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package bn256
import (
"math/big"
)
var half = new(big.Int).Rsh(Order, 1)
var curveLattice = &lattice{
vectors: [][]*big.Int{
{bigFromBase10("147946756881789319000765030803803410728"), bigFromBase10("147946756881789319010696353538189108491")},
{bigFromBase10("147946756881789319020627676272574806254"), bigFromBase10("-147946756881789318990833708069417712965")},
},
inverse: []*big.Int{
bigFromBase10("147946756881789318990833708069417712965"),
bigFromBase10("147946756881789319010696353538189108491"),
},
det: bigFromBase10("43776485743678550444492811490514550177096728800832068687396408373151616991234"),
}
var targetLattice = &lattice{
vectors: [][]*big.Int{
{bigFromBase10("9931322734385697761"), bigFromBase10("9931322734385697761"), bigFromBase10("9931322734385697763"), bigFromBase10("9931322734385697764")},
{bigFromBase10("4965661367192848881"), bigFromBase10("4965661367192848881"), bigFromBase10("4965661367192848882"), bigFromBase10("-9931322734385697762")},
{bigFromBase10("-9931322734385697762"), bigFromBase10("-4965661367192848881"), bigFromBase10("4965661367192848881"), bigFromBase10("-4965661367192848882")},
{bigFromBase10("9931322734385697763"), bigFromBase10("-4965661367192848881"), bigFromBase10("-4965661367192848881"), bigFromBase10("-4965661367192848881")},
},
inverse: []*big.Int{
bigFromBase10("734653495049373973658254490726798021314063399421879442165"),
bigFromBase10("147946756881789319000765030803803410728"),
bigFromBase10("-147946756881789319005730692170996259609"),
bigFromBase10("1469306990098747947464455738335385361643788813749140841702"),
},
det: new(big.Int).Set(Order),
}
type lattice struct {
vectors [][]*big.Int
inverse []*big.Int
det *big.Int
}
// decompose takes a scalar mod Order as input and finds a short, positive decomposition of it wrt to the lattice basis.
func (l *lattice) decompose(k *big.Int) []*big.Int {
n := len(l.inverse)
// Calculate closest vector in lattice to <k,0,0,...> with Babai's rounding.
c := make([]*big.Int, n)
for i := 0; i < n; i++ {
c[i] = new(big.Int).Mul(k, l.inverse[i])
round(c[i], l.det)
}
// Transform vectors according to c and subtract <k,0,0,...>.
out := make([]*big.Int, n)
temp := new(big.Int)
for i := 0; i < n; i++ {
out[i] = new(big.Int)
for j := 0; j < n; j++ {
temp.Mul(c[j], l.vectors[j][i])
out[i].Add(out[i], temp)
}
out[i].Neg(out[i])
out[i].Add(out[i], l.vectors[0][i]).Add(out[i], l.vectors[0][i])
}
out[0].Add(out[0], k)
return out
}
func (l *lattice) Precompute(add func(i, j uint)) {
n := uint(len(l.vectors))
total := uint(1) << n
for i := uint(0); i < n; i++ {
for j := uint(0); j < total; j++ {
if (j>>i)&1 == 1 {
add(i, j)
}
}
}
}
func (l *lattice) Multi(scalar *big.Int) []uint8 {
decomp := l.decompose(scalar)
maxLen := 0
for _, x := range decomp {
if x.BitLen() > maxLen {
maxLen = x.BitLen()
}
}
out := make([]uint8, maxLen)
for j, x := range decomp {
for i := 0; i < maxLen; i++ {
out[i] += uint8(x.Bit(i)) << uint(j)
}
}
return out
}
// round sets num to num/denom rounded to the nearest integer.
func round(num, denom *big.Int) {
r := new(big.Int)
num.DivMod(num, denom, r)
if r.Cmp(half) == 1 {
num.Add(num, big.NewInt(1))
}
}
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