Thousands of computers around the world keep a copy of the same ledger, and none of them is in charge. So how do they all agree on which transactions are valid and in what order? The answer is a consensus mechanism: a set of rules that lets independent nodes settle on a single shared history, even when some of them are faulty or trying to cheat.
You'll start with the Byzantine Generals Problem, the classic way to describe why agreement is hard in a distributed system. Then you'll study Proof of Work, the mining-based approach used by Bitcoin, and Proof of Stake, where validators put their own coins at risk. The module closes with other designs and the trade-offs between security, speed and decentralization, including what a 51% attack really means.
Once you know how blocks get agreed on, you'll be ready for the next module, Bitcoin & the UTXO Model, which looks at how Bitcoin applies these ideas in practice.
Why a network with no central authority needs a way to agree on one version of the truth, and how the Byzantine Generals Problem describes the challenge when some participants may be dishonest.
How miners compete to solve a hash puzzle, why the longest valid chain wins, and how the cost of doing work makes cheating expensive.
How validators lock up coins as a stake instead of burning energy, how they are chosen to propose blocks, and what happens to those who misbehave.
A look at Byzantine fault tolerant and delegated designs, the 51% attack, and how each mechanism balances security, speed and decentralization.