docs: public byte[] generateSignature(@NonNull byte[] content) {
          try {
              Mac mac = Mac.getInstance(HASH_ALGORITHM);
              mac.init(secretKeySpec);
              return mac.doFinal(content);
          } catch (NoSuchAlgorithmException | InvalidKeyException e) {
              // "HmacSHA256" is always supported in Java 8 platform.
              //   (see https://docs.oracle.com/javase/8/docs/api/javax/crypto/Mac.html)
              // All valid-SecretKeySpec-instance are not InvalidKey.
              //   (because the key for HmacSHA256 can be of any length. see RFC2104)
              throw new IllegalStateException(e);
          }
      }