Feather announced that it would be using a system called advanced checkpointing in an effort to guard its blockchain. The announcement said the mechanism could protect against a 51% attack.
Checkpointing works like this: Individual blocks along the chain are recorded and stored as code. Later, these checkpoints can be matched against the blockchain itself to determine whether it was been rewritten retroactively.
This all requires regular updating, which leaves a client vulnerable if it doesn’t do this. Feathercoin, to solve that problem, has separated the checkpoint record from the client software. Instead, there is a master node that will publish checkpoints against which clients can check.
Feathercoin has had at least one 51% attack, but its old checkpointing system minimized the damage.
Ideally, advanced checkpointing could be distributed to ensure the decentralization that is at the heart of cryptocurrencies. Instead, even with Bitcoin, nodes in one country record some blocks and nodes in another country record other blocks. In the event of an attack, an element of trust will come into play to determine the original constitution of the blockchain.
A 51% attack, which checkpointing is designed to protect against, is possible if a single person or organization contributed to the majority of the network’s mining hashrate, at which point it would have control over the network and could change the blockchain at will.
While that sounds catastrophic, the most a 51% attack could achieve is to prevent some transactions, reverse some transactions and prevent any miners from getting any blocks for a period of time.