Direct manipulation of the blockchain by 'hacking the blockchain' or corrupting the blockchain, is virtually impossible.
The USE of blockchains ONLY accomplishes the secure storage of information, not just limited to 'crypto' uses, but of ANY information. The information IN the blockchain is available publicly; Over 50,000 servers in the bitcoin network can read and modify the blockchain (by adding new blocks). The encryption of the blockchain is NOT what provides the security.
Any person can set up a node to 'dump' most of the contents of the blockchain. If you have the 'account number' of a specific user, any user, you can get everybit of information about that account, a record of all transactions, and with both the public and private 'account numbers' (keys) one can freely move money(sic. make debit or credit entries) both IN and OUT of that account, ie. perform a valid transaction, ledger modification.
The blockchain is simply a ledger, a ledger that is consistent across the network, with a lot of checks and balances to make sure that the ledger is valid. It is audited continuously by thousands of auditors (ie servers), and inconsistencies are caught, rejected, and not allowed.
Hacking the blockchain is NOT the threat. Somebody getting your private key IS - and that key is kept in your digital wallet, cold storage unit, or other places.
THAT is the point of weakness. If your computer or cell phone is hacked, the hacker can 'steal' everything in your account on the 'blockchain' ledger.
ANYBODY can set up a 'transaction processing' node. All they need is your 'keys' to enter VALID TRANSACTIONS into the blockchain. None of the auditors will ever catch or stop a 'valid transaction'.
IOTW: The real onus of security lies upon each and every 'owner' of a 'crypto currency'. If they get hacked, expose their 'keys' by letting someone look at their cold storage media, get intercepted providing those keys to someone by making a transaction, or the someone who receives that key for making a transaction with them (eg. Crypto exchanges) steals, records, or exposes that key.
That is where the REAL security issues with 'crypto' lie.
Just like credit and debit cards - when anyone gets your account number, that is where you get exposed. They don't have to hack the 'credit card' company's ledgers, they simply submit valid transactions with that card number.
The ledgers are secure in both worlds, and difficult/impossible to mess with. But that wont stop the thieves at all.
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good ...
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