// extendedToBigAffine converts projective x, y, and z field elements into
// affine x and y coordinates, and returns whether or not the x value
// returned is negative.
func (curve *TwistedEdwardsCurve) extendedToBigAffine(xi, yi,
zi *edwards25519.FieldElement) (*big.Int, *big.Int, bool) {
var recip, x, y edwards25519.FieldElement
// Normalize to Z=1.
edwards25519.FeInvert(&recip, zi)
edwards25519.FeMul(&x, xi, &recip)
edwards25519.FeMul(&y, yi, &recip)
isNegative := edwards25519.FeIsNegative(&x) == 1
return FieldElementToBigInt(&x), FieldElementToBigInt(&y), isNegative
}
// BigIntPointToEncodedBytes converts an affine point to a compressed
// 32 byte integer representation.
func BigIntPointToEncodedBytes(x *big.Int, y *big.Int) *[32]byte {
s := BigIntToEncodedBytes(y)
xB := BigIntToEncodedBytes(x)
xFE := new(edwards25519.FieldElement)
edwards25519.FeFromBytes(xFE, xB)
isNegative := edwards25519.FeIsNegative(xFE) == 1
if isNegative {
s[31] |= (1 << 7)
} else {
s[31] &^= (1 << 7)
}
return s
}
// RecoverXFieldElement recovers the X value for some Y value, for a coordinate
// on the Ed25519 curve given as a field element. Y value. Probably the fastest
// way to get your respective X from Y.
func (curve *TwistedEdwardsCurve) RecoverXFieldElement(xIsNeg bool,
y *edwards25519.FieldElement) *edwards25519.FieldElement {
// (y^2 - 1)
l := new(edwards25519.FieldElement)
edwards25519.FeSquare(l, y)
edwards25519.FeSub(l, l, &feOne)
// inv(d*y^2+1)
r := new(edwards25519.FieldElement)
edwards25519.FeSquare(r, y)
edwards25519.FeMul(r, r, &fed)
edwards25519.FeAdd(r, r, &feOne)
edwards25519.FeInvert(r, r)
x2 := new(edwards25519.FieldElement)
edwards25519.FeMul(x2, r, l)
// Get a big int so we can do the exponentiation.
x2Big := FieldElementToBigInt(x2)
// x = exp(x^2,(P+3)/8, P)
qp3 := new(big.Int).Add(curve.P, three)
qp3.Div(qp3, eight) // /= curve.H
xBig := new(big.Int).Exp(x2Big, qp3, curve.P)
// Convert back to a field element and do
// the rest.
x := BigIntToFieldElement(xBig)
// check (x^2 - x2) % q != 0
x22 := new(edwards25519.FieldElement)
edwards25519.FeSquare(x22, x)
xsub := new(edwards25519.FieldElement)
edwards25519.FeSub(xsub, x22, x2)
xsubBig := FieldElementToBigInt(xsub)
xsubBig.Mod(xsubBig, curve.P)
if xsubBig.Cmp(zero) != 0 {
xi := new(edwards25519.FieldElement)
edwards25519.FeMul(xi, x, &feI)
xiModBig := FieldElementToBigInt(xi)
xiModBig.Mod(xiModBig, curve.P)
xiMod := BigIntToFieldElement(xiModBig)
x = xiMod
}
xBig = FieldElementToBigInt(x)
xmod2 := new(big.Int).Mod(xBig, two)
if xmod2.Cmp(zero) != 0 {
// TODO replace this with FeSub
xBig.Sub(curve.P, xBig)
x = BigIntToFieldElement(xBig)
}
// We got the wrong x, negate it to get the right one.
isNegative := edwards25519.FeIsNegative(x) == 1
if xIsNeg != isNegative {
edwards25519.FeNeg(x, x)
}
return x
}
