crypto/elliptic: explicitly handle P+P, ∞+P and P+∞

These aren't needed for scalar multiplication, but since we export a
generic Add function we should handle it.

This change also corrects two bugs in p224Contract that it turned up.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6458076

src/pkg/crypto/ecdsa/ecdsa.go

@@ -140,14 +140,16 @@ func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
 	w := new(big.Int).ModInverse(s, N)
 
 	u1 := e.Mul(e, w)
+	u1.Mod(u1, N)
 	u2 := w.Mul(r, w)
+	u2.Mod(u2, N)
 
 	x1, y1 := c.ScalarBaseMult(u1.Bytes())
 	x2, y2 := c.ScalarMult(pub.X, pub.Y, u2.Bytes())
-	if x1.Cmp(x2) == 0 {
+	x, y := c.Add(x1, y1, x2, y2)
+	if x.Sign() == 0 && y.Sign() == 0 {
 		return false
 	}
-	x, _ := c.Add(x1, y1, x2, y2)
 	x.Mod(x, N)
 	return x.Cmp(r) == 0
 }

src/pkg/crypto/elliptic/elliptic.go

@@ -31,10 +31,10 @@ type Curve interface {
 	// Double returns 2*(x,y)
 	Double(x1, y1 *big.Int) (x, y *big.Int)
 	// ScalarMult returns k*(Bx,By) where k is a number in big-endian form.
-	ScalarMult(x1, y1 *big.Int, scalar []byte) (x, y *big.Int)
-	// ScalarBaseMult returns k*G, where G is the base point of the group and k
-	// is an integer in big-endian form.
-	ScalarBaseMult(scalar []byte) (x, y *big.Int)
+	ScalarMult(x1, y1 *big.Int, k []byte) (x, y *big.Int)
+	// ScalarBaseMult returns k*G, where G is the base point of the group
+	// and k is an integer in big-endian form.
+	ScalarBaseMult(k []byte) (x, y *big.Int)
 }
 
 // CurveParams contains the parameters of an elliptic curve and also provides
@@ -69,9 +69,24 @@ func (curve *CurveParams) IsOnCurve(x, y *big.Int) bool {
 	return x3.Cmp(y2) == 0
 }
 
+// zForAffine returns a Jacobian Z value for the affine point (x, y). If x and
+// y are zero, it assumes that they represent the point at infinity because (0,
+// 0) is not on the any of the curves handled here.
+func zForAffine(x, y *big.Int) *big.Int {
+	z := new(big.Int)
+	if x.Sign() != 0 || y.Sign() != 0 {
+		z.SetInt64(1)
+	}
+	return z
+}
+
 // affineFromJacobian reverses the Jacobian transform. See the comment at the
-// top of the file.
+// top of the file. If the point is ∞ it returns 0, 0.
 func (curve *CurveParams) affineFromJacobian(x, y, z *big.Int) (xOut, yOut *big.Int) {
+	if z.Sign() == 0 {
+		return new(big.Int), new(big.Int)
+	}
+
 	zinv := new(big.Int).ModInverse(z, curve.P)
 	zinvsq := new(big.Int).Mul(zinv, zinv)
 
@@ -84,14 +99,29 @@ func (curve *CurveParams) affineFromJacobian(x, y, z *big.Int) (xOut, yOut *big.
 }
 
 func (curve *CurveParams) Add(x1, y1, x2, y2 *big.Int) (*big.Int, *big.Int) {
-	z := new(big.Int).SetInt64(1)
-	return curve.affineFromJacobian(curve.addJacobian(x1, y1, z, x2, y2, z))
+	z1 := zForAffine(x1, y1)
+	z2 := zForAffine(x2, y2)
+	return curve.affineFromJacobian(curve.addJacobian(x1, y1, z1, x2, y2, z2))
 }
 
 // addJacobian takes two points in Jacobian coordinates, (x1, y1, z1) and
 // (x2, y2, z2) and returns their sum, also in Jacobian form.
 func (curve *CurveParams) addJacobian(x1, y1, z1, x2, y2, z2 *big.Int) (*big.Int, *big.Int, *big.Int) {
 	// See http://hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-3.html#addition-add-2007-bl
+	x3, y3, z3 := new(big.Int), new(big.Int), new(big.Int)
+	if z1.Sign() == 0 {
+		x3.Set(x2)
+		y3.Set(y2)
+		z3.Set(z2)
+		return x3, y3, z3
+	}
+	if z2.Sign() == 0 {
+		x3.Set(x1)
+		y3.Set(y1)
+		z3.Set(z1)
+		return x3, y3, z3
+	}
+
 	z1z1 := new(big.Int).Mul(z1, z1)
 	z1z1.Mod(z1z1, curve.P)
 	z2z2 := new(big.Int).Mul(z2, z2)
@@ -102,6 +132,7 @@ func (curve *CurveParams) addJacobian(x1, y1, z1, x2, y2, z2 *big.Int) (*big.Int
 	u2 := new(big.Int).Mul(x2, z1z1)
 	u2.Mod(u2, curve.P)
 	h := new(big.Int).Sub(u2, u1)
+	xEqual := h.Sign() == 0
 	if h.Sign() == -1 {
 		h.Add(h, curve.P)
 	}
@@ -119,17 +150,21 @@ func (curve *CurveParams) addJacobian(x1, y1, z1, x2, y2, z2 *big.Int) (*big.Int
 	if r.Sign() == -1 {
 		r.Add(r, curve.P)
 	}
+	yEqual := r.Sign() == 0
+	if xEqual && yEqual {
+		return curve.doubleJacobian(x1, y1, z1)
+	}
 	r.Lsh(r, 1)
 	v := new(big.Int).Mul(u1, i)
 
-	x3 := new(big.Int).Set(r)
+	x3.Set(r)
 	x3.Mul(x3, x3)
 	x3.Sub(x3, j)
 	x3.Sub(x3, v)
 	x3.Sub(x3, v)
 	x3.Mod(x3, curve.P)
 
-	y3 := new(big.Int).Set(r)
+	y3.Set(r)
 	v.Sub(v, x3)
 	y3.Mul(y3, v)
 	s1.Mul(s1, j)
@@ -137,16 +172,10 @@ func (curve *CurveParams) addJacobian(x1, y1, z1, x2, y2, z2 *big.Int) (*big.Int
 	y3.Sub(y3, s1)
 	y3.Mod(y3, curve.P)
 
-	z3 := new(big.Int).Add(z1, z2)
+	z3.Add(z1, z2)
 	z3.Mul(z3, z3)
 	z3.Sub(z3, z1z1)
-	if z3.Sign() == -1 {
-		z3.Add(z3, curve.P)
-	}
 	z3.Sub(z3, z2z2)
-	if z3.Sign() == -1 {
-		z3.Add(z3, curve.P)
-	}
 	z3.Mul(z3, h)
 	z3.Mod(z3, curve.P)
 
@@ -154,7 +183,7 @@ func (curve *CurveParams) addJacobian(x1, y1, z1, x2, y2, z2 *big.Int) (*big.Int
 }
 
 func (curve *CurveParams) Double(x1, y1 *big.Int) (*big.Int, *big.Int) {
-	z1 := new(big.Int).SetInt64(1)
+	z1 := zForAffine(x1, y1)
 	return curve.affineFromJacobian(curve.doubleJacobian(x1, y1, z1))
 }
 
@@ -219,40 +248,19 @@ func (curve *CurveParams) doubleJacobian(x, y, z *big.Int) (*big.Int, *big.Int,
 }
 
 func (curve *CurveParams) ScalarMult(Bx, By *big.Int, k []byte) (*big.Int, *big.Int) {
-	// We have a slight problem in that the identity of the group (the
-	// point at infinity) cannot be represented in (x, y) form on a finite
-	// machine. Thus the standard add/double algorithm has to be tweaked
-	// slightly: our initial state is not the identity, but x, and we
-	// ignore the first true bit in |k|.  If we don't find any true bits in
-	// |k|, then we return nil, nil, because we cannot return the identity
-	// element.
-
 	Bz := new(big.Int).SetInt64(1)
-	x := Bx
-	y := By
-	z := Bz
+	x, y, z := new(big.Int), new(big.Int), new(big.Int)
 
-	seenFirstTrue := false
 	for _, byte := range k {
 		for bitNum := 0; bitNum < 8; bitNum++ {
-			if seenFirstTrue {
-				x, y, z = curve.doubleJacobian(x, y, z)
-			}
+			x, y, z = curve.doubleJacobian(x, y, z)
 			if byte&0x80 == 0x80 {
-				if !seenFirstTrue {
-					seenFirstTrue = true
-				} else {
-					x, y, z = curve.addJacobian(Bx, By, Bz, x, y, z)
-				}
+				x, y, z = curve.addJacobian(Bx, By, Bz, x, y, z)
 			}
 			byte <<= 1
 		}
 	}
 
-	if !seenFirstTrue {
-		return nil, nil
-	}
-
 	return curve.affineFromJacobian(x, y, z)
 }
 

src/pkg/crypto/elliptic/elliptic_test.go

@@ -322,6 +322,44 @@ func TestGenericBaseMult(t *testing.T) {
 	}
 }
 
+func TestInfinity(t *testing.T) {
+	tests := []struct {
+		name  string
+		curve Curve
+	}{
+		{"p224", P224()},
+		{"p256", P256()},
+	}
+
+	for _, test := range tests {
+		curve := test.curve
+		x, y := curve.ScalarBaseMult(nil)
+		if x.Sign() != 0 || y.Sign() != 0 {
+			t.Errorf("%s: x^0 != ∞", test.name)
+		}
+		x.SetInt64(0)
+		y.SetInt64(0)
+
+		x2, y2 := curve.Double(x, y)
+		if x2.Sign() != 0 || y2.Sign() != 0 {
+			t.Errorf("%s: 2∞ != ∞", test.name)
+		}
+
+		baseX := curve.Params().Gx
+		baseY := curve.Params().Gy
+
+		x3, y3 := curve.Add(baseX, baseY, x, y)
+		if x3.Cmp(baseX) != 0 || y3.Cmp(baseY) != 0 {
+			t.Errorf("%s: x+∞ != x", test.name)
+		}
+
+		x4, y4 := curve.Add(x, y, baseX, baseY)
+		if x4.Cmp(baseX) != 0 || y4.Cmp(baseY) != 0 {
+			t.Errorf("%s: ∞+x != x", test.name)
+		}
+	}
+}
+
 func BenchmarkBaseMult(b *testing.B) {
 	b.ResetTimer()
 	p224 := P224()

src/pkg/crypto/elliptic/p224.go

@@ -80,10 +80,14 @@ func (p224Curve) Add(bigX1, bigY1, bigX2, bigY2 *big.Int) (x, y *big.Int) {
 
 	p224FromBig(&x1, bigX1)
 	p224FromBig(&y1, bigY1)
-	z1[0] = 1
+	if bigX1.Sign() != 0 || bigY1.Sign() != 0 {
+		z1[0] = 1
+	}
 	p224FromBig(&x2, bigX2)
 	p224FromBig(&y2, bigY2)
-	z2[0] = 1
+	if bigX2.Sign() != 0 || bigY2.Sign() != 0 {
+		z2[0] = 1
+	}
 
 	p224AddJacobian(&x3, &y3, &z3, &x1, &y1, &z1, &x2, &y2, &z2)
 	return p224ToAffine(&x3, &y3, &z3)
@@ -132,6 +136,44 @@ func (curve p224Curve) ScalarBaseMult(scalar []byte) (x, y *big.Int) {
 // exactly, making the reflections during a reduce much nicer.
 type p224FieldElement [8]uint32
 
+// p224P is the order of the field, represented as a p224FieldElement.
+var p224P = [8]uint32{1, 0, 0, 0xffff000, 0xfffffff, 0xfffffff, 0xfffffff, 0xfffffff}
+
+// p224IsZero returns 1 if a == 0 mod p and 0 otherwise.
+//
+// a[i] < 2**29
+func p224IsZero(a *p224FieldElement) uint32 {
+	// Since a p224FieldElement contains 224 bits there are two possible
+	// representations of 0: 0 and p.
+	var minimal p224FieldElement
+	p224Contract(&minimal, a)
+
+	var isZero, isP uint32
+	for i, v := range minimal {
+		isZero |= v
+		isP |= v - p224P[i]
+	}
+
+	// If either isZero or isP is 0, then we should return 1.
+	isZero |= isZero >> 16
+	isZero |= isZero >> 8
+	isZero |= isZero >> 4
+	isZero |= isZero >> 2
+	isZero |= isZero >> 1
+
+	isP |= isP >> 16
+	isP |= isP >> 8
+	isP |= isP >> 4
+	isP |= isP >> 2
+	isP |= isP >> 1
+
+	// For isZero and isP, the LSB is 0 iff all the bits are zero.
+	result := isZero & isP
+	result = (^result) & 1
+
+	return result
+}
+
 // p224Add computes *out = a+b
 //
 // a[i] + b[i] < 2**32
@@ -406,7 +448,7 @@ func p224Contract(out, in *p224FieldElement) {
 	// true.
 	top4AllOnes := uint32(0xffffffff)
 	for i := 4; i < 8; i++ {
-		top4AllOnes &= (out[i] & bottom28Bits) - 1
+		top4AllOnes &= out[i]
 	}
 	top4AllOnes |= 0xf0000000
 	// Now we replicate any zero bits to all the bits in top4AllOnes.
@@ -441,7 +483,7 @@ func p224Contract(out, in *p224FieldElement) {
 	out3Equal = ^uint32(int32(out3Equal<<31) >> 31)
 
 	// If out[3] > 0xffff000 then n's MSB will be zero.
-	out3GT := ^uint32(int32(n<<31) >> 31)
+	out3GT := ^uint32(int32(n) >> 31)
 
 	mask := top4AllOnes & ((out3Equal & bottom3NonZero) | out3GT)
 	out[0] -= 1 & mask
@@ -463,6 +505,9 @@ func p224AddJacobian(x3, y3, z3, x1, y1, z1, x2, y2, z2 *p224FieldElement) {
 	var z1z1, z2z2, u1, u2, s1, s2, h, i, j, r, v p224FieldElement
 	var c p224LargeFieldElement
 
+	z1IsZero := p224IsZero(z1)
+	z2IsZero := p224IsZero(z2)
+
 	// Z1Z1 = Z1²
 	p224Square(&z1z1, z1, &c)
 	// Z2Z2 = Z2²
@@ -480,6 +525,7 @@ func p224AddJacobian(x3, y3, z3, x1, y1, z1, x2, y2, z2 *p224FieldElement) {
 	// H = U2-U1
 	p224Sub(&h, &u2, &u1)
 	p224Reduce(&h)
+	xEqual := p224IsZero(&h)
 	// I = (2*H)²
 	for j := 0; j < 8; j++ {
 		i[j] = h[j] << 1
@@ -491,6 +537,11 @@ func p224AddJacobian(x3, y3, z3, x1, y1, z1, x2, y2, z2 *p224FieldElement) {
 	// r = 2*(S2-S1)
 	p224Sub(&r, &s2, &s1)
 	p224Reduce(&r)
+	yEqual := p224IsZero(&r)
+	if xEqual == 1 && yEqual == 1 && z1IsZero == 0 && z2IsZero == 0 {
+		p224DoubleJacobian(x3, y3, z3, x1, y1, z1)
+		return
+	}
 	for i := 0; i < 8; i++ {
 		r[i] <<= 1
 	}
@@ -524,6 +575,13 @@ func p224AddJacobian(x3, y3, z3, x1, y1, z1, x2, y2, z2 *p224FieldElement) {
 	p224Mul(&z1z1, &z1z1, &r, &c)
 	p224Sub(y3, &z1z1, &s1)
 	p224Reduce(y3)
+
+	p224CopyConditional(x3, x2, z1IsZero)
+	p224CopyConditional(x3, x1, z2IsZero)
+	p224CopyConditional(y3, y2, z1IsZero)
+	p224CopyConditional(y3, y1, z2IsZero)
+	p224CopyConditional(z3, z2, z1IsZero)
+	p224CopyConditional(z3, z1, z2IsZero)
 }
 
 // p224DoubleJacobian computes *out = a+a.
@@ -593,22 +651,19 @@ func p224CopyConditional(out, in *p224FieldElement, control uint32) {
 func p224ScalarMult(outX, outY, outZ, inX, inY, inZ *p224FieldElement, scalar []byte) {
 	var xx, yy, zz p224FieldElement
 	for i := 0; i < 8; i++ {
+		outX[i] = 0
+		outY[i] = 0
 		outZ[i] = 0
 	}
 
-	firstBit := uint32(1)
 	for _, byte := range scalar {
 		for bitNum := uint(0); bitNum < 8; bitNum++ {
 			p224DoubleJacobian(outX, outY, outZ, outX, outY, outZ)
 			bit := uint32((byte >> (7 - bitNum)) & 1)
 			p224AddJacobian(&xx, &yy, &zz, inX, inY, inZ, outX, outY, outZ)
-			p224CopyConditional(outX, inX, firstBit&bit)
-			p224CopyConditional(outY, inY, firstBit&bit)
-			p224CopyConditional(outZ, inZ, firstBit&bit)
-			p224CopyConditional(outX, &xx, ^firstBit&bit)
-			p224CopyConditional(outY, &yy, ^firstBit&bit)
-			p224CopyConditional(outZ, &zz, ^firstBit&bit)
-			firstBit = firstBit & ^bit
+			p224CopyConditional(outX, &xx, bit)
+			p224CopyConditional(outY, &yy, bit)
+			p224CopyConditional(outZ, &zz, bit)
 		}
 	}
 }
@@ -618,16 +673,8 @@ func p224ToAffine(x, y, z *p224FieldElement) (*big.Int, *big.Int) {
 	var zinv, zinvsq, outx, outy p224FieldElement
 	var tmp p224LargeFieldElement
 
-	isPointAtInfinity := true
-	for i := 0; i < 8; i++ {
-		if z[i] != 0 {
-			isPointAtInfinity = false
-			break
-		}
-	}
-
-	if isPointAtInfinity {
-		return nil, nil
+	if isPointAtInfinity := p224IsZero(z); isPointAtInfinity == 1 {
+		return new(big.Int), new(big.Int)
 	}
 
 	p224Invert(&zinv, z)