Delayed Proof of Work (dPoW)

Delayed proof of work (dPoW) is a security mechanism in which a blockchain regularly writes a fingerprint of its recent blocks onto another, more secure proof-of-work blockchain, and then refuses to accept any version of its own history that conflicts with those recorded fingerprints. The aim is to let a smaller network borrow some of the security of a much larger one. The method was developed by the Komodo project, which launched in 2016, and is closely associated with it.

dPoW is not a stand-alone consensus mechanism in the usual sense. The protected chain still produces blocks through its own process, typically proof of work, and dPoW adds a second layer of protection on top. The "delayed" in the name refers to the fact that this protection applies after a short lag: blocks become secured only once their fingerprint has been written to, and confirmed on, the larger chain. The approach is a form of checkpointing, an idea also used in other networks.

Imagine a small club that keeps its accounts in a notebook. Someone could, in theory, tear out pages and rewrite them. To prevent this, every few hours the club's trusted officers take a photo of the latest page and lodge it with a large public records office that almost nobody can tamper with. If anyone later tries to present a rewritten notebook, the club compares it with the lodged photo and rejects it.

dPoW works the same way. The small blockchain is the notebook, the large blockchain is the records office, and the "photo" is a short code that summarizes the latest blocks. Once the photo is lodged, changing anything before it would mean also attacking the much bigger chain, which is far harder.

1. Notary nodes. A group of designated operators, called notary nodes, monitors the protected chain. In Komodo's design these are elected by stakeholders for a fixed term.
2. Snapshot. At regular intervals the notaries take the hash of a recent block, a short value that changes completely if any earlier data is altered.
3. Notarization. A quorum of notaries signs a transaction that records this hash on the larger target chain, and then records proof of that transaction back on the protected chain.
4. Enforcement. Nodes on the protected chain follow a rule that they will not accept a chain reorganization reaching back beyond the most recent notarized block, even if an attacker presents a longer chain.

Because of this rule, dPoW departs from the plain "longest chain wins" logic of Bitcoin-style proof of work. Final settlement on the protected chain comes from the notarized checkpoints rather than only from accumulated mining work, giving users a firmer point after which a payment is considered safe than ordinary probabilistic finality provides.

Komodo began as a fork of Zcash and adopted dPoW from its launch, initially notarizing its blocks onto the Bitcoin blockchain. In June 2021 the project moved its notarizations to Litecoin, citing lower cost and Litecoin's shorter block time, at the price of relying on a chain with less mining power than Bitcoin. Komodo also offered dPoW as a service: independent blockchains built with its tools could notarize to Komodo, which in turn notarized to the larger chain, creating a layered chain of checkpoints. A number of smaller projects used the service to protect themselves at a time when several minor proof-of-work coins suffered damaging attacks.

The main purpose of dPoW is to defend small proof-of-work chains against a 51% attack. On a chain with little mining power, an attacker can rent enough hashrate to secretly mine a longer chain, spend coins on the public chain and then publish the hidden one, undoing those payments in a double spend. With dPoW, any attempt to rewrite blocks older than the last notarization is rejected by honest nodes, so the attacker's window is limited to the few blocks since the last checkpoint. Exchanges and merchants can therefore treat a payment as settled once it is notarized, rather than waiting for a large number of confirmations.

• Trust in notaries. The system depends on a fixed group of node operators. If enough notaries collude, are compromised or go offline, notarizations can stall or record the wrong history, so dPoW adds a degree of centralization.
• Protection is not instant. Blocks produced since the last notarization remain exposed, so recent transactions are only as secure as the protected chain's own mining power.
• Cost and dependence. Writing to the target chain costs fees, and the protected chain's security depends on the continued operation and security of that chain.
• Limited adoption. Outside the Komodo ecosystem, dPoW has seen little use, and many newer networks rely on proof-of-stake finality or on rollups that post data to larger chains instead.