How secure and private are blockchains?

Both public and private blockchains provide a certain level of out-of-the-box security for your data. The consensus mechanism is the main driver behind the security and correctness of the blockchain. With a public blockchain, all users need to abide by the consensus algorithm that verifies all transactions, and when doing so they need to prove that they made a sufficient amount of effort by solving a mathematical problem. In many cases, the first user to solve the problem, or who is chosen to solve the problem, gets rewarded. Each new solution then forms the basis for the next block of transactions to be solved. It becomes almost impossible to manipulate data that is confirmed in an earlier block, since it directly affects the blocks that were created after that block. A private blockchain is even more secure, since you need to have secured...