ckem: pass xof to elliptic.GenerateKey directly

[bwesterb]

No description provided.

[Lekensteyn]

Change looks reasonable, perhaps the commit message could include a further motivation.

[Lekensteyn]

Is this a bugfix? Previously, only enough randomness was provided for exactly one "private key". After this change, it could sample another key if the first secret was invalid:

func GenerateKey(curve Curve, rand io.Reader) (priv []byte, x, y *big.Int, err error) {
	N := curve.Params().N
	bitSize := N.BitLen()
	byteLen := (bitSize + 7) / 8
	priv = make([]byte, byteLen)

	for x == nil {
		_, err = io.ReadFull(rand, priv)
// ...
		// If the scalar is out of range, sample another random number.
		if new(big.Int).SetBytes(priv).Cmp(N) >= 0 {
			continue
		}

		x, y = curve.ScalarBaseMult(priv)
	}

For reference, sch.PrivateKeySize() ends up with the same size:

func (sch *cScheme) scSize() int {
	return (sch.curve.Params().N.BitLen() + 7) / 8
}
// ...
func (sch *cScheme) PrivateKeySize() int {
	return sch.scSize()
}