(defn hmacSHA256
  (memoize (fn [payload secret]
             (let [mac (Mac/getInstance "HMACSHA256")
                   secretKey (SecretKeySpec. (.getBytes secret) (.getAlgorithm mac))
                   byteResult (-> (doto mac
                                    (.init secretKey))
                                  (.doFinal (.getBytes payload)))
                   stringResult (->> byteResult
                                     (map #(format "%x" %))
                                     (apply str))]
               stringResult))))