What Are Zk-Rollups?

What Are Zk-Rollups?

Low blockchain scalability causes slow transaction speeds, high fees, and a poor user experience. However, these problems can be addressed through Layer 1 solutions, such as sharding, which aims to upgrade the blockchain itself, and Layer 2 solutions, such as rollups, which create a framework on top of the existing chain. Rollups are a popular Layer 2 solution that bundles transactions off-chain to speed up computation. Zk-rollups, a specific type of rollup implementation, utilize zero-knowledge proofs, a cryptographic technique that enables verification without revealing sensitive data, ensuring the privacy and security of transactions.


As cryptocurrencies gain traction, addressing blockchain scalability becomes crucial. Congestion in these networks leads to slower transaction times and increased fees. Two primary approaches aim to address this: Layer 1 and Layer 2 solutions.

  • Layer 1 strategies focus on enhancing the core structure of the blockchain, increasing its capacity for processing. Sharding, for instance, divides the blockchain into smaller segments, enabling parallel transactions and boosting throughput.
  • Layer 2 approaches work alongside the base blockchain, handling transactions off-chain and then incorporating them into the main chain in batches. Techniques include state channels, sidechains, and rollups. Among these, zero-knowledge rollups, or zk-rollups, use advanced cryptographic proofs to ensure security and efficiency.

Understanding Zk-Rollups and Zero-Knowledge Proofs

To comprehend zk-rollups, it's essential to know what rollups are and grasp the concept of zero-knowledge proofs.

What are rollups?

Rollups improve blockchain scalability by processing transaction data off-chain in bundles, then submitting the results to the main chain. This approach reduces blockchain congestion and lowers transaction costs, as processing is more efficient. There are two types of rollups: Optimistic rollups and zk-rollups.

  • Optimistic rollups assume that all rolled-up transactions are valid. Before these transactions are added to the blockchain, they undergo a delay, allowing the network to raise disputes if necessary. If a transaction is challenged, there's a process to resolve it. Optimism, Arbitrum, and opBNB are notable examples of this approach.
  • Zk-rollups, on the other hand, validate each transaction through zero-knowledge proofs, eliminating the need for a dispute resolution process. This speeds up transaction processing compared to Optimistic rollups but requires more complex implementation.

What are zero-knowledge proofs?

Zero-knowledge proofs (ZKPs) are cryptographic techniques allowing one party (the prover) to demonstrate the truth of a statement to another party (the verifier) without revealing the underlying information. A valid ZKP must meet three criteria:

  • Completeness: If the statement is true, a genuine proof will confirm it.
  • Soundness: A false prover should not be able to deceive a verifier into believing an incorrect statement.
  • Zero-Knowledge: The verifier learns only the truth of the statement, without gaining access to the details.

A ZKP operates in three stages:

  1. Witness: The prover offers a piece of hidden information (the witness) to demonstrate they have access to specific data without explicitly sharing it.
  2. Challenge: The verifier challenges the prover by selecting random questions from a predefined set.
  3. Response: The prover must correctly answer the verifier's questions to validate the proof.

Combining rollups with ZKPs, zk-rollups can achieve faster and more secure transaction processing by confirming each transaction's validity without revealing the details, thus streamlining blockchain scalability.

The Structure of Zk-Rollups

Zk-rollups rely on two primary components to function:

  • On-chain Contracts define the rules governing the zk-rollup protocol. This setup includes a main contract that holds rollup blocks, tracks deposits, and facilitates key updates, as well as a verifier contract that checks the validity of zero-knowledge proofs.
  • Off-chain Virtual Machines perform transaction execution away from the base Ethereum blockchain, operating within a secondary layer. These virtual machines run independently from the Ethereum chain, processing transactions off-chain to avoid overloading the main network.

Zk-rollups are deeply integrated with the Ethereum blockchain, yet they manage transaction data in a way that avoids congesting the base layer. Instead of sending detailed information to Ethereum, they summarize transactions into bundled proofs, keeping the main blockchain efficient and uncluttered.

Evaluating the Advantages and Disadvantages of Zk-Rollups

Zk-rollups offer a number of significant benefits, but they also come with some notable drawbacks. Here's an outline of the pros and cons associated with their use.

Advantages of Zk-Rollups

  • Increased Throughput: Zk-rollups process transactions off-chain, allowing for greater scalability as individual transactions are not verified on the base layer, resulting in higher throughput.
  • Reduced Congestion: By limiting the amount of data processed on the main blockchain, zk-rollups help reduce congestion and minimize the risk of blockchain bloat. Full nodes are required to store only zero-knowledge proofs rather than all transaction data.
  • Lower Fees: With less congestion on the base layer, transaction fees are generally lower, providing a more cost-effective solution.
  • Enhanced Security: Zk-rollups are designed with robust security features that ensure users can retrieve funds even if issues occur within the rollup network. This is an advantage over sidechains, which may compromise funds if the network fails.
  • Faster Challenge Period: Zk-rollups verify only the validity proofs rather than each transaction, leading to a shorter challenge period.

Disadvantages of Zk-Rollups

  • Complexity: Zk-rollups are inherently complex to implement and manage, requiring more sophisticated cryptographic techniques compared to optimistic rollups.
  • Base Layer Constraints: Despite their efficiencies, zk-rollups still rely on the underlying base layer, which can impose certain limitations.
  • Liquidity Fragmentation: Layer 2 solutions like zk-rollups can lead to liquidity being spread across different layers, potentially creating issues when liquidity in the base layer becomes sparse.

By weighing these pros and cons, developers and users can better understand whether zk-rollups are the right choice for their specific blockchain needs.

Comparing Optimistic Rollups and Zk-Rollups

Here is a comparison between optimistic rollups and zk-rollups, focusing on their key differences and characteristics.

Optimistic Rollups

  • Assumption: Transactions are presumed valid unless proven otherwise.
  • Challenge System: There is a challenge period during which the network can contest suspicious transactions. Fraud proofs are used to validate disputes.
  • Complexity: Implementation is relatively straightforward.
  • Adoption: More commonly adopted due to lower complexity.
  • Examples: Notable examples include Optimism, Arbitrum, and opBNB.


  • Assumption: Every transaction is validated through zero-knowledge proofs.
  • Challenge System: No challenge period is required, as all transactions are pre-validated.
  • Complexity: Implementation involves complex cryptographic techniques due to the use of zero-knowledge proofs.
  • Adoption: Less widely adopted compared to optimistic rollups.
  • Examples: zkSync and Starknet.

Each approach has unique benefits and trade-offs, with optimistic rollups favoring simplicity and broader adoption, while zk-rollups offer enhanced security and reduced potential for fraud.


Scalability has become a critical issue in blockchain technology, and it's often seen as the key to unlocking its full potential. A system that can't scale is of limited use. Rollups, both optimistic and zk, have emerged as effective solutions to address this challenge. Zk-rollups, with their unique method, offer greater speed, reduced congestion, and enhanced security. While they do come with increased complexity, their advantages are significant. Anyone interested in the future of digital currencies should understand zk-rollups. As we strive for improved blockchain performance, this technology will play a crucial role in shaping the future.

Optimistic Rollup
Zero-Knowledge Proof
Layer 2