Proof of Useful Work
The Internet Computer's membership and consensus model — dedicated standardised hardware doing real computation, governed by the NNS, instead of hashing or pure stake.
“Proof of useful work” (PoUW) is the Internet Computer’s answer to the membership-and-consensus problems that earlier decentralized networks solved with proof-of-work or proof-of-stake. It rests on dedicated, standardised node machines performing real application computation — not hashing — under a governance system that controls who is admitted and who is removed.
Background — the prior schemes
When Bitcoin was designed, two problems had to be solved at once. First, Sybil resistance: stopping an adversary from spinning up enough nodes to take over the network. Second, consensus: agreeing on the order of transactions in the ledger.
The original answer was proof-of-work (PoW). Miners run specialised hardware that produces large numbers of cryptographic hashes of candidate blocks until a “winning” hash is found, at which point the block is submitted. The share of blocks each miner produces is proportional to the hash rate of their hardware. PoW is both a Sybil-resistance scheme and a consensus protocol: the cost of acquiring enough hardware and electricity to dominate is prohibitive, and because winning hashes are random, a random miner is effectively assigned each block.
PoW produces a chain backed by stable, dedicated hardware — a giant decentralized hashing factory — but the scheme is in practice an arms race: aggregate spend on hardware and electricity trends towards the value of the block reward, and consensus is slow.
To escape that cost the industry moved to proof-of-stake (PoS), in which network nodes are joined by “staking” some cryptocurrency and earn rewards in proportion to the stake. Ethereum 2.0 migrated from PoW to PoS for both performance and energy reasons. But PoS has its own problems:
- Cloud concentration. Once dedicated hardware was no longer required, most PoS nodes ended up running in commercial clouds. When the Hetzner cloud suddenly banned Solana nodes, about 40% of the Solana network went dark in an instant. A network running in someone else’s cloud is not a sovereign network.
- Liquidity of stake. Cryptocurrency is highly liquid, so the distribution of network power can shift quickly. A clever DeFi manipulation, or an exchange compromise, can put enough stake in an attacker’s hands to break a network — with PoS frameworks that let nodes be spun up by script, an attacker can launch from cold start.
What proof of useful work is
PoUW is the Internet Computer’s response to those weaknesses. The network is produced by dedicated node machines built to a very similar standardised specification. They run sophisticated consensus protocols that lean on advanced cryptography — the chain-key engine. PoUW is a scheme about membership in the network.
As in PoW, the purchase, hosting, and operation of node-machine hardware acts as the stake. But these machines do not hash. They produce and process blocks of transactions that represent canister computations. Because each machine is built to the same spec, the question is not who can perform more work but whether each machine performs roughly the same amount of work. Statistical deviation — producing too many or too few blocks — is the failure condition. In PoUW, machines compete not to deviate, and can be punished for doing so.
A key ingredient is the Network Nervous System (NNS) — the permissionless, on-chain governance system integrated with the network’s protocols. The NNS configures the network and upgrades the software the node machines run. Among its responsibilities, it combines node machines into subnet networks, which are themselves combined into a single network using chain-key cryptography. Two properties follow:
- Adversaries cannot just buy in. The NNS selects nodes by provider, data center, geography, and jurisdiction — a scheme of deterministic decentralization. Even with unlimited funds, an adversary cannot simply add nodes to a subnet.
- Deviating nodes can be removed. Nodes that deviate statistically can be slashed by the NNS — removed from the subnet they were assigned to.
Through deterministic decentralization the network becomes a sovereign fabric of dedicated hardware that can be tightly held to correct behavior, with continued participation in block production — and the rewards that go to the providers — conditional on that behavior. The repetitive hashing of PoW, which exists primarily to secure the network, has been replaced by useful canister computation. Work that has to be done anyway is the work that secures the network.
Historical resources
Project founder Dominic Williams was an early researcher of crypto Sybil resistance and consensus. Some material that informs the design of PoUW:
- May 2015 talk at San Francisco Bitcoin Devs — the “3 Es of Sybil resistance”.
- DEVCON1 — scalability panel with Vitalik Buterin and Gavin Wood.
- DEVCON1 — introducing consensus theory.
Related
- Node Provider Documentation — the operators who provide and run the dedicated hardware.
- Node Provider Remuneration — how the rewards that follow from PoUW are calculated.
- Internet Computer Protocol — the network the protocol secures.
- Chain-key cryptography — the cryptographic substrate PoUW relies on.