// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
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 *big.Int
}
var curveB = new(big.Int).SetInt64(3)
// curveGen is the generator of G₁.
var curveGen = &curvePoint{
new(big.Int).SetInt64(1),
new(big.Int).SetInt64(2),
new(big.Int).SetInt64(1),
new(big.Int).SetInt64(1),
}
func newCurvePoint(pool *bnPool) *curvePoint {
return &curvePoint{
pool.Get(),
pool.Get(),
pool.Get(),
pool.Get(),
}
}
func (c *curvePoint) String() string {
c.MakeAffine(new(bnPool))
return "(" + c.x.String() + ", " + c.y.String() + ")"
}
func (c *curvePoint) Put(pool *bnPool) {
pool.Put(c.x)
pool.Put(c.y)
pool.Put(c.z)
pool.Put(c.t)
}
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 where c must be in affine form.
func (c *curvePoint) IsOnCurve() bool {
yy := new(big.Int).Mul(c.y, c.y)
xxx := new(big.Int).Mul(c.x, c.x)
xxx.Mul(xxx, c.x)
yy.Sub(yy, xxx)
yy.Sub(yy, curveB)
if yy.Sign() < 0 || yy.Cmp(P) >= 0 {
yy.Mod(yy, P)
}
return yy.Sign() == 0
}
func (c *curvePoint) SetInfinity() {
c.z.SetInt64(0)
}
func (c *curvePoint) IsInfinity() bool {
return c.z.Sign() == 0
}
func (c *curvePoint) Add(a, b *curvePoint, pool *bnPool) {
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
z1z1 := pool.Get().Mul(a.z, a.z)
z1z1.Mod(z1z1, P)
z2z2 := pool.Get().Mul(b.z, b.z)
z2z2.Mod(z2z2, P)
u1 := pool.Get().Mul(a.x, z2z2)
u1.Mod(u1, P)
u2 := pool.Get().Mul(b.x, z1z1)
u2.Mod(u2, P)
t := pool.Get().Mul(b.z, z2z2)
t.Mod(t, P)
s1 := pool.Get().Mul(a.y, t)
s1.Mod(s1, P)
t.Mul(a.z, z1z1)
t.Mod(t, P)
s2 := pool.Get().Mul(b.y, t)
s2.Mod(s2, P)
// 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 := pool.Get().Sub(u2, u1)
xEqual := h.Sign() == 0
t.Add(h, h)
// i = 4h²
i := pool.Get().Mul(t, t)
i.Mod(i, P)
// j = 4h³
j := pool.Get().Mul(h, i)
j.Mod(j, P)
t.Sub(s2, s1)
yEqual := t.Sign() == 0
if xEqual && yEqual {
c.Double(a, pool)
return
}
r := pool.Get().Add(t, t)
v := pool.Get().Mul(u1, i)
v.Mod(v, P)
// t4 = 4(s2-s1)²
t4 := pool.Get().Mul(r, r)
t4.Mod(t4, P)
t.Add(v, v)
t6 := pool.Get().Sub(t4, j)
c.x.Sub(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
t.Sub(v, c.x) // t7
t4.Mul(s1, j) // t8
t4.Mod(t4, P)
t6.Add(t4, t4) // t9
t4.Mul(r, t) // t10
t4.Mod(t4, P)
c.y.Sub(t4, t6)
// Set z = 2(u2-u1)·z1·z2 = 2h·z1·z2
t.Add(a.z, b.z) // t11
t4.Mul(t, t) // t12
t4.Mod(t4, P)
t.Sub(t4, z1z1) // t13
t4.Sub(t, z2z2) // t14
c.z.Mul(t4, h)
c.z.Mod(c.z, P)
pool.Put(z1z1)
pool.Put(z2z2)
pool.Put(u1)
pool.Put(u2)
pool.Put(t)
pool.Put(s1)
pool.Put(s2)
pool.Put(h)
pool.Put(i)
pool.Put(j)
pool.Put(r)
pool.Put(v)
pool.Put(t4)
pool.Put(t6)
}
func (c *curvePoint) Double(a *curvePoint, pool *bnPool) {
// See http://hyperelliptic.org/EFD/g1p/auto-code/shortw/jacobian-0/doubling/dbl-2009-l.op3
A := pool.Get().Mul(a.x, a.x)
A.Mod(A, P)
B := pool.Get().Mul(a.y, a.y)
B.Mod(B, P)
C_ := pool.Get().Mul(B, B)
C_.Mod(C_, P)
t := pool.Get().Add(a.x, B)
t2 := pool.Get().Mul(t, t)
t2.Mod(t2, P)
t.Sub(t2, A)
t2.Sub(t, C_)
d := pool.Get().Add(t2, t2)
t.Add(A, A)
e := pool.Get().Add(t, A)
f := pool.Get().Mul(e, e)
f.Mod(f, P)
t.Add(d, d)
c.x.Sub(f, t)
t.Add(C_, C_)
t2.Add(t, t)
t.Add(t2, t2)
c.y.Sub(d, c.x)
t2.Mul(e, c.y)
t2.Mod(t2, P)
c.y.Sub(t2, t)
t.Mul(a.y, a.z)
t.Mod(t, P)
c.z.Add(t, t)
pool.Put(A)
pool.Put(B)
pool.Put(C_)
pool.Put(t)
pool.Put(t2)
pool.Put(d)
pool.Put(e)
pool.Put(f)
}
func (c *curvePoint) Mul(a *curvePoint, scalar *big.Int, pool *bnPool) *curvePoint {
sum := newCurvePoint(pool)
sum.SetInfinity()
t := newCurvePoint(pool)
for i := scalar.BitLen(); i >= 0; i-- {
t.Double(sum, pool)
if scalar.Bit(i) != 0 {
sum.Add(t, a, pool)
} else {
sum.Set(t)
}
}
c.Set(sum)
sum.Put(pool)
t.Put(pool)
return c
}
func (c *curvePoint) MakeAffine(pool *bnPool) *curvePoint {
if words := c.z.Bits(); len(words) == 1 && words[0] == 1 {
return c
}
zInv := pool.Get().ModInverse(c.z, P)
t := pool.Get().Mul(c.y, zInv)
t.Mod(t, P)
zInv2 := pool.Get().Mul(zInv, zInv)
zInv2.Mod(zInv2, P)
c.y.Mul(t, zInv2)
c.y.Mod(c.y, P)
t.Mul(c.x, zInv2)
t.Mod(t, P)
c.x.Set(t)
c.z.SetInt64(1)
c.t.SetInt64(1)
pool.Put(zInv)
pool.Put(t)
pool.Put(zInv2)
return c
}
func (c *curvePoint) Negative(a *curvePoint) {
c.x.Set(a.x)
c.y.Neg(a.y)
c.z.Set(a.z)
c.t.SetInt64(0)
}
