docs: public void sign(PrivateKey key, String digest) throws NoSuchAlgorithmException, NoSuchPaddingException,
          InvalidKeyException, BadPaddingException, IllegalBlockSizeException {
          add(Feature.Create.stringFeature(DIGEST_KEY, digest));
          final var digester = MessageDigest.getInstance(digest);
          final var ser = unsigned();
          final var digestValue = digester.digest(ser);
          final var cipher = Cipher.getInstance(key.getAlgorithm());
          cipher.init(Cipher.ENCRYPT_MODE, key);
          final var signature = cipher.doFinal(digestValue);
          add(signature);
      }