What is Merged Mining?
Merged mining lets miners secure two blockchains at the same time using the same proof-of-work. Learn how it works and why projects like Namecoin use it.
Definition
Merged mining (also called auxiliary proof of work, or AuxPoW) is a technique that lets a miner use the same computational work to mine two or more blockchains simultaneously. The miner produces a single proof-of-work that satisfies the difficulty requirements of both a primary chain (typically Bitcoin) and one or more auxiliary chains — without any extra hash power.
The concept was first implemented in 2011 with Namecoin, a decentralized domain name system that merged-mine with Bitcoin. Today it is used by several smaller blockchains that need security but lack enough miners to defend against a 51% attack.
How It Works
In standard proof-of-work mining, a miner searches for a nonce that produces a block hash below the network’s current difficulty target. With merged mining, the process adds one extra step:
- Mine the auxiliary block. The miner constructs a block for the auxiliary chain and hashes it.
- Embed the proof. The hash of that auxiliary block is included inside the coinbase transaction of the primary chain’s block template (typically Bitcoin). This is the link that proves the miner did the work for both chains.
- Mine the primary block. The miner performs normal proof-of-work on the primary block. If the resulting hash meets the primary chain’s difficulty, the primary block is valid.
- Claim auxiliary rewards. If the same hash also happens to meet the auxiliary chain’s (usually much lower) difficulty, the miner submits the auxiliary block along with the primary block as proof. Both chains accept the work.
Because auxiliary chains typically have far less hash rate than Bitcoin, their difficulty is much lower. A hash that is too high to be a valid Bitcoin block can still easily qualify for the auxiliary chain. This is why merged mining adds no extra computational cost — the miner is simply checking two difficulty thresholds against the same hash.
Why It Matters
Merged mining solves a real bootstrapping problem. A new proof-of-work blockchain is vulnerable because it attracts little hash rate at launch. An attacker with modest hardware could overpower the network. By piggybacking on Bitcoin’s massive mining ecosystem, an auxiliary chain inherits real security without convincing miners to switch away from Bitcoin.
For miners, the appeal is straightforward: extra revenue with no additional electricity or hardware. A miner running SHA-256 ASICs can earn both the Bitcoin block reward and any auxiliary chain rewards from the same hashes.
For mining pools, merged mining requires pool-side support. The pool must build block templates that include the auxiliary chain’s data and distribute the extra rewards. Not all pools support it, so miners interested in merged mining need to choose a compatible pool.
Limitations
Merged mining is not without trade-offs:
- Complexity. Setting up merged mining requires configuring additional software and connecting to the auxiliary chain’s nodes. It is more involved than mining a single chain.
- Pool dependency. Solo miners rarely benefit because they would need to run nodes for both chains and handle the proof-linking logic themselves. Most merged mining happens through pools.
- Asymmetric benefit. The auxiliary chain gets security, but miners only get extra revenue if the auxiliary coin has market value. If the coin trades near zero, the extra income is negligible.
- Trust assumptions. The auxiliary chain must trust that miners on the primary chain are honestly including the auxiliary proofs, though this is enforced by consensus rules on both sides.
Notable Examples
- Namecoin (NMC) — The original merged-mined coin with Bitcoin. Used for decentralized DNS (.bit domains).
- RSK (Rootstock) — A Bitcoin sidechain that brings smart contract functionality. Merged-mined with Bitcoin through participating pools.
- Elastos (ELA) — A decentralized internet platform that supports merged mining with Bitcoin.