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Addition chain for scalar exponentiation.
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@armfazh
armfazh committed Aug 26, 2020
1 parent 3f3c87b commit e375be0
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Showing 2 changed files with 82 additions and 46 deletions.
9 changes: 0 additions & 9 deletions internal/ted448/constants.go
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Expand Up @@ -44,15 +44,6 @@ var (
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3f,
}
orderMinusTwo = Scalar{
0xf1, 0x44, 0x58, 0xab, 0x92, 0xc2, 0x78, 0x23,
0x55, 0x8f, 0xc5, 0x8d, 0x72, 0xc2, 0x6c, 0x21,
0x90, 0x36, 0xd6, 0xae, 0x49, 0xdb, 0x4e, 0xc4,
0xe9, 0x23, 0xca, 0x7c, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3f,
}
// residue448 is 2^448 mod order.
residue448 = [4]uint64{
0x721cf5b5529eec34, 0x7a4cf635c8e9c2ab, 0xeec492d944a725bf, 0x20cd77058,
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119 changes: 82 additions & 37 deletions internal/ted448/scalar.go
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Expand Up @@ -60,6 +60,39 @@ func (z *scalar64) cmov(x *scalar64, b uint64) {
}
}

// mul calculates z = x * y.
func (z *scalar64) mul(x, y *scalar64) *scalar64 {
var t scalar64
prod := (&[_N + 1]uint64{})[:]
mulWord(prod, x[:], y[_N-1])
copy(t[:], prod[:_N])
t.reduceOneWord(prod[_N])
for i := _N - 2; i >= 0; i-- {
h := t.leftShift(0)
t.reduceOneWord(h)
mulWord(prod, x[:], y[i])
c := add(t[:], t[:], prod[:_N])
t.reduceOneWord(prod[_N] + c)
}
*z = t
return z
}

// sqrn calculates z = x^(2^n).
func (z *scalar64) sqrn(x *scalar64, n uint) *scalar64 {
t := *x
for i := uint(0); i < n; i++ {
t.mul(&t, &t)
}
*z = t
return z
}

// sqrnmul calculates z = x^(2^n) * y.
func (z *scalar64) sqrnmul(x *scalar64, n uint, y *scalar64) *scalar64 {
return z.mul(z.sqrn(x, n), y)
}

// 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
Expand Down Expand Up @@ -203,49 +236,61 @@ func (z *Scalar) Mul(x, y *Scalar) {
var z64, x64, y64 scalar64
x64.fromScalar(x)
y64.fromScalar(y)
coremul(&z64, &x64, &y64)
z64.mul(&x64, &y64)
z64.modOrder()
z64.toScalar(z)
}

func coremul(z64, x64, y64 *scalar64) {
var p64 scalar64
prod := (&[_N + 1]uint64{})[:]
mulWord(prod, x64[:], y64[_N-1])
copy(p64[:], prod[:_N])
p64.reduceOneWord(prod[_N])
for i := _N - 2; i >= 0; i-- {
h := p64.leftShift(0)
p64.reduceOneWord(h)
mulWord(prod, x64[:], y64[i])
c := add(p64[:], p64[:], prod[:_N])
p64.reduceOneWord(prod[_N] + c)
}
*z64 = p64
}

// Inv calculates z = 1/x mod order.
// Inv calculaxes z = 1/x mod order.
func (z *Scalar) Inv(x *Scalar) {
var x64 scalar64
x64.fromScalar(x)

var T [16]scalar64
T[0] = scalar64{1}
for i := 1; i < 16; i++ {
coremul(&T[i], &T[i-1], &x64)
}

t := &scalar64{1}
for i := 8*len(orderMinusTwo) - 4; i >= 0; i -= 4 {
b := (orderMinusTwo[i/8] >> uint(i%8)) & 0xF
coremul(t, t, t)
coremul(t, t, t)
coremul(t, t, t)
coremul(t, t, t)
if b != 0 {
coremul(t, t, &T[b])
}
}
t.modOrder()
t.toScalar(z)
x10 := (&scalar64{}).mul(&x64, &x64) // x10 = 2 * 1
x11 := (&scalar64{}).mul(x10, &x64) // x11 = 1 + x10
x100 := (&scalar64{}).mul(x11, &x64) // x100 = 1 + x11
x101 := (&scalar64{}).mul(x100, &x64) // x101 = 1 + x100
x1001 := (&scalar64{}).mul(x100, x101) // x1001 = x100 + x101
x1011 := (&scalar64{}).mul(x10, x1001) // x1011 = x10 + x1001
x1101 := (&scalar64{}).mul(x10, x1011) // x1101 = x10 + x1011
x1111 := (&scalar64{}).mul(x10, x1101) // x1111 = x10 + x1101
x10001 := (&scalar64{}).mul(x10, x1111) // x10001 = x10 + x1111
x10011 := (&scalar64{}).mul(x10, x10001) // x10011 = x10 + x10001
x10101 := (&scalar64{}).mul(x10, x10011) // x10101 = x10 + x10011
x10111 := (&scalar64{}).mul(x10, x10101) // x10111 = x10 + x10101
x11001 := (&scalar64{}).mul(x10, x10111) // x11001 = x10 + x10111
x11011 := (&scalar64{}).mul(x10, x11001) // x11011 = x10 + x11001
x11101 := (&scalar64{}).mul(x10, x11011) // x11101 = x10 + x11011
x11111 := (&scalar64{}).mul(x10, x11101) // x11111 = x10 + x11101
x111110 := (&scalar64{}).mul(x11111, x11111) // x111110 = 2 * x11111
x1111100 := (&scalar64{}).mul(x111110, x111110) // x1111100 = 2 * x111110
var i24, i41, i73, i129, t = &scalar64{}, &scalar64{}, &scalar64{},
&scalar64{}, &scalar64{}
var x222, i262, i279, i298, i312, i331, i343, i365,
i375, i396, i411, i431, i444, i464, i478, i498, iret *scalar64 = t, t,
t, t, t, t, t, t, t, t, t, t, t, t, t, t, t

i24.sqrnmul(x1111100, 5, x1111100) // i24 = x1111100 << 5 + x1111100
i41.sqrnmul(i41.sqrnmul(i24, 4, x111110), 11, i24) // i41 = (i24 << 4 + x111110) << 11 + i24
i73.sqrnmul(i73.sqrnmul(i41, 4, x111110), 26, i41) // i73 = (i41 << 4 + x111110) << 26 + i41
i129.sqrnmul(i73, 55, i73) // i129 = i73 << 55 + i73
x222.mul(x222.sqrnmul(i129, 110, x11), i129) // x222 = i129 << 110 + x11 + i129
i262.sqrn(i262.sqrnmul(i262.sqrnmul(x222, 6, x11111), 7, x11001), 6) // i262 = ((x222 << 6 + x11111) << 7 + x11001) << 6
i279.sqrnmul(i279.sqrnmul(i279.mul(x10001, i262), 8, x11111), 6, x10011) // i279 = ((x10001 + i262) << 8 + x11111) << 6 + x10011
i298.sqrn(i298.sqrnmul(i298.sqrnmul(i279, 5, x10001), 8, x10011), 4) // i298 = ((i279 << 5 + x10001) << 8 + x10011) << 4
i312.sqrnmul(i312.sqrnmul(i312.mul(x1011, i298), 6, x11011), 5, x1001) // i312 = ((x1011 + i298) << 6 + x11011) << 5 + x1001
i331.sqrn(i331.sqrnmul(i331.sqrnmul(i312, 6, x1101), 6, x11101), 5) // i331 = ((i312 << 6 + x1101) << 6 + x11101) << 5
i343.sqrnmul(i343.sqrnmul(i343.mul(x10101, i331), 5, x10001), 4, x1011) // i343 = ((x10101 + i331) << 5 + x10001) << 4 + x1011
i365.sqrn(i365.sqrnmul(i365.sqrnmul(i343, 5, x1001), 7, &x64), 8) // i365 = ((i343 << 5 + x1001) << 7 + 1) << 8
i375.sqrnmul(i375.sqrnmul(i375.mul(x1011, i365), 6, x11001), 1, &x64) // i375 = 2*((x1011 + i365) << 6 + x11001) + 1
i396.sqrn(i396.sqrnmul(i396.sqrnmul(i375, 9, x10011), 4, x1001), 6) // i396 = ((i375 << 9 + x10011) << 4 + x1001) << 6
i411.sqrnmul(i411.sqrnmul(i411.mul(x10001, i396), 5, x10111), 7, x1011) // i411 = ((x10001 + i396) << 5 + x10111) << 7 + x1011
i431.sqrn(i431.sqrnmul(i431.sqrnmul(i411, 7, x1111), 6, x10101), 5) // i431 = ((i411 << 7 + x1111) << 6 + x10101) << 5
i444.sqrnmul(i444.sqrnmul(i444.mul(x1001, i431), 8, x11011), 2, x11) // i444 = ((x1001 + i431) << 8 + x11011) << 2 + x11
i464.sqrn(i464.sqrnmul(i464.sqrnmul(i444, 5, x11), 7, x101), 6) // i464 = ((i444 << 5 + x11) << 7 + x101) << 6
i478.sqrnmul(i478.sqrnmul(i478.mul(x1001, i464), 6, x10101), 5, x1101) // i478 = ((x1001 + i464) << 6 + x10101) << 5 + x1101
i498.sqrn(i498.sqrnmul(i498.sqrnmul(i478, 3, x11), 9, x10001), 6) // i498 = ((i478 << 3 + x11) << 9 + x10001) << 6
iret.sqrnmul(iret.mul(x1111, i498), 4, &x64) // z = (x1111 + i498) << 4 + 1
iret.modOrder()
iret.toScalar(z)
}

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