transaction log can't be changed after the fact. if anyone tries to modify the data blog that's visible in every copy of the ledger. and any participant can check it at any time. change technology is designed to be secure transparent and tamper proof. of the history if you don't change began with bitcoin. these days crypto currency such as pick quine ripple and ether are accepted as payment in many shops and restaurants. and on yeah things of every transaction is stored by every computer in the network in an encrypted data block and then link to the chain. none of this requires a bank which is why crypto currency and block chain technology could help break up the banking monopoly. but not all crypto currency is based. in technology.