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Bitcoin Data GuideBitcoin Reference

Security & Resilience

What could go wrong, and what stops it

Bitcoin is often described as secure without saying what that means. This hub collects the pages that say it precisely: what a miner can do with the block it finds and what it cannot, how a double spend is attempted and why confirmation depth is the defence, what a majority of hash rate would actually buy an attacker, how a single node can be isolated from the network, and what happens when the network is split or the hash rate falls. The recurring theme is that the protocol's protection is a cost imposed on the attacker rather than a wall that cannot be climbed.

Security writing about Bitcoin tends to run to one of two extremes. Either the network is unbreakable, or it is one clever attack away from collapse. Neither is useful. The honest position is that Bitcoin replaces a trusted operator with a set of rules and an economic cost, and that both can be described exactly. A miner that produces an invalid block loses the reward it spent electricity to earn. An attacker that wants to rewrite a confirmed transaction has to outrun the entire honest hash rate for as long as the rewrite takes. A node that is isolated from its peers can be deceived, which is why the number of confirmations a merchant waits for is a risk decision rather than a protocol constant.

Each page in this section names the attack, states the assumption the attacker needs, and says what the defence costs the defender. Where a figure is an estimate rather than a measurement — the cost of mounting a majority attack, the size of the security budget in future epochs — the page says so rather than presenting it as a settled number. Where a calculation would genuinely help, the writing points to a tool that does that work properly rather than embedding a calculator in the reference material.

What a Miner Can Do

Can the people who find blocks break the rules?

A miner is the only participant who gets to choose what goes into a block, so the natural question is what that power is worth. The answer is narrower than it first appears. A miner can order the transactions it includes, it can leave transactions out, and it can pay the block reward to an address of its choosing — but it cannot create coins, spend someone else's output, or make an invalid block acceptable. Every other node checks the block against the same consensus rules and discards it if it fails. This strand works through that boundary: what the block template actually controls, why a double spend is a race against confirmation rather than a rewrite of history, and what a majority of hash rate would and would not let an attacker do.

Start with Can Miners Cheat?

Attacks on the Node

What happens when a node cannot see the honest network?

Not every attack targets the chain. Some target the individual node, and the most direct of them is to surround it with peers the attacker controls so that it sees only the attacker's view of the network. An eclipsed node can be fed a stale or fabricated chain, and a merchant relying on that node alone can be shown a payment that the rest of the network never accepted. This strand covers the isolation attack and the peer-management defences that make it harder, then turns to the two ordinary disruptions that are not attacks at all: a node going offline, and a network that splits into two groups which cannot see each other. In both cases the protocol's recovery is the same — the chain with the most accumulated work wins, and the node that was behind catches up.

Start with Eclipse Attacks

Paying for Security

What funds the hash rate that makes rewriting history expensive?

Bitcoin's resistance to a rewrite is not a property of the software; it is a cost imposed on anyone who tries. That cost is set by the hash rate, and the hash rate is paid for out of the block reward — the subsidy plus the fees collected in each block. This strand follows the money. It sets out how the two components divide the reward today, why the split matters as the subsidy halves on its fixed schedule, and what a fall in hash rate actually does to the network: block times lengthen, the difficulty adjustment follows within a retarget period, and the system returns to its ten-minute target without anyone deciding that it should. The pages here are careful to separate what is measured from what is projected.

Start with The Security Budget

Every page in this section

The strands above are the way in. This is the full contents of the section, grouped by whether the page describes an attack on the chain or the network's response to being stressed.

The reference pages behind the strands

The strands above explain the attacks. These pages hold the underlying record and the methodology, for when you want to check a figure or follow a source.

Where the security argument meets the record

The cost of attacking Bitcoin is set by the reward paid to miners. These pages carry the measured record of that reward and the supply schedule behind it.