Solving scalar constant-time operations.

Author: armfazh

ecc/goldilocks/decaf.go

@@ -101,7 +101,7 @@ func (e *Elt) UnmarshalBinary(data []byte) error {
 	fp.Mul(y, y, t0)                    // y = (1 - a*s^2)*isr*den
 
 	isValid := isPositiveS && isLessThanP && isQR
-	b := *((*uint)(unsafe.Pointer(&isValid)))
+	b := uint(*((*byte)(unsafe.Pointer(&isValid))))
 	fp.Cmov(&e.p.x, x, b)
 	fp.Cmov(&e.p.y, y, b)
 	fp.Cmov(&e.p.ta, x, b)

ecc/goldilocks/isogeny_test.go

@@ -10,7 +10,9 @@ import (
 func randomPoint() *Point {
 	var k Scalar
 	_, _ = rand.Read(k[:])
-	return Curve{}.ScalarBaseMult(&k)
+	P := &Point{}
+	Curve{}.ScalarBaseMult(P, &k)
+	return P
 }
 
 func TestIsogeny(t *testing.T) {

ecc/goldilocks/point.go

@@ -123,7 +123,7 @@ func (P *Point) UnmarshalBinary(data []byte) error {
 	fp.Cmov(x, u, uint(signX^(x[0]&1))) // if signX != x mod 2
 
 	isValid := isLessThanP && isQR && isValidXSign
-	b := *((*uint)(unsafe.Pointer(&isValid)))
+	b := uint(*((*byte)(unsafe.Pointer(&isValid))))
 	fp.Cmov(&P.x, x, b)
 	fp.Cmov(&P.y, y, b)
 	fp.Cmov(&P.ta, x, b)

ecc/goldilocks/scalar.go

@@ -33,6 +33,25 @@ func (z *scalar64) toScalar(x *Scalar) {
 	binary.LittleEndian.PutUint64(x[6*8:7*8], z[6])
 }
 
+// isZero returns 1 if z=0.
+func (z *scalar64) isZero() uint {
+	z.modOrder()
+	var z64 uint64
+	for i := range z {
+		z64 |= z[i]
+	}
+	z32 := uint32(z64&0xFFFFFFFF) | (uint32(z64>>32) & 0xFFFFFFF)
+	return uint((uint64(z32) - 1) >> 63)
+}
+
+// cmov moves x into z if b=1.
+func (z *scalar64) cmov(x *scalar64, b uint64) {
+	m := -(b & 1)
+	for i := range z {
+		z[i] = (z[i] &^ m) | (x[i] & m)
+	}
+}
+
 // add calculates z = x + y. Assumes len(z) > max(len(x),len(y)).
 func add(z, x, y []uint64) uint64 {
 	l, L, zz := len(x), len(y), y
@@ -81,14 +100,6 @@ func mulWord(z, x []uint64, y uint64) {
 	z[len(x)] = carry
 }
 
-// Cmov moves x into z if b=1.
-func (z *scalar64) Cmov(b uint64, x *scalar64) {
-	m := uint64(0) - b
-	for i := range z {
-		z[i] = (z[i] &^ m) | (x[i] & m)
-	}
-}
-
 // leftShift shifts to the left the words of z returning the more significant word.
 func (z *scalar64) leftShift(low uint64) uint64 {
 	high := z[_N-1]
@@ -108,6 +119,15 @@ func (z *scalar64) reduceOneWord(x uint64) {
 	add(z[:], z[:], prod)
 }
 
+// Sub calculates z = x-y mod order.
+func (z *scalar64) sub(x, y *scalar64) {
+	var t scalar64
+	c := sub(z[:], x[:], y[:])
+	sub(t[:], z[:], residue448[:])
+	z.cmov(&t, c)
+	z.modOrder()
+}
+
 // modOrder reduces z mod order.
 func (z *scalar64) modOrder() {
 	var o64, x scalar64
@@ -116,7 +136,7 @@ func (z *scalar64) modOrder() {
 	// At most 8 (eight) iterations reduce 3 bits by subtracting.
 	for i := 0; i < 8; i++ {
 		c := sub(x[:], z[:], o64[:]) // (c || x) = z-order
-		z.Cmov(1-c, &x)              // if c != 0 { z = x }
+		z.cmov(&x, 1-c)              // if c != 0 { z = x }
 	}
 }
 
@@ -159,20 +179,17 @@ func (z *Scalar) Add(x, y *Scalar) {
 	y64.fromScalar(y)
 	c := add(z64[:], x64[:], y64[:])
 	add(t[:], z64[:], residue448[:])
-	z64.Cmov(c, &t)
+	z64.cmov(&t, c)
 	z64.modOrder()
 	z64.toScalar(z)
 }
 
 // Sub calculates z = x-y mod order.
 func (z *Scalar) Sub(x, y *Scalar) {
-	var z64, x64, y64, t scalar64
+	var z64, x64, y64 scalar64
 	x64.fromScalar(x)
 	y64.fromScalar(y)
-	c := sub(z64[:], x64[:], y64[:])
-	sub(t[:], z64[:], residue448[:])
-	z64.Cmov(c, &t)
-	z64.modOrder()
+	z64.sub(&x64, &y64)
 	z64.toScalar(z)
 }
 
@@ -195,6 +212,3 @@ func (z *Scalar) Mul(x, y *Scalar) {
 	z64.modOrder()
 	z64.toScalar(z)
 }
-
-// IsZero returns true if z=0.
-func (z *Scalar) IsZero() bool { z.Red(); return *z == Scalar{} }

ecc/goldilocks/twist.go

@@ -3,7 +3,6 @@ package goldilocks
 import (
 	"crypto/subtle"
 	"math/bits"
-	"unsafe"
 
 	"github.com/cloudflare/circl/internal/conv"
 	"github.com/cloudflare/circl/math"
@@ -66,14 +65,12 @@ func subYDiv16(x *scalar64, y int64) {
 	x[6] = (x6 >> 4)
 }
 
-func recodeScalar(d *[113]int8, k *Scalar) {
-	var k64 scalar64
-	k64.fromScalar(k)
+func recodeScalar(d *[113]int8, k *scalar64) {
 	for i := 0; i < 112; i++ {
-		d[i] = int8((k64[0] & 0x1f) - 16)
-		subYDiv16(&k64, int64(d[i]))
+		d[i] = int8((k[0] & 0x1f) - 16)
+		subYDiv16(k, int64(d[i]))
 	}
-	d[112] = int8(k64[0])
+	d[112] = int8(k[0])
 }
 
 // ScalarMult calculates R = kP.
@@ -82,16 +79,16 @@ func (e twistCurve) ScalarMult(R *twistPoint, k *Scalar, P *twistPoint) {
 	var S preTwistPointProy
 	var d [113]int8
 
-	kk := *k
-	isZero := kk.IsZero()
-	isZeroInt := *(*int)(unsafe.Pointer(&isZero))
-	subtle.ConstantTimeCopy(isZeroInt, kk[:], order[:])
+	var k64, _k64, order64 scalar64
+	k64.fromScalar(k)
+	order64.fromScalar(&order)
+	k64.cmov(&order64, uint64(k64.isZero()))
+
+	isEven := 1 - int(k64[0]&0x1)
+	_k64.sub(&order64, &k64)
+	k64.cmov(&_k64, uint64(isEven))
 
-	minusK := kk
-	isEven := 1 - int(kk[0]&0x1)
-	minusK.Neg()
-	subtle.ConstantTimeCopy(isEven, kk[:], minusK[:])
-	recodeScalar(&d, &kk)
+	recodeScalar(&d, &k64)
 
 	P.oddMultiples(TabP[:])
 	Q := e.Identity()

ecc/goldilocks/twist_basemult.go

@@ -24,20 +24,18 @@ func (e twistCurve) ScalarBaseMult(R *twistPoint, k *Scalar) {
 		panic("not extended")
 	}
 
-	kk := *k
-
-	var isZero int
-	if kk.IsZero() {
-		isZero = 1
-	}
-
-	subtle.ConstantTimeCopy(isZero, kk[:], order[:])
-
-	minusK := kk
-	isEven := 1 - int(kk[0]&0x1)
-	minusK.Neg()
-	subtle.ConstantTimeCopy(isEven, kk[:], minusK[:])
-	c, err := m.Encode(kk[:])
+	var k64, _k64, order64 scalar64
+	k64.fromScalar(k)
+	order64.fromScalar(&order)
+	k64.cmov(&order64, uint64(k64.isZero()))
+
+	isEven := 1 - int(k64[0]&0x1)
+	_k64.sub(&order64, &k64)
+	k64.cmov(&_k64, uint64(isEven))
+	var scalar Scalar
+	k64.toScalar(&scalar)
+
+	c, err := m.Encode(scalar[:])
 	if err != nil {
 		panic(err)
 	}