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A **hard fork** is a radical, non‑backward‑compatible upgrade to a blockchain’s protocol. When a hard fork occurs, the new set of consensus rules diverges from the old ones, so nodes that continue running the old software will reject blocks created under the new rules (and vice‑versa). As a result, the blockchain splits into two separate chains: | Aspect | Pre‑fork chain | Post‑fork chain | |--------|----------------|-----------------| | Consensus rules | Old rules | New rules (e.g., block s

A **51 % attack** (also called a majority attack) is a scenario in which a single entity—or a coalition of entities—gains control of more than half of the total mining (or staking) power of a proof‑of‑work (PoW) or proof‑of‑stake (PoS) blockchain. ### Why > 50 % matters - **Block creation authority**: With > 50 % of the network’s hash rate (PoW) or stake (PoS), the attacker can reliably produce the next block faster than the rest of the network combined. - **Chain re‑org capability**: The attac