Sui Bridge Explained: How It Moves Assets to Sui

Sui Bridge is the native cross-chain bridge implemented within the Sui protocol, enabling users to transfer assets like ETH, USDC, and wrapped BTC from Ethereum to Sui and vice versa. Contrary to bridges built by external teams, the logic of Sui Bridge is embedded into the Sui codebase and is protected by the same validator committee that operates the Sui blockchain.

For a nascent Layer 1 network, a reliable bridge is often the key to establishing liquidity on the blockchain. Sui Bridge ensures that Ethereum users have a protocol-level way to migrate their assets to Sui, while providing the Sui ecosystem with access to liquid assets that have already established themselves on Ethereum.

What Is Sui Bridge?

Sui Bridge went live on the Sui mainnet on September 30, 2024, after a testnet phase that began in June of the same year. The bridge was developed by Mysten Labs, the company behind the Sui protocol, in collaboration with the Sui Foundation. Unlike most bridges, Sui Bridge is an integral part of the core Sui protocol, rather than an additional application layered over it.

Wormhole Bridge was the only choice for transferring assets between Ethereum and Sui before Sui Bridge appeared. While Wormhole is still operational, it relies on its own independent validator network --- guardians — to observe and attest to the events happening on different chains. Sui Bridge, in turn, leverages the validator committee that already guarantees the operation of Sui's consensus layer.

Why Sui Needed a Bridge

Sui is a Layer 1 blockchain, which is a product of Mysten Labs, the company whose founding members worked on the Diem project of Meta. The blockchain relies on the Move programming language that was initially developed for Diem. In Sui, all assets — coins, NFTs, etc. — are organized as separate objects, each belonging to a specific account. Together with the Mysticeti consensus protocol, this allows for processing of many transactions in parallel and for achieving consensus rapidly.

As a young Layer 1 network, Sui has little liquidity of its own. Bringing over established assets such as ETH and USDC will enable users of Sui to trade, borrow, and provide liquidity on Sui-based DApps right away. A bridge is necessary to accomplish this task without the need to sell Ethereum tokens in order to purchase new ones.

How Sui Bridge Works

Sui Bridge uses the lock-and-mint design, also referred to as the burn-and-mint design, depending on the direction of the transfer. This design is used by a great many blockchain bridges, although methods of asset verification differ greatly between projects.

Transfer of Assets From Ethereum To Sui

There are several steps to transferring an asset from Ethereum to Sui.

• A user sends a request to the Sui Bridge smart contract deployed on Ethereum.

• The contract locks or burns the asset.

• The transaction gets finalized and emits an event on Ethereum.

• The Sui validator committee independently observes the event through a trusted Ethereum full node and signs an attestation verifying the event.

• The signatures are aggregated and sent to the Sui blockchain.

• The bridge package on the Sui blockchain verifies the signatures.

• The corresponding asset is minted or unlocked on Sui and made claimable.

• The user or, more accurately, anyone who has the transaction details, claims the asset in the destination Sui wallet.

Transfer of Assets From Sui to Ethereum

The procedure is almost identical. An asset is locked or burned on Sui, the validator committee observes the event through a trusted Sui full node, and the aggregated signatures are sent to the Ethereum smart contract, which verifies the signatures and unlocks/mints the asset for the destination Ethereum wallet.

Since both transfer directions use the same validator committee, Sui Bridge does not introduce any extra trust assumption — a common vulnerability of bridges built by external teams.

The Bridge Validator Committee

The validators who attest to Sui Bridge events are the same ones that guarantee the operation of Sui's consensus layer. The reuse of the same validator set is the very essence of a native bridge: compromising the Sui Bridge would mean compromising the entire Sui network, rather than a separate, sometimes smaller, subset of validators.

It stands in contrast to bridges with their own guardian networks, oracle committees, or multisig wallets — where the signing/verification process can be compromised without compromising the entire blockchain. It is this reduced attack surface that is one of the reasons why bridges became frequent targets of hacking attacks in the past. Wormhole and Ronin bridges have lost hundreds of millions of dollars of assets due to a compromise of their signing process, and not because of flaws in the corresponding blockchains.

The Limiter: A Safety Valve

Sui Bridge includes the limiter functionality — which puts a cap on the total value of assets that can be withdrawn from Sui Bridge within a rolling twenty-four-hour period. The value tracking is done hourly, and the cap is adjusted separately for each direction, as the threat of withdrawing assets from Ethereum is different from the threat of withdrawing assets from Sui.

Validators of Sui Bridge set and adjust the caps via a governance vote, which is publicly announced on the official channels of Sui. The limiter also limits the value of a single transaction to limit how many assets an attacker can withdraw even in a case of a compromise of the signing process.

Supported Assets

Sui Bridge was launched with support for ETH and WETH — wrapped ETH --- transfers from Ethereum to Sui and vice versa. Since then, Sui Foundation expanded the list of supported assets to include wrapped BTC, USDC, and USDT. More assets will likely be added as the bridge gains traction.

However, native USDC on Sui does not go through Sui Bridge. Sui was integrated by Circle, the issuer of USDC, into the Cross-Chain Transfer Protocol — CCTP — which burns and mints USDC directly, without creating a wrapped asset. It is a separate system that performs the same task as Sui Bridge, even though both solve the same underlying problem of moving value across chains.

Native Bridging Versus Wrapped Assets

In terms of usability, there is one notable difference between Sui Bridge and third-party bridges. Sui Bridge is designed so that the ETH transferred from Ethereum looks more native inside the Sui ecosystem. A bridge like Wormhole's Portal Bridge transfers a wrapped token --- e.g., WETH — rather than a native asset.

That difference matters in terms of composability. A wrapped asset requires all applications supporting it to integrate with it explicitly. Moreover, the liquidity of the wrapped asset can be separated from the liquidity of other versions of the same token. Fewer and more consistent representations of an asset make it easier for a Sui-based application to offer a lot of liquidity for the asset without fragmentation of that liquidity.

Sui Bridge Versus Third-Party Bridges

Several other bridges connect assets to Sui, and each relies on its own trust model.

• Wormhole provides access to Sui through its Portal Bridge interface, connecting it to more than twenty other chains through its guardian network that observes and signs cross-chain events.

• deBridge and Celer's cBridge are liquidity-based bridges, relying on their own relayer and validator infrastructure, and the pooled liquidity in the form of the assets held on both sides of the transfer.

• Sui Bridge works only with Ethereum and Sui currently, but benefits from the security provided by Sui's own validator set and from being audited as an integral part of the core protocol, rather than as a separate application.

The choice between the options often comes down to which chain one wants to move assets from, which representation of the asset one wants to get inside Sui, and what degree of trust one is willing to place in a separate operator network.

Fees and Bridging Time

Upon the launch, Sui Bridge did not charge a separate protocol fee for using it. Users have to pay gas fees on Ethereum, which are necessary for initiating a transfer and, in some cases, claiming the transferred asset. The gas fees on Sui were, at times, paid by the network itself in order to lower the barrier for newcomers testing the bridge.

The bridging time depends mostly on the Ethereum side. The transfer of assets from Ethereum to Sui requires the Ethereum finality to occur before the Sui validator committee is able to verify the event. In practice, it usually takes about fifteen to twenty minutes. Transfers from Sui to Ethereum benefit from Sui's fast finality on the Sui side, but still depend on the confirmation of the corresponding transaction on the Ethereum side.

How to Use Sui Bridge

Transferring assets via Sui Bridge follows a common pattern among bridges.

• Conducting a connection to the Sui Bridge site from an Ethereum wallet — for example, MetaMask — and a Sui wallet — e.g., Slush Wallet.

• Selection of the asset and the direction of the transfer.

• Initiating the transfer and paying the necessary gas fee.

• Waiting for the finality and the processing of the attestation by the validator committee.

• Claiming the asset when the bridge website marks it as available, delivering it to the destination wallet address.

The bridge typically shows the status of the transaction going through several phases: initiated, verified, claimed, delivered, and so on. However, it is always possible to check the details on a blockchain explorer if one wishes to do so.

Audits and Ongoing Development

Sui Bridge underwent independent security audits. The Sui Foundation also conducts a bug bounty program for the discovery and disclosure of vulnerabilities — which is a standard practice for infrastructure managing a significant amount of value. Audits and a bug bounty program provide an additional layer of protection on top of the code review conducted by the core development team.

The bridge is still evolving. Its early versions were designed to support a limited number of assets and work with only one counterpart chain — Ethereum — to ensure stability and ease of monitoring. According to the Sui documentation and roadmap, future versions of the bridge will support custom cross-chain messages, which would allow for triggering actions on one chain from the smart contracts on another. Support for additional chains may be added afterwards.

Risks and Practical Considerations

Bridges, by definition, are risky, as they involve smart contracts issuing assets on the basis of events happening on other chains. Sui Bridge minimizes several common risks of bridges by tying its security to Sui's validator set, conducting audits, offering bug bounties, and having the limiter functionality.

However, there are still ways to reduce personal risks associated with the usage of any bridges. First of all, testing a small transfer amount first is always advisable — especially when using a bridge for the first time or transferring a new asset pair. Also, it is important to remember that a bridged asset — either a native or a wrapped version of it — may behave differently in different Sui applications.