The leaks · 51 / 86 · 2 min
51% attacks and chain halts
A chain can fail as a chain. An attacker with enough mining power or stake can rewrite recent history. A small validator set can halt, or be ordered to halt.
A small validator set can halt, or be told to halt. Decentralised is a measurement.
A worked case
A chain is described as unstoppable. Most of the stake sits with a handful of companies. Those companies go offline together, or receive an order. Transfers wait. The slogan did not restart the chain.
Rewriting the recent past
Payments that looked confirmed can be replaced if the attacker builds a heavier history. Exchanges wait for many confirmations because of this. A merchant who accepts one confirmation on a small chain is exposed.
A halt
If validators stop, your transfer waits. If a few companies run the validators, a court or a government can speak to those companies. Decentralised is a measurement, not a sticker.
The check is who can stop the chain, not the adjective in the marketing.
Enough of the keys, or enough of the operators
A 51 percent attack is the old bitcoin fear: someone with most of the mining power rewrites recent history and double-spends. It has happened on smaller proof-of-work chains. Ethereum Classic suffered repeated reorganisations in 2019 and 2020. The coins did not become fake. The recent payments became optional for the attacker. Bitcoin's size is the defence, and it is a measurement of cost, not a law of nature.
A halt is the other failure. Solana stopped producing blocks for hours on more than one occasion, and the foundation and the press described restarts. Ronin's bridge did not need a majority of a global mining network. It needed five of nine keys. Decentralised is a count. If you cannot do the count, the homepage has done it for you, which means you do not have a count.
Your transaction during a halt simply waits. Nobody owes you the time. A chain that can be ordered to halt by a handful of companies can be ordered by a court as well. That is not a slogan against those chains. It is the check.
Read it yourself
These links are the record. They are not a recommendation, and they are not instructions. A news story or a court paper can still be wrong about a detail. The check does not change because a famous name is in the story.
Apply the check
Open a question. The line is about this topic. It is not a verdict that anything is safe.
The attacker, if they can rewrite recent history. Nobody, during a halt.
Whoever has the majority of hash or stake, or the few companies that run validators.
That majority. Your transaction simply waits.
Check yourself
Does a logo that says decentralised prove a chain cannot be halted?
No. Count who actually produces the blocks.
After this you can ask how many independent parties would have to stop before this chain stops.