Key Takeaways
ZK-rollups are a Layer 2 scaling solution that bundles many transactions off-chain and submits a compact cryptographic proof to the Ethereum main chain.
They use zero-knowledge proofs to confirm that transactions are valid without sharing the details of each one, allowing for fast finality and strong security.
Compared to optimistic rollups, ZK-rollups do not need a lengthy dispute period, which means funds can potentially be withdrawn faster.
Ethereum's EIP-4844 upgrade in March 2024 introduced a new data storage format called blobs, which substantially reduced transaction costs across ZK-rollup networks.
ZK-rollups are more complex to build than optimistic rollups, but they are becoming more practical as zkEVM technology matures.
Introduction
Blockchains like Ethereum can only process a limited number of transactions per second. This scalability challenge leads to network congestion, slower confirmation times, and higher transaction fees when demand increases. Solving this problem is one of theimportant goals in the blockchain space.
Two main approaches exist: Layer 1 and Layer 2 solutions. Layer 1 solutions upgrade the blockchain itself. Layer 2 solutions build a separate framework on top of the existing chain, handling transactions off-chain before settling results on the base layer. ZK-rollups are one of the more promising Layer 2 approaches available today.
What Are ZK-Rollups?
ZK-rollups are a type of Layer 2 solution that bundles, or "rolls up," hundreds of transactions into a single batch. This batch is processed off the main Ethereum chain, and only a summary along with a cryptographic proof is submitted on-chain. The proof confirms that all transactions in the batch are valid without revealing the full details of each one.
The "ZK" in ZK-rollups stands for zero-knowledge. This refers to the type of cryptographic proof used, called a zero-knowledge proof. These proofs allow one party to confirm the truth of a statement to another party without disclosing any additional information about it.
Because ZK-rollups verify every transaction using these proofs before committing to the main chain, they do not require a challenge period. This makes final settlement faster than with optimistic rollups, which assume transactions are valid and rely on fraud challenges after the fact.
What Are Zero-Knowledge Proofs?
Zero-knowledge proofs (ZKPs) are a cryptographic technique that lets a prover convince a verifier that a statement is true without sharing any extra information. For a proof to be valid, it must satisfy three conditions.
Completeness: if the statement is true and both parties are honest, the proof will always confirm it.
Soundness: a dishonest prover should not be able to convince an honest verifier that a false statement is true.
Zero-knowledge: the verifier learns only that the statement is true, nothing else about the underlying data.
ZKPs are used in ZK-rollups to prove that a batch of transactions was processed correctly. The proof itself is compact and can be verified quickly on-chain, which is what makes ZK-rollups efficient.
How Do ZK-Rollups Work?
ZK-rollups use two main components: smart contracts on the main chain and virtual machines off-chain. The on-chain contracts store rollup blocks, handle deposits, and verify the cryptographic proofs submitted by the rollup. The off-chain virtual machines handle all transaction execution away from Ethereum.
When users submit transactions to a ZK-rollup, a network operator collects them into a batch. The operator processes the batch off-chain and generates a validity proof, which confirms all the transactions were executed correctly. This proof is then sent to Ethereum's main chain for verification.
Verification on Ethereum is quick because the chain only needs to check the proof, not replay every transaction. Once accepted, the new state of the rollup is recorded on Ethereum, giving it the same security guarantees as the main chain.
ZK-Rollups After EIP-4844
In March 2024, Ethereum activated EIP-4844, also known as proto-danksharding. This upgrade introduced a new type of data storage called blobs, designed specifically for rollup data. Blobs are cheaper to store than regular transaction data and are automatically deleted from the chain after a set period.
The impact on ZK-rollups was significant. Transaction fees on major ZK-rollup networks dropped substantially after EIP-4844, in some cases by more than 80%. This made ZK-rollup networks more practical for everyday use, including small transactions that were previously not cost-effective.
Pros and Cons of ZK-Rollups
Advantages
Faster finality: transactions are verified immediately using validity proofs, without a waiting period for disputes.
Strong security: ZK-rollups inherit Ethereum's security because proofs are verified on-chain. Funds cannot be moved unless a valid proof is submitted.
Higher throughput: by processing transactions off-chain in batches, ZK-rollups can handle far more transactions per second than the base layer.
Lower fees: batching reduces the per-transaction cost, especially after the EIP-4844 upgrade reduced data storage costs.
Limitations
Complexity: ZK-rollups are technically demanding to build and require specialized cryptographic knowledge.
EVM compatibility challenges: not all ZK-rollup implementations fully support the Ethereum Virtual Machine, which can limit the types of applications that can run on them. This is improving as zkEVM technology advances.
Centralization risk: many ZK-rollup networks still rely on a single operator or sequencer to batch transactions, which can create a centralization point.
ZK-Rollups vs. Optimistic Rollups
The main alternative to ZK-rollups is optimistic rollups. Both bundle transactions off-chain, but they handle verification differently. Optimistic rollups assume all transactions are valid by default. They publish transaction data and allow a challenge window, typically around seven days, during which anyone can submit a fraud proof to dispute an invalid transaction.
ZK-rollups take the opposite approach. Every batch includes a validity proof that confirms all transactions were processed correctly. There is no challenge period, so funds can potentially be withdrawn to the main chain faster once the proof is accepted.
Optimistic rollups are generally simpler to build and have been more widely adopted, but ZK-rollups offer advantages in security and finality. As zkEVM technology matures, ZK-rollups are becoming a more practical choice for general-purpose smart contract execution.
ZK-Rollup Examples
Several ZK-rollup networks have launched on Ethereum. zkSync Era opened to the public in March 2023 and introduced a native zkEVM, allowing standard Ethereum smart contracts to run on the rollup. StarkNet uses a different proof system called STARKs and supports a wide range of decentralized applications. Polygon zkEVM launched in 2023 and is designed to be compatible with existing Ethereum tooling.
On the BNB Chain, zkBNB is a ZK-rollup built specifically to support non-fungible tokens (NFTs) and trading applications with lower costs and higher throughput than the main chain.
These networks each make different trade-offs in proof generation speed, EVM compatibility, and decentralization. The ecosystem continues to develop, and new implementations may offer further improvements in cost and performance.
FAQ
What is the main difference between ZK-rollups and optimistic rollups?
ZK-rollups verify every transaction with a cryptographic proof before committing to the main chain. Optimistic rollups assume transactions are valid and only check them if someone raises a dispute during a challenge window. ZK-rollups generally offer faster finality, while optimistic rollups are currently simpler to implement.
Are ZK-rollups safe to use?
ZK-rollups rely on mathematical proofs verified on the main chain, which provides a strong security model. However, most current ZK-rollup networks still depend on centralized sequencers for transaction ordering, and smart contract bugs can present risks. As with any technology in active development, users should do their own research before using any specific network.
How did EIP-4844 affect ZK-rollup fees?
EIP-4844, activated on Ethereum in March 2024, introduced blob data storage that is cheaper than regular on-chain data. Since ZK-rollups publish compressed transaction data to Ethereum, they benefit directly from this cost reduction. Many ZK-rollup networks saw fees drop significantly following this upgrade.
What is a zkEVM?
A zkEVM is a ZK-rollup that is compatible with the Ethereum Virtual Machine. This means developers can deploy Ethereum smart contracts on the rollup without major changes to their code. Different projects achieve different levels of EVM compatibility, with trade-offs in performance and proof generation speed.
Closing Thoughts
ZK-rollups represent one of the most technically sophisticated approaches to blockchain scalability. By using zero-knowledge proofs to verify transaction batches, they can offer fast finality, strong security, and meaningfully lower fees compared to transacting directly on Ethereum.
The technology has matured considerably since 2022. EIP-4844 reduced data costs significantly, zkEVM implementations have become more capable, and multiple ZK-rollup networks are now live with substantial user activity. Challenges remain around decentralization and full EVM compatibility, but development is ongoing.
For anyone interested in how Ethereum scaling works, understanding ZK-rollups provides useful context for where the ecosystem may be heading.
Further Reading
Optimistic vs. Zero-Knowledge Rollups: What's the Difference?
What Is a zkEVM and How Can It Enhance the Ethereum Ecosystem?
What Is Zero-Knowledge Proof and How Does It Impact Blockchain?
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