not safe for concurrent calls
 func (s *Suite) ValidatePairing(p1, p2, inv1, inv2 kyber.Point) bool {
-	p2.(*pointG2).g.MakeAffine()
-	inv2.(*pointG2).g.MakeAffine()
-	return s.Pair(p1, p2).Equal(s.Pair(inv1, inv2))
+	p2Norm := p2.Clone()
+	inv2Norm := inv2.Clone()
+	p2Norm.(*pointG2).g.MakeAffine()
+	inv2Norm.(*pointG2).g.MakeAffine()
+	return s.Pair(p1, p2Norm).Equal(s.Pair(inv1, inv2Norm))
 }
 
 // Not used other than for reflect.TypeOf()

pairing/bn254/twist.go

@@ -177,24 +177,27 @@ func (c *twistPoint) Mul(a *twistPoint, scalar *big.Int) {
 	c.Set(sum)
 }
 
+// NB: Not safe for concurrent calls
 func (c *twistPoint) MakeAffine() {
-	if c.z.IsOne() {
+	g := c.Clone()
+	if g.z.IsOne() {
 		return
-	} else if c.z.IsZero() {
-		c.x.SetZero()
-		c.y.SetOne()
-		c.t.SetZero()
+	} else if g.z.IsZero() {
+		g.x.SetZero()
+		g.y.SetOne()
+		g.t.SetZero()
 		return
 	}
 
-	zInv := (&gfP2{}).Invert(&c.z)
-	t := (&gfP2{}).Mul(&c.y, zInv)
+	zInv := (&gfP2{}).Invert(&g.z)
+	t := (&gfP2{}).Mul(&g.y, zInv)
 	zInv2 := (&gfP2{}).Square(zInv)
-	c.y.Mul(t, zInv2)
-	t.Mul(&c.x, zInv2)
-	c.x.Set(t)
-	c.z.SetOne()
-	c.t.SetOne()
+	g.y.Mul(t, zInv2)
+	t.Mul(&g.x, zInv2)
+	g.x.Set(t)
+	g.z.SetOne()
+	g.t.SetOne()
+	c.Set(g)
 }
 
 func (c *twistPoint) Neg(a *twistPoint) {