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edwards25519: make SqrtRatio slightly more efficient

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Inspired by https://twitter.com/jxxf/status/1339930877497925632.
This commit is contained in:
Filippo Valsorda 2020-12-18 15:04:48 +01:00
parent 700f4f4a67
commit c5477978af
1 changed files with 3 additions and 5 deletions
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8
fe.go
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@ -393,12 +393,10 @@ var sqrtM1 = &fieldElement{1718705420411056, 234908883556509,
func (r *fieldElement) SqrtRatio(u, v *fieldElement) (rr *fieldElement, wasSquare int) {
var a, b fieldElement
v3 := a.Multiply(a.Square(v), v) // v^3 = v^2 * v
v7 := b.Multiply(b.Square(v3), v) // v^7 = (v^3)^2 * v
// r = (u * v3) * (u * v7)^((p-5)/8)
uv3 := a.Multiply(u, v3) // (u * v3)
uv7 := b.Multiply(u, v7) // (u * v7)
v2 := a.Square(v)
uv3 := b.Multiply(u, b.Multiply(v2, v))
uv7 := a.Multiply(uv3, a.Square(v2))
r.Multiply(uv3, r.Pow22523(uv7))
check := a.Multiply(v, a.Square(r)) // check = v * r^2