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220 lines
3.7 KiB
Go
220 lines
3.7 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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// For details of the algorithms used, see "Multiplication and Squaring on
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// Pairing-Friendly Fields, Devegili et al.
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// http://eprint.iacr.org/2006/471.pdf.
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import (
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"math/big"
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)
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// gfP2 implements a field of size p² as a quadratic extension of the base
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// field where i²=-1.
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type gfP2 struct {
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x, y *big.Int // value is xi+y.
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}
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func newGFp2(pool *bnPool) *gfP2 {
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return &gfP2{pool.Get(), pool.Get()}
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}
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func (e *gfP2) String() string {
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x := new(big.Int).Mod(e.x, p)
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y := new(big.Int).Mod(e.y, p)
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return "(" + x.String() + "," + y.String() + ")"
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}
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func (e *gfP2) Put(pool *bnPool) {
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pool.Put(e.x)
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pool.Put(e.y)
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}
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func (e *gfP2) Set(a *gfP2) *gfP2 {
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e.x.Set(a.x)
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e.y.Set(a.y)
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return e
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}
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func (e *gfP2) SetZero() *gfP2 {
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e.x.SetInt64(0)
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e.y.SetInt64(0)
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return e
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}
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func (e *gfP2) SetOne() *gfP2 {
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e.x.SetInt64(0)
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e.y.SetInt64(1)
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return e
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}
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func (e *gfP2) Minimal() {
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if e.x.Sign() < 0 || e.x.Cmp(p) >= 0 {
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e.x.Mod(e.x, p)
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}
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if e.y.Sign() < 0 || e.y.Cmp(p) >= 0 {
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e.y.Mod(e.y, p)
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}
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}
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func (e *gfP2) IsZero() bool {
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return e.x.Sign() == 0 && e.y.Sign() == 0
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}
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func (e *gfP2) IsOne() bool {
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if e.x.Sign() != 0 {
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return false
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}
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words := e.y.Bits()
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return len(words) == 1 && words[0] == 1
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}
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func (e *gfP2) Conjugate(a *gfP2) *gfP2 {
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e.y.Set(a.y)
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e.x.Neg(a.x)
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return e
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}
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func (e *gfP2) Negative(a *gfP2) *gfP2 {
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e.x.Neg(a.x)
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e.y.Neg(a.y)
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return e
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}
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func (e *gfP2) Add(a, b *gfP2) *gfP2 {
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e.x.Add(a.x, b.x)
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e.y.Add(a.y, b.y)
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return e
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}
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func (e *gfP2) Sub(a, b *gfP2) *gfP2 {
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e.x.Sub(a.x, b.x)
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e.y.Sub(a.y, b.y)
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return e
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}
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func (e *gfP2) Double(a *gfP2) *gfP2 {
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e.x.Lsh(a.x, 1)
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e.y.Lsh(a.y, 1)
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return e
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}
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func (c *gfP2) Exp(a *gfP2, power *big.Int, pool *bnPool) *gfP2 {
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sum := newGFp2(pool)
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sum.SetOne()
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t := newGFp2(pool)
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for i := power.BitLen() - 1; i >= 0; i-- {
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t.Square(sum, pool)
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if power.Bit(i) != 0 {
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sum.Mul(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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// See "Multiplication and Squaring in Pairing-Friendly Fields",
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// http://eprint.iacr.org/2006/471.pdf
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func (e *gfP2) Mul(a, b *gfP2, pool *bnPool) *gfP2 {
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tx := pool.Get().Mul(a.x, b.y)
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t := pool.Get().Mul(b.x, a.y)
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tx.Add(tx, t)
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tx.Mod(tx, p)
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ty := pool.Get().Mul(a.y, b.y)
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t.Mul(a.x, b.x)
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ty.Sub(ty, t)
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e.y.Mod(ty, p)
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e.x.Set(tx)
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pool.Put(tx)
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pool.Put(ty)
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pool.Put(t)
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return e
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}
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func (e *gfP2) MulScalar(a *gfP2, b *big.Int) *gfP2 {
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e.x.Mul(a.x, b)
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e.y.Mul(a.y, b)
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return e
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}
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// MulXi sets e=ξa where ξ=i+3 and then returns e.
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func (e *gfP2) MulXi(a *gfP2, pool *bnPool) *gfP2 {
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// (xi+y)(i+3) = (3x+y)i+(3y-x)
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tx := pool.Get().Lsh(a.x, 1)
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tx.Add(tx, a.x)
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tx.Add(tx, a.y)
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ty := pool.Get().Lsh(a.y, 1)
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ty.Add(ty, a.y)
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ty.Sub(ty, a.x)
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e.x.Set(tx)
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e.y.Set(ty)
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pool.Put(tx)
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pool.Put(ty)
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return e
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}
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func (e *gfP2) Square(a *gfP2, pool *bnPool) *gfP2 {
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// Complex squaring algorithm:
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// (xi+b)² = (x+y)(y-x) + 2*i*x*y
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t1 := pool.Get().Sub(a.y, a.x)
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t2 := pool.Get().Add(a.x, a.y)
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ty := pool.Get().Mul(t1, t2)
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ty.Mod(ty, p)
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t1.Mul(a.x, a.y)
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t1.Lsh(t1, 1)
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e.x.Mod(t1, p)
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e.y.Set(ty)
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pool.Put(t1)
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pool.Put(t2)
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pool.Put(ty)
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return e
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}
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func (e *gfP2) Invert(a *gfP2, pool *bnPool) *gfP2 {
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// See "Implementing cryptographic pairings", M. Scott, section 3.2.
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// ftp://136.206.11.249/pub/crypto/pairings.pdf
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t := pool.Get()
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t.Mul(a.y, a.y)
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t2 := pool.Get()
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t2.Mul(a.x, a.x)
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t.Add(t, t2)
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inv := pool.Get()
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inv.ModInverse(t, p)
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e.x.Neg(a.x)
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e.x.Mul(e.x, inv)
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e.x.Mod(e.x, p)
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e.y.Mul(a.y, inv)
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e.y.Mod(e.y, p)
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pool.Put(t)
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pool.Put(t2)
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pool.Put(inv)
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return e
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}
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