What is a blockchain checkpoint?

A **blockchain checkpoint** is a reference point in a blockchain’s history that is treated as a trusted, immutable snapshot of the ledger. ### Why checkpoints are used - **Security:** By hard‑coding a known good block hash (or block height) into the client software, nodes can quickly verify that they are on the correct chain and reject any alternative chain that diverges before that point. This helps protect against attacks such as long‑range forks or malicious re‑organizations. - **Efficiency:** When a node syncs from scratch, it can start verification from the checkpoint instead of re‑checking every transaction from genesis, dramatically speeding up the initial sync. - **Consensus stability:** Checkpoints act as “anchors” that make it computationally infeasible for an attacker to rewrite history prior to the checkpoint without controlling a majority of the network’s hash power. ### How they work 1. **Selection:** Developers or the community choose a block that is deep enough in the chain (often many thousands of blocks old) and that is widely accepted as valid. 2. **Embedding:** The block’s hash (or height) is hard‑coded into the client’s source code or configuration. 3. **Verification:** During normal operation, a node checks that the block at that height matches the stored hash. If it doesn’t, the node will reject the chain as invalid. ### Examples - **Bitcoin:** Early Bitcoin clients included checkpoints at various block heights (e.g., block 111,111, 333,333, etc.) to speed up syncing and guard against attacks. - **Ethereum:** Some Ethereum clients use “trusted block” configurations for similar purposes, especially in light‑client or archival‑node setups. ### Limitations - **Centralization risk:** Hard‑coding checkpoints introduces a small trust assumption in the developers who choose them. - **Staleness:** As the blockchain grows, checkpoints need to be updated periodically; otherwise they become less useful for new nodes. In summary, a blockchain checkpoint is a pre‑agreed, trusted block that serves as a security and performance anchor, allowing nodes to verify they are on the correct chain without re‑processing the entire history.