bug: btcec.PrivateKey.Sign creates invalid signatures for curves other than S256()

[benma]

func unhex(s string) []byte {
	b, err := hex.DecodeString(s)
	if err != nil {
		panic(err)
	}
	return b
}

privateKey, publicKey := btcec.PrivKeyFromBytes(
		elliptic.P256(),
		unhex("9b1a4d293a6eef1960d8afab5e58dd581b135152ec3399bde9268fa23051321b"),
)

Using privateKey.Sign() does not give a valid signature for the P256() curve.

Likely cause:

btcd/btcec/privkey.go

Line 62 in f3ec130

return signRFC6979(p, hash)

calls

btcd/btcec/signature.go

Line 422 in f3ec130

func signRFC6979(privateKey *PrivateKey, hash []byte) (*Signature, error) {

which assumes S256() here:

btcd/btcec/signature.go

Line 425 in f3ec130

N := S256().N

[cubancorona]

I apologize for my previous statement, now deleted, which was inappropriately premature. My apologies to every person who worked on this significant modification.