code: public static byte[] sha3(byte[] input, int start, int length) {
          Keccak256 digest =  new Keccak256();
          digest.update(input, start, length);
          return digest.digest();
      }
docs: hashing chunk of the data
  @param input - data for hash
  @param start - start of hashing chunk
  @param length - length of hashing chunk
  @return - keccak hash of the chunk