Unveiling ZK in Blockchain: A Comprehensive Overview of ZKEVM and Innovative Projects

ZK Rollup is the star of Layer 2 projects, and its difficulty is also exceptionally high. The basic idea is to utilize zk technology to submit the execution results of Layer 2 along with a proof to Layer 1, which then verifies and puts it on-chain.

In Layer 2, a virtual machine (VM) is required to execute bundled transactions. This type of VM based on zk technology is called zkVM. If this virtual machine is compatible with Ethereum smart contracts, it is referred to as zkEVM.

EVM

First, let’s understand the execution process of an EVM contract: The contract is compiled into bytecode, and then the bytecode is executed through the EVM.

What is zkEVM?

zkEVM is a virtual machine that supports the execution of Solidity contracts through zk technology.

According to Vitalik’s classification, there are four types of zkEVM:

*https://vitalik.ca/general/2022/08/04/zkevm.html

• Type 1, fully Ethereum-compatible, refers to an EVM that can directly run on the Ethereum chain, and it is the most challenging to implement. PSE (formerly known as appliedzkp) and Taiko are currently working on it;
• Type 2, fully EVM-equivalent, means that it is compatible with the EVM but not necessarily with Ethereum. It can run EVM contracts, but the data structure and state tree may be different. It can operate on a chain separate from Ethereum. Currently, projects like Scroll and Polygon are working on Type 2 zkEVM implementations;
• Type 2.5 refers to an implementation of zkEVM that is nearly EVM-equivalent, with the only difference being the consumption of gas fees. It closely resembles the EVM in functionality, but the gas fee mechanism may vary;
• Type 3 refers to an implementation of zkEVM that is nearly EVM-equivalent but with some functional differences. Type 3 is considered a transitional type, where projects like Scroll and Polygon currently fall under this category. However, the ultimate goal for them is to achieve either Type 2 or Type 2.5 zkEVM, which means being fully EVM-equivalent while offering lower gas consumption;
• Type 4 refers to a language-compatible ZK-EVM, which can compile Solidity or other languages smart contracts but has little connection to the underlying EVM. The prominent projects in this category are StarkWare and zkSync. These projects focus on providing a high-level language compatibility for developing smart contracts while utilizing zk technology, but they are not directly based on the EVM.

Vitalik’s four classifications are viewed from a product perspective. From an implementation perspective, there are generally two approaches to implementing zkEVM:

• Language-level compatibility with EVM: which is similar to Type 4, where only the smart contract itself is supported at the language level, but the underlying implementation has little connection to the EVM. Products following this approach include StarkWare’s StarkNet and zkSync 2.0. The characteristic of this approach is that the underlying virtual machine instruction set differs from the EVM, which means some existing development tools cannot be directly used. However, it is more ZK-friendly and enables more efficient generation of zero-knowledge proofs.
• Bytecode-level, which is similar to Type 2 or Type 2.5. In this approach, the high-level language is first compiled into bytecode similar to the EVM, and then each bytecode is implemented using zk circuits to represent its logic. The main products following this approach are Polygon ZK-EVM and Scroll. The characteristic of this approach is the realization of a fully compatible Ethereum extension with zero-knowledge (ZK) capabilities. All existing smart contracts, developer tools, and wallets can seamlessly operate on it, but it also introduces increased complexity.

Zero-knowledge proof algorithms

Currently, there are two mainstream zero-knowledge proof (zkp) algorithms.

• ZK-SNARK (Zero-Knowledge Succinct Non-Interactive Argument of Knowledge)
• ZK-STARK(Zero-Knowledge Scalable Transparent Argument of Knowledge)

Among them, ZK-SNARK has a trusted setup process, which involves precomputing a set of parameters using secret information. If this secret information is leaked, it can potentially compromise the security of the system. Therefore, this secret information is also referred to as “toxic waste,” as it poses a security risk.

For ZK-STARK, one of its design goals is to eliminate the need for a trusted setup. It utilizes advanced mathematical techniques such as polynomial interpolation and low-degree pseudo-random functions. These techniques make the process of constructing STARK parameters more transparent, decentralized, and independent of toxic waste. This advantage of STARK in terms of verifiability and scalability provides higher security and trustworthiness in the deployment and usage of the system.

What is the relationship between zkp and zkEVM?

• zkEVM is a product that supports the execution of smart contracts;
• The zkp algorithm is a technology, and zkEVM implements one of these zkp algorithms.

Related projects to zkEVM

Applied ZKP

The Ethereum Foundation’s zk project, aiming for Type 1 zkEVM implementation, which means achieving full Ethereum compatibility.

Open-source repository:?https://github.com/privacy-scaling-explorations/zkevm-specs

Polygon zkEVM

Polygon zkEVM is a result of Polygon’s acquisition and integration of a ZKR (Zero-Knowledge Rollup) project called Hermez. It operates by implementing EVM opcodes through circuits.

Open-source repository:?https://github.com/orgs/0xPolygonHermez/repositories

Currently, Polygon zkEVM is classified as Type 3, but the ultimate goal is to achieve Type 2 to Type 2.5 compatibility.

Scroll

Scroll is also open-source and can be found at?https://github.com/scroll-tech

Similar to Polygon, Scroll operates by implementing EVM opcodes through circuits.

Currently, Scroll is categorized as Type 3, but the ultimate goal is to achieve Type 2 to Type 2.5 compatibility.

Taiko

Taiko’s goal is also to achieve Type 1 compatibility, which means aiming for full Ethereum compatibility in their implementation.

Open-source repository:?https://github.com/taikoxyz

zkSync

zkSync belongs to Type 4. Currently, it is open-source. Thanks for Porter Adams’ reminder.?@porteradams12

https://github.com/matter-labs/zksync

zkSync is the collective term for the entire zkRollup protocol developed by Matter Labs.

zkSync Era is a chain that runs zkVM.

The approach of zkSync involves using its own LLVM compiler to compile contract code, such as Solidity code, into the desired bytecode format (instead of EVM opcodes). This compiled bytecode is then passed to zkVM for execution.

Starkware

StarkWare is an Israeli technology company that specializes in building scalable solutions based on zkSTARK (Zero-Knowledge Scalable Transparent Argument of Knowledge).

StarkNet is the blockchain operated by StarkWare.

SNVM (StarkNet Virtual Machine) is the contract virtual machine running on StarkNet. The SNVM virtual machine utilizes zkSTARK technology to enable smart contract execution.

Cairo is the programming language used to write smart contracts that run on StarkNet.

Originally, StarkNet and zkEVM were not directly related, as StarkNet primarily focused on zkVM. However, Nethermind’s Warp project provides a compiler that can compile Solidity code into Cairo, effectively transforming StarkWare’s StarkNet into a Type 4 implementation.

Below is a comparison of various products:

The further to the right, the better the EVM compatibility, but also the greater the implementation difficulty.

Ref

• https://foresightnews.pro/article/detail/5364
• https://ethereum.org/en/developers/docs/scaling/zk-rollup
• https://drive.google.com/file/d/1dhZ5GtSK4sHCNcklGzNHfR-lSzplpD8J/view
• https://vitalik.ca/general/2022/08/04/zkevm.html