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Merge pull request #1 from nspcc-dev/use_stdlib_formula_for_decompress_point

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Refactor decompress Y point
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Evgeniy Kulikov 2019-10-24 12:56:25 +03:00 committed by GitHub
commit 40d65daa2e
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1 changed files with 8 additions and 12 deletions
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20
ecdsa.go
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@ -70,26 +70,22 @@ func unmarshalXY(data []byte) (x *big.Int, y *big.Int) {
return
}
// decompressPoints using formula y^2 = x^3 + ax + b mod p
// decompressPoints using formula y² = x³ - 3x + b
// crypto/elliptic/elliptic.go:55
func decompressPoints(x *big.Int, yBit uint) (*big.Int, *big.Int) {
params := curve.Params()
// x^3 mod P
x3 := new(big.Int).Exp(x, new(big.Int).SetInt64(3), params.P)
x3 := new(big.Int).Mul(x, x)
x3.Mul(x3, x)
// a * x mod P
ax := new(big.Int).Mul(x, new(big.Int).SetInt64(-3))
ax.Mod(ax, params.P)
threeX := new(big.Int).Lsh(x, 1)
threeX.Add(threeX, x)
// x^3 + a * x mod P
x3.Add(x3, ax)
x3.Mod(x3, params.P)
// x^3 + a * x + b mod P
x3.Sub(x3, threeX)
x3.Add(x3, params.B)
x3.Mod(x3, params.P)
// y = sqrt(x^3 + ax + b mod p) mod P
// y = √(x³ - 3x + b) mod p
y := new(big.Int).ModSqrt(x3, params.P)
// big.Int.Jacobi(a, b) can return nil