When we use purely peer-to-peer version of electronic cash, we must not interfere from third-party.
But In real world We have to interfere from third-party bacause of double-spending. (someone have to check sent or not)
Satoshi Nakamoto Point out this Problem.
"How we can prevent the double-spending problem without third-party?"
And he proposes a solution. "Using a peer-to-peer network"
The network transactions will be hasing, and put into a chain of hash-based proof-of-work.
Like this, all transactions saved.
So, If someone wants to hack this data, then need As long as a majority of CPU power
Every message and transaction historys are broadcast on a best effort basis.
The nodes that saved transctions data can leave whenever, can join whenever.
If they rejoin, the longest proof-of-work chain will proof of empty history.
