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74c9252 Sep 26, 2017
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231 lines (190 sloc) 4.33 KB
package bn256
import (
"math/big"
)
// curvePoint implements the elliptic curve y²=x³+3. Points are kept in Jacobian
// form and t=z² when valid. G₁ is the set of points of this curve on GF(p).
type curvePoint struct {
x, y, z, t gfP
}
var curveB = newGFp(3)
// curveGen is the generator of G₁.
var curveGen = &curvePoint{
x: *newGFp(1),
y: *newGFp(-2),
z: *newGFp(1),
t: *newGFp(1),
}
func (c *curvePoint) String() string {
c.MakeAffine()
x, y := &gfP{}, &gfP{}
montDecode(x, &c.x)
montDecode(y, &c.y)
return "(" + x.String() + ", " + y.String() + ")"
}
func (c *curvePoint) Set(a *curvePoint) {
c.x.Set(&a.x)
c.y.Set(&a.y)
c.z.Set(&a.z)
c.t.Set(&a.t)
}
// IsOnCurve returns true iff c is on the curve.
func (c *curvePoint) IsOnCurve() bool {
c.MakeAffine()
if c.IsInfinity() {
return true
}
y2, x3 := &gfP{}, &gfP{}
gfpMul(y2, &c.y, &c.y)
gfpMul(x3, &c.x, &c.x)
gfpMul(x3, x3, &c.x)
gfpAdd(x3, x3, curveB)
return *y2 == *x3
}
func (c *curvePoint) SetInfinity() {
c.x = gfP{0}
c.y = *newGFp(1)
c.z = gfP{0}
c.t = gfP{0}
}
func (c *curvePoint) IsInfinity() bool {
return c.z == gfP{0}
}
func (c *curvePoint) Add(a, b *curvePoint) {
if a.IsInfinity() {
c.Set(b)
return
}
if b.IsInfinity() {
c.Set(a)
return
}
// See http://hyperelliptic.org/EFD/g1p/auto-code/shortw/jacobian-0/addition/add-2007-bl.op3
// Normalize the points by replacing a = [x1:y1:z1] and b = [x2:y2:z2]
// by [u1:s1:z1·z2] and [u2:s2:z1·z2]
// where u1 = x1·z2², s1 = y1·z2³ and u1 = x2·z1², s2 = y2·z1³
z12, z22 := &gfP{}, &gfP{}
gfpMul(z12, &a.z, &a.z)
gfpMul(z22, &b.z, &b.z)
u1, u2 := &gfP{}, &gfP{}
gfpMul(u1, &a.x, z22)
gfpMul(u2, &b.x, z12)
t, s1 := &gfP{}, &gfP{}
gfpMul(t, &b.z, z22)
gfpMul(s1, &a.y, t)
s2 := &gfP{}
gfpMul(t, &a.z, z12)
gfpMul(s2, &b.y, t)
// Compute x = (2h)²(s²-u1-u2)
// where s = (s2-s1)/(u2-u1) is the slope of the line through
// (u1,s1) and (u2,s2). The extra factor 2h = 2(u2-u1) comes from the value of z below.
// This is also:
// 4(s2-s1)² - 4h²(u1+u2) = 4(s2-s1)² - 4h³ - 4h²(2u1)
// = r² - j - 2v
// with the notations below.
h := &gfP{}
gfpSub(h, u2, u1)
xEqual := *h == gfP{0}
gfpAdd(t, h, h)
// i = 4h²
i := &gfP{}
gfpMul(i, t, t)
// j = 4h³
j := &gfP{}
gfpMul(j, h, i)
gfpSub(t, s2, s1)
yEqual := *t == gfP{0}
if xEqual && yEqual {
c.Double(a)
return
}
r := &gfP{}
gfpAdd(r, t, t)
v := &gfP{}
gfpMul(v, u1, i)
// t4 = 4(s2-s1)²
t4, t6 := &gfP{}, &gfP{}
gfpMul(t4, r, r)
gfpAdd(t, v, v)
gfpSub(t6, t4, j)
gfpSub(&c.x, t6, t)
// Set y = -(2h)³(s1 + s*(x/4h²-u1))
// This is also
// y = - 2·s1·j - (s2-s1)(2x - 2i·u1) = r(v-x) - 2·s1·j
gfpSub(t, v, &c.x) // t7
gfpMul(t4, s1, j) // t8
gfpAdd(t6, t4, t4) // t9
gfpMul(t4, r, t) // t10
gfpSub(&c.y, t4, t6)
// Set z = 2(u2-u1)·z1·z2 = 2h·z1·z2
gfpAdd(t, &a.z, &b.z) // t11
gfpMul(t4, t, t) // t12
gfpSub(t, t4, z12) // t13
gfpSub(t4, t, z22) // t14
gfpMul(&c.z, t4, h)
}
func (c *curvePoint) Double(a *curvePoint) {
// See http://hyperelliptic.org/EFD/g1p/auto-code/shortw/jacobian-0/doubling/dbl-2009-l.op3
A, B, C := &gfP{}, &gfP{}, &gfP{}
gfpMul(A, &a.x, &a.x)
gfpMul(B, &a.y, &a.y)
gfpMul(C, B, B)
t, t2 := &gfP{}, &gfP{}
gfpAdd(t, &a.x, B)
gfpMul(t2, t, t)
gfpSub(t, t2, A)
gfpSub(t2, t, C)
d, e, f := &gfP{}, &gfP{}, &gfP{}
gfpAdd(d, t2, t2)
gfpAdd(t, A, A)
gfpAdd(e, t, A)
gfpMul(f, e, e)
gfpAdd(t, d, d)
gfpSub(&c.x, f, t)
gfpAdd(t, C, C)
gfpAdd(t2, t, t)
gfpAdd(t, t2, t2)
gfpSub(&c.y, d, &c.x)
gfpMul(t2, e, &c.y)
gfpSub(&c.y, t2, t)
gfpMul(t, &a.y, &a.z)
gfpAdd(&c.z, t, t)
}
func (c *curvePoint) Mul(a *curvePoint, scalar *big.Int) {
sum, t := &curvePoint{}, &curvePoint{}
sum.SetInfinity()
for i := scalar.BitLen(); i >= 0; i-- {
t.Double(sum)
if scalar.Bit(i) != 0 {
sum.Add(t, a)
} else {
sum.Set(t)
}
}
c.Set(sum)
}
func (c *curvePoint) MakeAffine() {
if c.z == *newGFp(1) {
return
} else if c.z == *newGFp(0) {
c.x = gfP{0}
c.y = *newGFp(1)
c.t = gfP{0}
return
}
zInv := &gfP{}
zInv.Invert(&c.z)
t, zInv2 := &gfP{}, &gfP{}
gfpMul(t, &c.y, zInv)
gfpMul(zInv2, zInv, zInv)
gfpMul(&c.x, &c.x, zInv2)
gfpMul(&c.y, t, zInv2)
c.z = *newGFp(1)
c.t = *newGFp(1)
}
func (c *curvePoint) Neg(a *curvePoint) {
c.x.Set(&a.x)
gfpNeg(&c.y, &a.y)
c.z.Set(&a.z)
c.t = gfP{0}
}