What is a ZK Rollup & Which is Best?

Scalability has been a longstanding concern for Ethereum. In a live stream during Ethereum’s Shanghai upgrade, the blockchain’s co-founder Vitalik Buterin said:

If we don’t fix [Ethereum] scaling before the next bull run, people are going to be stuck paying $500 transaction fees.

Layer 2 (L2) solutions like state channels, sidechains, and rollups are popping up left & right to make Ethereum more scalable (and thus cheaper) without compromising security or decentralization.

In this race to Ethereum scalability, zero-knowledge rollups (or simply ZK rollups) are considered one of the most promising solutions.

But, what are zero-knowledge rollups in the first place? How do they improve Ethereum scalability? And which one’s the best?

In this blog post, we will explore what a ZK rollup is, how it works, and more importantly, how popular ZK rollups fare against each other.

Let’s dive in.

What is a zero-knowledge rollup?

A zero-knowledge (ZK) rollup is a layer-2 scaling solution that takes contract computation and transaction execution off-chain to make Ethereum more scalable. Upon execution, ZK rollups batch transactions and publish a validity proof of those transactions on Ethereum.

We can break down the concept of ZK rollups into two parts: zero-knowledge proofs (ZKPs) and blockchain rollups.

Let’s check out these individual concepts to better understand what ZK rollups are.

What are Zero-knowledge proofs (or ZKP)

Zero-knowledge proof or ZKP is a cryptographic technique used to prove the ownership or validity of certain information without revealing the information itself. This ability of ZKP technology is the reason why it’s called zero-knowledge

Zero Knowledge Proofs (ZKPs) have many use cases in blockchain, including identity verification, private transactions (as seen in zCash), and blockchain scalability.

Rollups

Blockchain rollups are L2 blockchains that process transactions on a separate network, roll-up transactions into a batch, and publish them on Ethereum — the L1 network.

The basic premise of rollups is combining multiple letters (transactions) into an envelope (batch) to be posted (recorded on a blockchain).

Rollups reduce the computational pressure on the Ethereum network by doing two things:

• Computing transactions and moving state storage off-chain,
• Compressing data for every transaction that gets stored on Ethereum. For example, a simple ETH transfer takes ~110 bytes on Ethereum but only takes about ~12 bytes on a rollup.

These two concepts combine to make zero-knowledge rollups an efficient and secure scaling solution for Ethereum.

How do zero-knowledge rollups work?

To understand how zero-knowledge rollups work, let us divide the process into two parts — core infrastructure and execution process. The former consists of the moving parts that make the rollups while the latter is the rollup’s route to scaling Ethereum.

Core infrastructure

The core infrastructure of a ZK rollup has two parts to it: the on-chain contracts and the off-chain virtual machine.

On-chain contracts

 ZK rollups are operated by smart contracts running on Ethereum. The main contract stores rollup blocks, tracks deposits, and monitors state updates coming from the ZK rollup. Another verifier contract is used to validate the ZK proofs submitted by block producers.

Off-chain virtual machine

This is the off-chain execution engine of ZK rollups responsible for the computation and storage of transactions and chain state. The rollups use ZKPs to help Ethereum-based verifier contracts to verify the accuracy and legitimacy of all computations in this off-chain Virtual Machine.

Execution process

The execution process of ZK rollups can be split into three parts: sequencing, proof generation, and proof verification.

Sequencing

 Similar to node operators on Ethereum, L2s have sequencers that execute transactions, roll them up into batches, and submit them to the ZK rollup contract. This process is called sequencing.

Proof generation

ZK rollup operators produce validity proof to verify the correctness of a batched transaction.

Proof verification

Validation mechanisms, namely zk SNARKs or zk STARKs are used to verify the integrity of the zk proofs submitted on-chain. The proof needs to prove a sequence of valid transactions from the L1’s pre-state root to the ZK rollup’s post-state root. If valid, the zk rollup’s post-state root is accepted as ‘valid’ and is published on Ethereum.

Types of ZK Rollups: ZK-SNARKs vs ZK-STARKs

Here’s a brief overview of how STARKs and SNARKs  fare against each other:

What is a ZK-SNARK

ZK SNARKs are the more popular of the two types of ZK rollups, mainly since they are smaller, consume less gas to generate proofs and verify proofs quicker. However, they can be prone to centralization risks as they require a trusted setup.

What is a ZK-STARK?

ZK STARKs, on the other hand, are much more decentralized by nature. They are also quantum attack resistant, making them better suited for security.

What’s better: ZK -SNARK or ZK-STARK

There is no black-or-white answer to this question since ZK-STARKs and ZK-SNARKs have different benefits and limitations. If you’d rather opt for performance, you’ll probably want to use ZK-SNARKs, and if security is more important to you, you’re better off with ZK-STARKs.

ZK rollups vs Optimistic Rollups: what's the difference?

Zero-knowledge (ZK) rollups and Optimistic rollups are both Layer 2 scaling solutions designed to enhance Ethereum’s scalability, but they differ significantly in their approach to transaction validation and security. On a high level, the difference is simple. ZK rollups work on a ‘guilty until found innocent’ principle, considering every transaction fraudulent by default and requiring the network to prove otherwise. 

Diving a bit deeper, ZK rollups rely on cryptographic validity proofs (either ZK-SNARKs or ZK-STARKs) to verify authenticity of transactions. Conversely, optimistic rollups verify transactions without proof, and instead rely on validators who must challenge incorrect transactions within a dispute period. 

ZK rollups also achieve a near-instantaneous finality, thanks to each transaction batch including a validity proof; as opposed to optimistic rollups which use a withdrawal delay due to the dispute mechanism. Finally, ZK rollups are also more secure than optimistic rollups. However, they may be more computationally expensive.

Due to these differences, ZK rollups are often favored for applications requiring high security and fast settlement, while Optimistic rollups are preferred for general-purpose scalability due to their lower computational costs.

Challenges and Limitations of ZK Rollups

While ZK rollups present a compelling solution to Ethereum’s scalability issues, they still face several challenges:

Computational Complexity

Generating ZK proofs requires significant computational resources, making it expensive and limiting accessibility.

Developer Adoption

Some rollups, like StarkNet, require developers to learn new programming languages such as Cairo, creating barriers to entry.

Smart Contract Compatibility

While solutions like Polygon zkEVM and Scroll zkEVM strive for full compatibility with Ethereum, others still struggle to support all existing Ethereum dApps seamlessly.

Centralization Risks

 Many current ZK rollup implementations rely on centralized sequencers and provers, which could undermine decentralization until further decentralization efforts materialize.

Addressing these limitations is crucial for the widespread adoption of ZK rollups.

ZK Rollup comparison: which is best?

In the Ethereum-compatible ZK rollup category, four major proponents are building promising solutions: Polygon, Scroll, zkSync, and StarkWare. Let’s check out all four of them.

Polygon ZK tech

Polygon is one of the earliest projects to work on ZK tech and adopt them as L2 scaling solutions. Their $1 billion commitment towards zero knowledge tech-related efforts is a testament to their belief in ZK tech.

The ZK suite of Polygon currently has 4 products powered by the Polygon zkEVM

• Hermez: Hermez is Polygon’s decentralized ZK rollup protocol that employs ‘Proof of Efficiency’, allowing any user to become a sequencer (to batch transactions) or aggregator (to generate ZK proofs). This reduces centralization and its associated risks.
• Miden: Miden is Polygon’s EVM- compatible general-purpose ZK rollup that’s supercharged with local execution of smart contracts. Only proofs need to be submitted to the network, enabling parallel transactions (improves efficiency).
• Nightfall: Polygon Nightfall is the union of optimistic rollups and ZK technology to cater to enterprise-level scaling. Nightfall embraces the efficiency of optimistic rollups alongside the privacy elements of ZK technology. It thus opens avenues of enterprise use cases where sensitive data is in play.
• Zero: Polygon Zero is a fully EVM-compatible ZK rollup solution. It employs Plonky2 — a recursive proof system easing the time and resources needed for validity proof generation.

With these products, Polygon offers a wide array of ZK rollup solutions that are:

1. Fast: Plonky2 generates a ZK proof in 0.17 seconds
2. Efficient: Hermes 2.0 aims to minimize proof sizes.
3. Economical: Gas costs to reduce from 5M to 350K

These are solutions to watch out for in the race to Ethereum scalability.

Build web3 apps on Polygon zkEVM:

Scroll zkEVM

Scroll is a layer-2 scaling solution for Ethereum that uses zk-rollup technology. It is a general-purpose zk-rollup, which means that it can be used to support any type of Ethereum application.

Scroll has a three-part architecture:

• Scroll node: This is the bridge between layer 2 and layer 1. It constructs blocks from user transactions on layer 2 and submits them to Ethereum.
• Roller network: This network of nodes generates zero-knowledge (ZK) validity proofs to prove the correctness of the transactions in the blocks.
• Rollup and bridge contract: This contract bridges fund transfers between Ethereum and Scroll, verifies ZK validity proofs, and ensures data availability.

In addition to its architecture, Scroll has two unique features that help scale Ethereum:

• Decentralized proving: Anyone can become a roller by staking SCR, the native token of the Scroll network. The amount of SCR staked determines the probability of a roller being chosen to generate proofs. This ensures that proving is decentralized and not controlled by a small group of entities.
• Low cost of proving: Scroll uses a novel technique to reduce the cost of proving. This makes it possible for anyone to participate in the roller network, even if they do not have a lot of computing resources.

Since its launch in February 2023, Scroll has attracted over 1 million unique addresses and 2+ million transactions.

Build web3 apps on Scroll zkEVM:

zkSync Era

zkSync, developed by Matter Labs, is an EVM-compatible ZK rollup that has undergone significant upgrades. Initially launched as zkSync 1.0 (now known as zkSync Lite) with limited functionalities, it has recently evolved into zkSync Era, a full-scale zkEVM. This latest chain introduces new features like native account abstraction and an LLVM compiler.

Since its upgrade, zkSync Era has demonstrated impressive performance, processing over 16 Million transactions using ZKPs in less than two months. It has also amassed a total value locked (TVL) of more than $100 million, showcasing its growing popularity.

The advantages of using zkSync Era are:

• Data compression: zkSync prioritizes data availability (DA) and its cost, which ultimately determines transaction costs. By publishing only state differences on-chain, zkSync can improve DA and reduce the costs of transactions with large input data.
• Account abstraction: Account abstraction is native on zkSync to improve user and developer experience. The initial use case is limited to paying gas fees in any currency of choice. More use cases like social recovery, subsidizing gas, and batching complex transactions are possible with account abstraction on zkSync.

These features supercharge zkSync Era into a powerful engine for scaling Ethereum.

Build web3 apps on zkSync Era:

StarkNet

StarkNet is a general-purpose, EVM-compatible layer-2 scaling solution for Ethereum. It is being developed by StarkWare, a company founded by Eli Ben-Sasson — the co-inventor of ZK STARK proofs and the founding scientist at the privacy-focused crypto payment network Zcash.

By using StarkWare’s ZK technology, StarkNet can scale to millions of transactions per day while still maintaining the security of Ethereum.

StarkNet is still under development, but it has the potential to be a major scaling solution for Ethereum. It has already attracted a number of developers and projects, including dYdX, Immutable, and DeversiFi.

Unlike other ZK protocols, StarkNet uses STARK (Succinct Non-interactive ARgument of Knowledge) proofs instead of the commonly used SNARK (Scalable Transparent ARgument of Knowledge) proofs.

However, on the EVM-compatibility spectrum, StarkWare’s StarkNet is the least compatible ZK rollup. That’s because StarkWare is developing StarkNet to also function as a standalone chain and not be completely reliant on Ethereum.

Get started with ZK Rollups

You can start deploying smart contracts and building dApps on any of the above zkEVMs — and 700+ EVM chains — with thirdweb’s web3 developer tools, available for free. Simply search for the network that you want to build on in our Chainlist & start building:

Are ZK rollups and zkEVMs the same?

The difference between ZK rollups and zkEVMs is the same as the difference between Ethereum and EVM. The former is the blockchain that records the transactions while the latter is the computation environment where the smart contract computation happens.

ZK Rollups: Potential and Future

ZK rollups strike a fine balance between efficiency and privacy, enabling a wider array of use cases for blockchain applications. The future of ZK rollups looks promising, with ongoing research and development focused on improving efficiency and accessibility.

Advancements in recursive proof aggregation, such as Plonky3 and Nova. to make proof generation faster and more cost-efficient. 

And other efforts, like Layer 3 solutions and shared liquidity networks, could enable smoother interactions between different rollups.

Further, as Ethereum moves down its scalability roadmap, ZK rollups will benefit from reduced data availability costs, making them more viable long-term. As these technologies mature, ZK rollups are poised to become a dominant solution for Ethereum’s scalability, potentially even outpacing Optimistic rollups in adoption and efficiency.

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