I read somewhere that Private Blockchain has the risk of 51% attack, while Public Blockchain is pretty safe. So, my question is - can we protect private chain with public chain? Can we go around it the following way - Save block height and block hash, every 100 block. We save 1,101,201... block height and hash on public chain, and we verify it. But it still has risk, before 101 block is protected, 2-99 blocks still have risk. Is there any better way to solve it?