// makeHmac:
//	non-exportable helper to do SHA256 HMAC
func makeHmac(data string, key string) string {
	byteKey, _ := hex.DecodeString(key)
	byteData := []byte(data)
	sig := hmac.New(sha256.New, byteKey)
	sig.Write([]byte(byteData))
	return base64.StdEncoding.EncodeToString(sig.Sum(nil))
}