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279 lines
5.3 KiB
Go
279 lines
5.3 KiB
Go
// Copyright 2012 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package bn256
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import (
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"math/big"
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)
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// curvePoint implements the elliptic curve y²=x³+3. Points are kept in
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// Jacobian form and t=z² when valid. G₁ is the set of points of this curve on
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// GF(p).
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type curvePoint struct {
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x, y, z, t *big.Int
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}
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var curveB = new(big.Int).SetInt64(3)
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// curveGen is the generator of G₁.
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var curveGen = &curvePoint{
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new(big.Int).SetInt64(1),
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new(big.Int).SetInt64(-2),
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new(big.Int).SetInt64(1),
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new(big.Int).SetInt64(1),
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}
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func newCurvePoint(pool *bnPool) *curvePoint {
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return &curvePoint{
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pool.Get(),
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pool.Get(),
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pool.Get(),
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pool.Get(),
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}
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}
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func (c *curvePoint) String() string {
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c.MakeAffine(new(bnPool))
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return "(" + c.x.String() + ", " + c.y.String() + ")"
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}
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func (c *curvePoint) Put(pool *bnPool) {
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pool.Put(c.x)
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pool.Put(c.y)
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pool.Put(c.z)
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pool.Put(c.t)
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}
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func (c *curvePoint) Set(a *curvePoint) {
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c.x.Set(a.x)
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c.y.Set(a.y)
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c.z.Set(a.z)
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c.t.Set(a.t)
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}
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// IsOnCurve returns true iff c is on the curve where c must be in affine form.
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func (c *curvePoint) IsOnCurve() bool {
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yy := new(big.Int).Mul(c.y, c.y)
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xxx := new(big.Int).Mul(c.x, c.x)
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xxx.Mul(xxx, c.x)
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yy.Sub(yy, xxx)
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yy.Sub(yy, curveB)
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if yy.Sign() < 0 || yy.Cmp(p) >= 0 {
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yy.Mod(yy, p)
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}
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return yy.Sign() == 0
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}
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func (c *curvePoint) SetInfinity() {
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c.z.SetInt64(0)
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}
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func (c *curvePoint) IsInfinity() bool {
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return c.z.Sign() == 0
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}
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func (c *curvePoint) Add(a, b *curvePoint, pool *bnPool) {
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if a.IsInfinity() {
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c.Set(b)
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return
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}
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if b.IsInfinity() {
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c.Set(a)
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return
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}
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// See http://hyperelliptic.org/EFD/g1p/auto-code/shortw/jacobian-0/addition/add-2007-bl.op3
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// Normalize the points by replacing a = [x1:y1:z1] and b = [x2:y2:z2]
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// by [u1:s1:z1·z2] and [u2:s2:z1·z2]
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// where u1 = x1·z2², s1 = y1·z2³ and u1 = x2·z1², s2 = y2·z1³
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z1z1 := pool.Get().Mul(a.z, a.z)
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z1z1.Mod(z1z1, p)
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z2z2 := pool.Get().Mul(b.z, b.z)
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z2z2.Mod(z2z2, p)
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u1 := pool.Get().Mul(a.x, z2z2)
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u1.Mod(u1, p)
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u2 := pool.Get().Mul(b.x, z1z1)
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u2.Mod(u2, p)
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t := pool.Get().Mul(b.z, z2z2)
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t.Mod(t, p)
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s1 := pool.Get().Mul(a.y, t)
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s1.Mod(s1, p)
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t.Mul(a.z, z1z1)
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t.Mod(t, p)
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s2 := pool.Get().Mul(b.y, t)
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s2.Mod(s2, p)
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// Compute x = (2h)²(s²-u1-u2)
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// where s = (s2-s1)/(u2-u1) is the slope of the line through
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// (u1,s1) and (u2,s2). The extra factor 2h = 2(u2-u1) comes from the value of z below.
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// This is also:
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// 4(s2-s1)² - 4h²(u1+u2) = 4(s2-s1)² - 4h³ - 4h²(2u1)
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// = r² - j - 2v
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// with the notations below.
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h := pool.Get().Sub(u2, u1)
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xEqual := h.Sign() == 0
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t.Add(h, h)
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// i = 4h²
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i := pool.Get().Mul(t, t)
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i.Mod(i, p)
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// j = 4h³
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j := pool.Get().Mul(h, i)
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j.Mod(j, p)
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t.Sub(s2, s1)
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yEqual := t.Sign() == 0
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if xEqual && yEqual {
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c.Double(a, pool)
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return
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}
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r := pool.Get().Add(t, t)
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v := pool.Get().Mul(u1, i)
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v.Mod(v, p)
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// t4 = 4(s2-s1)²
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t4 := pool.Get().Mul(r, r)
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t4.Mod(t4, p)
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t.Add(v, v)
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t6 := pool.Get().Sub(t4, j)
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c.x.Sub(t6, t)
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// Set y = -(2h)³(s1 + s*(x/4h²-u1))
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// This is also
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// y = - 2·s1·j - (s2-s1)(2x - 2i·u1) = r(v-x) - 2·s1·j
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t.Sub(v, c.x) // t7
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t4.Mul(s1, j) // t8
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t4.Mod(t4, p)
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t6.Add(t4, t4) // t9
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t4.Mul(r, t) // t10
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t4.Mod(t4, p)
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c.y.Sub(t4, t6)
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// Set z = 2(u2-u1)·z1·z2 = 2h·z1·z2
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t.Add(a.z, b.z) // t11
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t4.Mul(t, t) // t12
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t4.Mod(t4, p)
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t.Sub(t4, z1z1) // t13
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t4.Sub(t, z2z2) // t14
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c.z.Mul(t4, h)
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c.z.Mod(c.z, p)
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pool.Put(z1z1)
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pool.Put(z2z2)
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pool.Put(u1)
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pool.Put(u2)
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pool.Put(t)
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pool.Put(s1)
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pool.Put(s2)
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pool.Put(h)
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pool.Put(i)
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pool.Put(j)
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pool.Put(r)
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pool.Put(v)
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pool.Put(t4)
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pool.Put(t6)
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}
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func (c *curvePoint) Double(a *curvePoint, pool *bnPool) {
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// See http://hyperelliptic.org/EFD/g1p/auto-code/shortw/jacobian-0/doubling/dbl-2009-l.op3
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A := pool.Get().Mul(a.x, a.x)
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A.Mod(A, p)
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B := pool.Get().Mul(a.y, a.y)
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B.Mod(B, p)
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C := pool.Get().Mul(B, B)
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C.Mod(C, p)
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t := pool.Get().Add(a.x, B)
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t2 := pool.Get().Mul(t, t)
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t2.Mod(t2, p)
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t.Sub(t2, A)
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t2.Sub(t, C)
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d := pool.Get().Add(t2, t2)
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t.Add(A, A)
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e := pool.Get().Add(t, A)
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f := pool.Get().Mul(e, e)
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f.Mod(f, p)
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t.Add(d, d)
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c.x.Sub(f, t)
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t.Add(C, C)
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t2.Add(t, t)
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t.Add(t2, t2)
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c.y.Sub(d, c.x)
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t2.Mul(e, c.y)
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t2.Mod(t2, p)
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c.y.Sub(t2, t)
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t.Mul(a.y, a.z)
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t.Mod(t, p)
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c.z.Add(t, t)
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pool.Put(A)
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pool.Put(B)
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pool.Put(C)
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pool.Put(t)
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pool.Put(t2)
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pool.Put(d)
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pool.Put(e)
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pool.Put(f)
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}
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func (c *curvePoint) Mul(a *curvePoint, scalar *big.Int, pool *bnPool) *curvePoint {
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sum := newCurvePoint(pool)
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sum.SetInfinity()
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t := newCurvePoint(pool)
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for i := scalar.BitLen(); i >= 0; i-- {
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t.Double(sum, pool)
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if scalar.Bit(i) != 0 {
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sum.Add(t, a, pool)
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} else {
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sum.Set(t)
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}
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}
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c.Set(sum)
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sum.Put(pool)
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t.Put(pool)
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return c
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}
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func (c *curvePoint) MakeAffine(pool *bnPool) *curvePoint {
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if words := c.z.Bits(); len(words) == 1 && words[0] == 1 {
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return c
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}
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zInv := pool.Get().ModInverse(c.z, p)
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t := pool.Get().Mul(c.y, zInv)
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t.Mod(t, p)
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zInv2 := pool.Get().Mul(zInv, zInv)
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zInv2.Mod(zInv2, p)
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c.y.Mul(t, zInv2)
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c.y.Mod(c.y, p)
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t.Mul(c.x, zInv2)
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t.Mod(t, p)
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c.x.Set(t)
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c.z.SetInt64(1)
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c.t.SetInt64(1)
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pool.Put(zInv)
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pool.Put(t)
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pool.Put(zInv2)
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return c
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}
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func (c *curvePoint) Negative(a *curvePoint) {
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c.x.Set(a.x)
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c.y.Neg(a.y)
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c.z.Set(a.z)
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c.t.SetInt64(0)
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}
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