FromECDSAPub: Use curve from input pub instead of fixed S256()?

[matthiasgeihs]

go-ethereum/crypto/crypto.go

Lines 175 to 180 in ec2ec2d

	func FromECDSAPub(pub *ecdsa.PublicKey) []byte {
	if pub == nil || pub.X == nil || pub.Y == nil {
	return nil
	}
	return elliptic.Marshal(S256(), pub.X, pub.Y)
	}

pub.Curve may be different from S256(). However, here marshaling is fixed to S256().

This may be problematic if the function is used externally with different curves.

Any thoughts?

[fjl]

We do not support any elliptic curves beyond secp256k1 in package crypto. Probably this information should be added to the documentation.