rsa.go
func (sk *RsaPrivateKey) Sign(message []byte) ([]byte, error) {
	hashed := sha256.Sum256(message)
	return rsa.SignPKCS1v15(rand.Reader, sk.sk, crypto.SHA256, hashed[:])
}
func (sk *RsaPrivateKey) Sign(message []byte) ([]byte, error) {
	hashed := sha256.Sum256(message)
	return rsa.SignPKCS1v15(rand.Reader, sk.sk, crypto.SHA256, hashed[:])
}
// Sign returns a signature of the input data