internal/bigmod: sync sdk #200

internal/bigmod/nat.go

@@ -318,14 +318,48 @@ type Modulus struct {
 // rr returns R*R with R = 2^(_W * n) and n = len(m.nat.limbs).
 func rr(m *Modulus) *Nat {
 	rr := NewNat().ExpandFor(m)
-	// R*R is 2^(2 * _W * n). We can safely get 2^(_W * (n - 1)) by setting the
-	// most significant limb to 1. We then get to R*R by shifting left by _W
-	// n + 1 times.
-	n := len(rr.limbs)
-	rr.limbs[n-1] = 1
-	for i := n - 1; i < 2*n; i++ {
-		rr.shiftIn(0, m) // x = x * 2^_W mod m
+	n := uint(len(rr.limbs))
+	mLen := uint(m.BitLen())
+	logR := _W * n
+
+	// We start by computing R = 2^(_W * n) mod m. We can get pretty close, to
+	// 2^⌊log₂m⌋, by setting the highest bit we can without having to reduce.
+	rr.limbs[n-1] = 1 << ((mLen - 1) % _W)
+	// Then we double until we reach 2^(_W * n).
+	for i := mLen - 1; i < logR; i++ {
+		rr.Add(rr, m)
 	}
+
+	// Next we need to get from R to 2^(_W * n) R mod m (aka from one to R in
+	// the Montgomery domain, meaning we can use Montgomery multiplication now).
+	// We could do that by doubling _W * n times, or with a square-and-double
+	// chain log2(_W * n) long. Turns out the fastest thing is to start out with
+	// doublings, and switch to square-and-double once the exponent is large
+	// enough to justify the cost of the multiplications.
+
+	// The threshold is selected experimentally as a linear function of n.
+	threshold := n / 4
+
+	// We calculate how many of the most-significant bits of the exponent we can
+	// compute before crossing the threshold, and we do it with doublings.
+	i := bits.UintSize
+	for logR>>i <= threshold {
+		i--
+	}
+	for k := uint(0); k < logR>>i; k++ {
+		rr.Add(rr, m)
+	}
+
+	// Then we process the remaining bits of the exponent with a
+	// square-and-double chain.
+	for i > 0 {
+		rr.montgomeryMul(rr, rr, m)
+		i--
+		if logR>>i&1 != 0 {
+			rr.Add(rr, m)
+		}
+	}
+
 	return rr
 }
 
@@ -775,26 +809,21 @@ func (out *Nat) Exp(x *Nat, e []byte, m *Modulus) *Nat {
 	return out.montgomeryReduction(m)
 }
 
-// ExpShort calculates out = x^e mod m.
+// ExpShortVarTime calculates out = x^e mod m.
 //
 // The output will be resized to the size of m and overwritten. x must already
-// be reduced modulo m. This leaks the exact bit size of the exponent.
-func (out *Nat) ExpShort(x *Nat, e uint, m *Modulus) *Nat {
-	xR := NewNat().Set(x).montgomeryRepresentation(m)
-
-	out.resetFor(m)
-	out.limbs[0] = 1
-	out.montgomeryRepresentation(m)
-
+// be reduced modulo m. This leaks the exponent through timing side-channels.
+func (out *Nat) ExpShortVarTime(x *Nat, e uint, m *Modulus) *Nat {
 	// For short exponents, precomputing a table and using a window like in Exp
-	// doesn't pay off. Instead, we do a simple constant-time conditional
-	// square-and-multiply chain, skipping the initial run of zeroes.
-	tmp := NewNat().ExpandFor(m)
-	for i := bits.UintSize - bitLen(e); i < bits.UintSize; i++ {
+	// doesn't pay off. Instead, we do a simple conditional square-and-multiply
+	// chain, skipping the initial run of zeroes.
+	xR := NewNat().Set(x).montgomeryRepresentation(m)
+	out.Set(xR)
+	for i := bits.UintSize - bitLen(e) + 1; i < bits.UintSize; i++ {
 		out.montgomeryMul(out, out, m)
-		k := (e >> (bits.UintSize - i - 1)) & 1
-		tmp.montgomeryMul(out, xR, m)
-		out.assign(ctEq(k, 1), tmp)
+		if k := (e >> (bits.UintSize - i - 1)) & 1; k != 0 {
+			out.montgomeryMul(out, xR, m)
+		}
 	}
 	return out.montgomeryReduction(m)
 }

internal/bigmod/nat_test.go

@@ -303,7 +303,7 @@ func TestExpShort(t *testing.T) {
 	m := modulusFromBytes([]byte{13})
 	x := &Nat{[]uint{3}}
 	out := &Nat{[]uint{0}}
-	out.ExpShort(x, 12, m)
+	out.ExpShortVarTime(x, 12, m)
 	expected := &Nat{[]uint{1}}
 	if out.Equal(expected) != 1 {
 		t.Errorf("%+v != %+v", out, expected)
@@ -383,6 +383,13 @@ func makeBenchmarkExponent() []byte {
 	return e
 }
 
+func BenchmarkRR256(b *testing.B) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		makeBenchmarkModulus(4)
+	}
+}
+
 func BenchmarkModAdd(b *testing.B) {
 	x := makeBenchmarkValue(32)
 	y := makeBenchmarkValue(32)