ZK Rollups Explained
Published on: May 12, 2024

ZK Rollups Explained

Ethereum has long grappled with the blockchain scalability trilemma of balancing security, decentralization, and scalability. As the number of users grows, Ethereum prioritizes security and decentralization, leading to scalability challenges. Transactions slow down, and gas prices rise, creating a bottleneck that impedes Ethereum’s adoption at scale. Enter validity rollups, a solution that scales Ethereum without compromising its core tenets.

Note: Validity rollups are often mistakenly referred to as Zero Knowledge (ZK) rollups, but this is not accurate. ZK is a misnomer in this context: Most validity rollups do not use ZK proofs and are not utilized for the purpose of ensuring privacy. They use “validity proofs” to establish the computational integrity of transactions, hence the term “validity rollup” is more accurate.

What are Validity Rollups?

At their core, validity rollups involve executing transactions offchain, bundling them into a single proof, and subsequently submitting that proof for verification and settlement on the Ethereum mainnet. This offchain computation allows scaling Ethereum without compromising its security and decentralization.

So, how do validity rollups work? The process involves three main steps:

  1. Bundling transactions offchain: Thousands of transactions are executed offchain and then batched together. This reduces the load on the Ethereum mainnet and allows for faster processing times.
  2. Generating validity proofs: After executing the transactions offchain, a validity proof is created. The validity proof is a cryptographic assurance that the state change proposed by the rollup is indeed the result of properly executing a certain batch of transactions. Validity proofs are very easy to verify, and this verification is done on Ethereum.
  3. Submitting proofs and data onchain: Once the validity proof is generated, the rollup operators post a summary of the changes describing all the transactions in the batch, along with the validity proof, to the Ethereum mainnet for verification and settlement.

Once the proof is on Ethereum mainnet, it becomes an immutable part of Ethereum.

Validity Rollups vs. Optimistic Rollups

When comparing Validity Rollups to Optimistic Rollups, another popular Layer 2 scaling solution, there are a few key differences. While Optimistic Rollups rely on fraud proofs and assume transactions are valid by default, Validity Rollups use Validity Proofs to guarantee the correctness of transactions mathematically for every transaction. This allows for faster finality and shorter withdrawal times in Validity Rollups, as there is no need for a challenge period. However, Validity Rollups are not natively EVM-compatible, whereas Optimistic Rollups are.

The Starknet Validity Rollup

One notable example of a validity rollup is Starknet, which utilizes its own virtual machine, the Cairo VM (CVM). Producing a Validity Proof, even on L2 where computation is cheap, necessitates a significant amount of computational effort and time.The CVM is designed specifically for the efficient generation of Validity Proofs, prioritizing performance and scalability over EVM compatibility. Starknet also supports Solidity smart contracts through the Warp transpiler and the Kakarot zkEVM, making it easier for developers to migrate their existing contracts.

Most Validity Rollups use SNARK proofs, which require trust assumptions and make use of elliptic curve cryptography, which is not secure from attacks by quantum computers. However, StarkWare’s Starknet and StarkEx use STARKs instead of SNARKs, which do not require a trusted setup, are secure against quantum computers, and offer greater scalability than SNARKs.

Several other ZK Rollup projects are also making significant contributions to the Ethereum ecosystem. These include zkSync, Scroll, Miden, and Hermez, each with their unique features and approaches to scaling.


In summary, Validity Rollups emerge as a beacon of hope in Ethereum’s quest for scalability. Through a blend of advanced cryptography and off-chain computation, Validity Rollups offer a viable solution to Ethereum’s scalability woes without compromising its security and decentralization. As Validity Rollups continue to evolve and gain traction, they pave the way for Ethereum to accommodate a burgeoning user base and foster broader adoption of decentralized applications (dApps).


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