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extra: explain the absence of SetMontgomeryBytes (#30)

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Filippo Valsorda 2022-07-26 10:28:42 -04:00 committed by GitHub
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extra.go
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@ -79,6 +79,12 @@ func isOnCurve(X, Y, Z, T *field.Element) bool {
// Note that BytesMontgomery only encodes the u-coordinate, so v and -v encode
// to the same value. If v is the identity point, BytesMontgomery returns 32
// zero bytes, analogously to the X25519 function.
//
// The lack of an inverse operation (such as SetMontgomeryBytes) is deliberate:
// while every valid edwards25519 point has a unique u-coordinate Montgomery
// encoding, X25519 accepts inputs on the quadratic twist, which don't correspond
// to any edwards25519 point, and every other X25519 input corresponds to two
// edwards25519 points.
func (v *Point) BytesMontgomery() []byte {
// This function is outlined to make the allocations inline in the caller
// rather than happen on the heap.