code: func signingKey(clientSecret string, timestamp string) []byte {
  	h := hmac.New(sha256.New, []byte(clientSecret))
  	h.Write([]byte(timestamp))
  	key := make([]byte, base64.StdEncoding.EncodedLen(32))
  	base64.StdEncoding.Encode(key, h.Sum(nil))
  	return key
  }
docs: // signingKey makes a signing key by HMAC'ing the timestamp
  // using a client secret as the key.