The secret sauce of Blockchain security is the way these blocks are linked to each other. Even if a malicious player tries to make changes to the past data, they will change that block's hash number. And since blocks are connected to each other through the previous block's hash number, it will change the entire blockchain completely. Apart from this, there are some other factors that make blockchains a safe haven for your data.   The data is protected by cryptography. That means only the intended recipient can decipher it.   Secondly, each participant has their own private and public key. No transactions can go through unless the private key is used to sign that transaction.   Signatures become invalid as soon as the records are altered.