code: func SaveECDSA(file string, key *ecdsa.PrivateKey) error {
  	k := hex.EncodeToString(FromECDSA(key))
  	return ioutil.WriteFile(file, []byte(k), 0600)
  }
docs: // SaveECDSA saves a secp256k1 private key to the given file with
  // restrictive permissions. The key data is saved hex-encoded.