Merged Mining

Merged mining lets a miner produce valid proof-of-work for two (or more) SHA-256 blockchains simultaneously, without doing extra hashing. The miner's work nonce satisfies both chains' difficulty requirements; if it does, both chains get a new block from the same effort.

The canonical example is Bitcoin + Namecoin, set up in 2011 when Namecoin's hash rate was too low to be secure on its own. Namecoin's block-header validation accepts a Bitcoin coinbase transaction as proof of work (encoded with a commitment to the Namecoin block), letting Bitcoin miners mine Namecoin essentially for free.

How the mechanics work:

1. The miner constructs a Namecoin block (with its own transactions, merkle root, etc.).
2. The miner includes a commitment to the Namecoin block's hash inside their Bitcoin coinbase transaction.
3. The miner hashes the Bitcoin block header normally.
4. If the resulting hash meets Bitcoin's difficulty, they win a Bitcoin block. If it also meets Namecoin's (lower) difficulty, they win a Namecoin block too.
5. The Namecoin block is constructed with a proof showing the Bitcoin coinbase committed to it, and Namecoin validators accept it.

Other merged-mined chains over Bitcoin's history: Namecoin, Devcoin, Ixcoin, Rootstock (RSK), Syscoin. All are smaller chains that benefit from Bitcoin's enormous hash rate without competing for it.

What merged mining buys you:

• Hash rate for free, from the perspective of the smaller chain.
• Higher security floor for the smaller chain (now requires attacking a fraction of Bitcoin's hash rather than just its own).
• Optional revenue for Bitcoin miners who choose to include the commitment, if the smaller chain's rewards are worth claiming.

What it doesn't fix:

• The smaller chain still has its own consensus rules, validators, and trust model.
• Bitcoin miners aren't required to merge-mine; if they collectively decided to stop, the smaller chain's security would drop.
• The smaller chain inherits some Bitcoin-mining concentration risks at the level of pool operators.

Merged mining is an interesting technical trick but mostly a footnote in Bitcoin's own story. It matters more for the chains that depend on Bitcoin's hash than for Bitcoin itself.