Broadcast and verification

The winner tells everyone#

As soon as a miner finds a valid hash, they send the complete block to their peers, who pass it on. Within a few seconds, it has reached the whole network. Every other miner then stops working on their own candidate, which can no longer win, and starts on a new one that builds on the block just found.

Every node checks for itself#

No node takes the winner’s word for it. Each one checks, independently, among other rules:

• that the hash of the header is below the target, with a single SHA-256d;
• that the header points to the previous block it knows;
• that every transaction is valid: correct signatures, no coin spent twice;
• that the coinbase does not claim more than the subsidy plus the fees.

A block that breaks a single rule is rejected, and all the work behind it is lost. Cheating does not pay: it is detected immediately, by everyone.

Blocks are chained by their hashes#

Each header contains the hash of the previous block. Modify an old block, and its hash changes; the next block now points to a hash that no longer exists. Edit a block below to see it:

To rewrite history, an attacker would have to redo the proof of work of the modified block and of every block after it, faster than the rest of the network keeps adding new ones. The deeper a block is, the more impossible that becomes.

When two blocks arrive at once#

Sometimes two miners find a block at nearly the same moment. For a short while, the network is split between two versions. It resolves itself as soon as the next block is found: everyone follows the chain with the most accumulated work, and the other block is abandoned.

In Mini Miner#