substrate-rollup

This repository serves as a Proof of Concept (POC) for a prover which can serve as a zkrollup of a Substrate-based chain. Our goal is to explore the possibilities of enabling Substrate/rust developers to leverage zk proofs generally, and not necessarily within the context of rollups, without the difficulty of circuit design, and at any level throughout the Substrate stack.

We try to exemplify this through a zk-prover of an extrinsic: balance transfers, for use in a zkrollup. Substrate developers are in a unique position to selectively roll up parts of their chain and enshrine the rollup results simply and in pasteable code similar to the pallet that we show due to Substrate's modularity. In the future, it may be possible to rollup any substrate pallet through some process that generates the appropriate guest code for it. In lieau of such a process, we show that this is a simple process to perform manually.

Technology and Terms

Risc0

Risc0 is a general purpose STARK-based zero-knowledge virtual machine(ZKVM). Developers can write programs in languages such as Rust and C++ using Risc0, and produce a proof based on the program's execution which allows a separate party to verify that program's execution without access to the inputs that were given to it. For a more technically accurate and detailed explanation, see the Risc0 docs. In this project's documentation, we make claims about ease and accessibility around zk proofs and developer's ability to generate them from pure rust - that is all enabled by Risc0's technology.

Substrate

Substrate is a blockchain development framework written in Rust. Substrate provides out-of-the box client modules for networking, database, consensus, and a WASM runtime which boasts more modularity in the form of a concept of "pallets". Pallets are a notable feature because they give the blockchain developer a different mindset around deploying onchain logic compared to smart contracts due to the level of control and access they have over the blockchain. You can find the Substrate documentation website here.

This level of control is demonstrated in this project, as we demonstrate giving trust to the verified proofs generated by the known prover + known program to execute large state changes onchain, in a way we would not be able to on other blockchains. This is somewhat of a Substrate analogue to enshrined rollups, given in a few lines of code.

Goals

Our project has the following objectives:

• Empowering Substrate/Rust Developers: We want to provide an alternative approach for Substrate developers to build zk rollups using the familiar Rust programming language, rather than dealing with circuit design.

• Understanding Risc0 and Rollup Architecture: Through this project, we will dive deep into the Risc0 technology and explore its potential for building robust and scalable rollup systems.

• Performance Evaluation: We will evaluate the performance of Risc0 proving for various computations and assess whether it meets the requirements for processing Substrate extrinsics efficiently.

Project Overview

Our project comprises the following components:

• Prover: This component onchain state and requested transactions as input, computes the transactions, and outputs the STARK proof and updated state. As of this writing, the onchain state consists of a set of account balances, the requested transactions constist of a set of balance transfers.

  The input is given via local json file: transactions.json. The host-side of the prover receives the transactions and verifies the signatures associated with each.

  The transaction processing and proving occurs inside of Risc0's zkvm. The output consists of a Risc0 journal and receipt. The receipt contains the STARK proof, and the journal contains values we've committed to, i.e. our updated onchain state values.

  Finally, the host receives the receipt and journal from the Risc0 zkvm, constructs a transaction for the custom pallet with them, and sends it to a locally running Substrate node which has the custom pallet, via Subxt.

  You can find this code in ./provers/transfer. The code which runs inside of the Risc0 zkvm/guest can be found in ./provers/transfer/methods/guest/src/bin/transfer.rs

• Substrate Node with Verification Pallet: This is a substrate template node with a custom verification pallet. The verification pallet is responsible for validating the STARK proof sent by the prover, ensuring the integrity and correctness of the computed transfers, and then updating the chain's state in totality with the results included with the proof.

The typical Substrate structure of ./node, ./runtime, and ./pallets exists in this project. The custom pallet can be found in ./pallets/template

Performance

Some initial testing(not actual benchmarks) Macbook i7 16GB RAM(without metal feature)

• 3 transfer extrinsics: 12 secs
• 30 transfer extrinsics: 21 secs
• 50 transfer extrinsics: 28 secs

Development

Local development is based around the main workflow of sending proofs to the locally running Substrate node.

Pre-requisites

Install Rust Substrate

Start local node

1. In the root of the directory:

cargo build --release

2. Start the node with local dev options:

./target/release/node-template --dev

Run Prover

1. Navigate to prover directory

cd ./provers/transfer

2. Build:

cargo build --release

3. Run prover

../../target/release/prover-host

The prover will prove the transactions in ./provers/transfer/transactions.json, send the proofs to the chain, which will verify and change the balances state, if the proof is verified.

When making changes: ensure you keep the image id and subxt metadata up-to-date to avoid errors. See provers/transfer/README.md