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Scroll in an EVM-compatible ZK-Rollup that will help scale Ethereum Mainnet through the standardization of the layer-2 proof outsourcing market.

Technology

Scroll aims to be a layer-2 blockchain protocol that provides scaling solutions for the Ethereum mainnet. At its technical core, Scroll is being developed as a zero-knowledge rollup (ZK-rollup). ZK-rollups create large bundles of transactions for off-chain computation in the form of cryptographic proofs known as SNARKs. The SNARKs serve as a validating proof between the layer-2 data and the mainnet, giving a rollup like Scroll the advantage of maintaining layer-1 security with layer-2 finality speeds.

Scroll will support direct mainnet Ethereum validation through a single succinct proof. In many ways, Scroll operates in reverse of typical zk-rollups and is utilizing the EVM with custom optimizations to significantly boost Scroll block finality speeds while remaining compatible with all Ethereum infrastructure.

System Architecture

All the core infrastructure of the Scroll network relies on the success of the zkEVM. Prior to Scroll’s attempt, a successful native EVM circuit had never been constructed. Scroll is aiming to be the first by identifying and addressing previously unsolved issues with the concept.

Broad overview of zkEVM architecture | Source​​​​​​

Scroll's system architecture surrounding the zkEVM design has been made possible through recent breakthroughs, including:

• Polynomial Commitment - This allows a system to operate a circuit without size constraints that previously made circuits inefficient
• Customized Gadgets - The usage of lookup tables and customized gadgets reduce overall overhead for EVM circuit design
• Overall Hardware Acceleration - Advancements if GPU / CPU speed and other hardware components are making systems like EVM circuits more feasible, including a custom write-up from the Scroll team outlining a 5x improvement over Filecoin's GPU

These advancements allow for the zkEVM design to function, paving the way forward for the layer-2 architecture of the system to be built around it. In this architecture, we find three core pieces of infrastructure.

Those three pieces are:

• Scroll Node
• Roller Network
• Rollup & Bridge Contracts

The Scroll node is what formally creates the blocks and commits them to the Ethereum Mainnet layer. In other words, the nodes on Scroll are the messengers between the layer-1 and layer-2 networks. The nodes have three different components: sequencer, coordinator, & relayer.

Scroll architecture.

Sequencers receive the layer-2 transactions in batches and then generate layer-2 blocks with that information. After the sequencer generates a new block, the coordinator is notified and executes a trace before dispatching the execution trace to a roller. The roller is a part of the Roller Network, which operates through a decentralized, random leader selection algorithm to generate a proof.

Rollers serve as the ‘provers’ of the system in that they hold the responsibility of generating validation proofs for the rollup. Each zkEVM circuit has a generated proof from the Roller Network. Prior to releasing these proofs, they are combined into a single block proof to speed up block finality & overall efficiency. This idea also relies on the rollers utilizing top of the line GPUs and other hardware.

Roller workflow.

Two sets of smart contracts make the Scroll rollup functionality possible: rollup contracts and bridge contracts. The rollup contracts receive the layer-2 blocks from the sequencer and then store the state roots in the Ethereum state in the form of calldata. This does two things:

• Ensures layer-2 block data is always available
• Gives Scroll the advantage of using Ethereum's security

The other side of this process is the bridge contracts. These are deployed on both systems (Scroll & Ethereum) as a network messaging protocol. On top of this messaging protocol rests an ERC-20 token bridging protocol that users can leverage to transfer tokens in either direction.

Scroll workflow.

UniPass Smart Contract Wallet

Scroll has expanded its functionality in Q4 2022 with the release of the UniPass smart contract wallet integration. The wallet allows anyone to sign up via their existing email or web2 social logins and interact with the Scroll network. 

Source

Gas and network fees can also be paid in any token, meaning that UniPass wallet users have the ability to transact without restriction on the Scroll network. This is the first wallet integration of its kind. A demo version of the wallet integration is available for users to try

Vulnerabilities 

There are a handful of key vulnerabilities that are worth noting on the Scroll architecture & infrastructure. The first of these is the initial reliance on a centralized sequencer node or nodes. There is a degree of centralization required in the launch of such a system as it must be in a trusted state. These initial developers (i.e., the Scroll team) will have to be trusted by first adopters. Scroll has stated that there are plans to decentralize the sequencing nodes at a later date. 

Outside of initial centralization issues, zk-proofs tend to be slow and require a high amount of data optimization, creating a barrier to entry for participation in the layer-2 proof verification market. This can create a mover’s first advantage for Scroll but only if their in-house GPU design proves to be a competitive advantage in the data computation market. 

Zk-proof execution speeds are also reliant on novel breakthroughs in specialized hardware (similar to what Scroll’s in-house hardware design relies on). Two of these breakthroughs that are notable for this circumstance are the Field Programmable Gate Arrays (FPGAs) and Application Specific Integrated Circuits (ASICs). An ASIC is at the heart of Scroll’s hardware designs and it has been theorized the execution speeds of zk-proofs are anywhere between 10x and 1000x.

Because Scroll has not gone live yet, there is a lack of concrete data to support the theories surrounding the architecture of the layer-2 system. The successful launch of a zkEVM engine will be an industry first and immediately introduce layer-2 compatibility to all existing decentralized applications existing on the Ethereum mainnet. So, applications like Aave could easily migrate to layer-2 without the complexities of learning a unique programming language (i.e., zk). With more data and documentation, these theories can begin to materialize into verifiable open source for developers to vindicate.