2. Bitcoin's robustness

Bitcoins do not exist materially, but only on the peer-to-peer network. They are the result of a consensus between users who, thanks to the information present on the network and which everyone can consult and control, indicate which sums of money are in their accounts. All accounts are stored in a file – the blockchain – accessible to all. More precisely, the blockchain contains all transactions (validated by page or "block") since the beginning of bitcoin, from which we can deduce the bitcoin content of each account.

Only the leader of a pool of miners controls the correction of transactions and their registration on the blockchain. He must have downloaded the blockchain (no mean feat!), of which he keeps a copy. He updates it every ten minutes by adding a new page (block). The other miners in a pool just work to increase its hash computing power (§