func edwardsToMontgomeryX(outX, y *edwards25519.FieldElement) {
// We only need the x-coordinate of the curve25519 point, which I'll
// call u. The isomorphism is u=(y+1)/(1-y), since y=Y/Z, this gives
// u=(Y+Z)/(Z-Y). We know that Z=1, thus u=(Y+1)/(1-Y).
var oneMinusY edwards25519.FieldElement
edwards25519.FeOne(&oneMinusY)
edwards25519.FeSub(&oneMinusY, &oneMinusY, y)
edwards25519.FeInvert(&oneMinusY, &oneMinusY)
edwards25519.FeOne(outX)
edwards25519.FeAdd(outX, outX, y)
edwards25519.FeMul(outX, outX, &oneMinusY)
}
func montgomeryXToEdwardsY(out, x *edwards25519.FieldElement) {
var t, tt edwards25519.FieldElement
edwards25519.FeOne(&t)
edwards25519.FeAdd(&tt, x, &t) // u+1
edwards25519.FeInvert(&tt, &tt) // 1/(u+1)
edwards25519.FeSub(&t, x, &t) // u-1
edwards25519.FeMul(out, &tt, &t) // (u-1)/(u+1)
}
// ScalarBaseMult computes a curve25519 public key from a private key and also
// a uniform representative for that public key. Note that this function will
// fail and return false for about half of private keys.
// See http://elligator.cr.yp.to/elligator-20130828.pdf.
func ScalarBaseMult(publicKey, representative, privateKey *[32]byte) bool {
var maskedPrivateKey [32]byte
copy(maskedPrivateKey[:], privateKey[:])
maskedPrivateKey[0] &= 248
maskedPrivateKey[31] &= 127
maskedPrivateKey[31] |= 64
var A edwards25519.ExtendedGroupElement
edwards25519.GeScalarMultBase(&A, &maskedPrivateKey)
var inv1 edwards25519.FieldElement
edwards25519.FeSub(&inv1, &A.Z, &A.Y)
edwards25519.FeMul(&inv1, &inv1, &A.X)
edwards25519.FeInvert(&inv1, &inv1)
var t0, u edwards25519.FieldElement
edwards25519.FeMul(&u, &inv1, &A.X)
edwards25519.FeAdd(&t0, &A.Y, &A.Z)
edwards25519.FeMul(&u, &u, &t0)
var v edwards25519.FieldElement
edwards25519.FeMul(&v, &t0, &inv1)
edwards25519.FeMul(&v, &v, &A.Z)
edwards25519.FeMul(&v, &v, &sqrtMinusA)
var b edwards25519.FieldElement
edwards25519.FeAdd(&b, &u, &edwards25519.A)
var c, b3, b8 edwards25519.FieldElement
edwards25519.FeSquare(&b3, &b) // 2
edwards25519.FeMul(&b3, &b3, &b) // 3
edwards25519.FeSquare(&c, &b3) // 6
edwards25519.FeMul(&c, &c, &b) // 7
edwards25519.FeMul(&b8, &c, &b) // 8
edwards25519.FeMul(&c, &c, &u)
q58(&c, &c)
var chi edwards25519.FieldElement
edwards25519.FeSquare(&chi, &c)
edwards25519.FeSquare(&chi, &chi)
edwards25519.FeSquare(&t0, &u)
edwards25519.FeMul(&chi, &chi, &t0)
edwards25519.FeSquare(&t0, &b) // 2
edwards25519.FeMul(&t0, &t0, &b) // 3
edwards25519.FeSquare(&t0, &t0) // 6
edwards25519.FeMul(&t0, &t0, &b) // 7
edwards25519.FeSquare(&t0, &t0) // 14
edwards25519.FeMul(&chi, &chi, &t0)
edwards25519.FeNeg(&chi, &chi)
var chiBytes [32]byte
edwards25519.FeToBytes(&chiBytes, &chi)
// chi[1] is either 0 or 0xff
if chiBytes[1] == 0xff {
return false
}
// Calculate r1 = sqrt(-u/(2*(u+A)))
var r1 edwards25519.FieldElement
edwards25519.FeMul(&r1, &c, &u)
edwards25519.FeMul(&r1, &r1, &b3)
edwards25519.FeMul(&r1, &r1, &sqrtMinusHalf)
var maybeSqrtM1 edwards25519.FieldElement
edwards25519.FeSquare(&t0, &r1)
edwards25519.FeMul(&t0, &t0, &b)
edwards25519.FeAdd(&t0, &t0, &t0)
edwards25519.FeAdd(&t0, &t0, &u)
edwards25519.FeOne(&maybeSqrtM1)
edwards25519.FeCMove(&maybeSqrtM1, &edwards25519.SqrtM1, edwards25519.FeIsNonZero(&t0))
edwards25519.FeMul(&r1, &r1, &maybeSqrtM1)
// Calculate r = sqrt(-(u+A)/(2u))
var r edwards25519.FieldElement
edwards25519.FeSquare(&t0, &c) // 2
edwards25519.FeMul(&t0, &t0, &c) // 3
edwards25519.FeSquare(&t0, &t0) // 6
edwards25519.FeMul(&r, &t0, &c) // 7
edwards25519.FeSquare(&t0, &u) // 2
edwards25519.FeMul(&t0, &t0, &u) // 3
edwards25519.FeMul(&r, &r, &t0)
edwards25519.FeSquare(&t0, &b8) // 16
edwards25519.FeMul(&t0, &t0, &b8) // 24
edwards25519.FeMul(&t0, &t0, &b) // 25
edwards25519.FeMul(&r, &r, &t0)
edwards25519.FeMul(&r, &r, &sqrtMinusHalf)
edwards25519.FeSquare(&t0, &r)
edwards25519.FeMul(&t0, &t0, &u)
edwards25519.FeAdd(&t0, &t0, &t0)
edwards25519.FeAdd(&t0, &t0, &b)
edwards25519.FeOne(&maybeSqrtM1)
edwards25519.FeCMove(&maybeSqrtM1, &edwards25519.SqrtM1, edwards25519.FeIsNonZero(&t0))
edwards25519.FeMul(&r, &r, &maybeSqrtM1)
var vBytes [32]byte
edwards25519.FeToBytes(&vBytes, &v)
vInSquareRootImage := feBytesLE(&vBytes, &halfQMinus1Bytes)
edwards25519.FeCMove(&r, &r1, vInSquareRootImage)
edwards25519.FeToBytes(publicKey, &u)
edwards25519.FeToBytes(representative, &r)
return true
}