Bitmex API key signature generation in Clojure

bitmex.clj

;;; www.Quanto.ga
;;;;;;;;;;;;;;;;;

;; Test from https://www.bitmex.com/app/apiKeysUsage#Full-sample-calculation
(letfn [(sign [^String api-secret ^String s]
          (let [h (com.google.common.hash.Hashing/hmacSha256 (.getBytes api-secret))]
            (.toString (.hashString h s com.google.common.base.Charsets/UTF_8))))]
  (let [apiKey "LAqUlngMIQkIUjXMUreyu3qn"
        apiSecret "chNOOS4KvNXR_Xq4k4c9qsfoKWvnDecLATCRlcBwyKDYnWgO"]
    ;; Simple GET
    (let [verb "GET"
          path "/api/v1/instrument"
          expires 1518064236 ;; 2018-02-08T04:30:36Z
          data ""]
      (assert (= "c7682d435d0cfe87c16098df34ef2eb5a549d4c5a3c2b1f0f77b8af73423bf00"
                 (sign apiSecret (str verb path expires data))))

      ;; POST
      (let [verb "POST"
            path "/api/v1/order"
            expires 1518064238 ;; 2018-02-08T04:30:38Z
            data "{\"symbol\":\"XBTM15\",\"price\":219.0,\"clOrdID\":\"mm_bitmex_1a/oemUeQ4CAJZgP3fjHsA\",\"orderQty\":98}"]
        (assert (= "1749cd2ccae4aa49048ae09f0b95110cee706e0944e6a14ad0b3a8cb45bd336b"
                   (sign apiSecret (str verb path expires data))))))))