3. Transactions

Let's get down to the nitty-gritty of how bitcoin works. We're particularly interested in bitcoin because it's the main cryptocurrency, far more important than any of the others, and because it's the best way to understand what a blockchain is, since the other blockchains are simply variants of bitcoin's, and therefore require a precise understanding.

When Alice wants to make a payment in bitcoins to Bernard (in exchange for a book, for example), Alice's computer will perform a series of operations. These operations are carried out automatically by the software (wallet) she has installed on her computer or smartphone, which manages communications between her machine and the bitcoin peer-to-peer network. The latter will accept or refuse the transaction, checking that it complies with the protocol and, in particular, that the account debited actually holds the bitcoins...