The landscape of trustless bridges on Polkadot

With research and writing from Oliver Brett, Adrian Catangiu, and Aidan Musnitzky, this article explores the rich environment of bridge building, both within Polkadot and to external ecosystems.

Any Web3 protocol with true aspirations of interoperability needs to consider the development and deployment of bridges to external networks, and in this sense Polkadot is no different.

Blockchain bridges↗ are, in essence, mechanisms for two sovereign chains with different technological foundations to open up a channel through which they can exchange assets and data. But no two bridges are quite the same as each other, and they sit on a scale of centralization.

At one extreme, you have fully centralized bridges controlled by a third party that each chain must trust, while at the other there are those that are fully trustless and decentralized. It is perhaps no surprise that Polkadot tends to favor the latter path (though there is nothing that would, in principle, prevent a team from building a centralized option).It is well known that centralized bridges have historically posed a significant risk of theft because intermediaries are prone to misuse funds, either deliberately or accidentally. Early iterations of such bridges have been compromised by their reliance on third-party data and the absence of regulatory oversight.

Why does Polkadot need external bridging?

In order for Polkadot to thrive across Web3, and not just by supporting and leveraging its own ecosystem, bridges are essential.

They allow Polkadot to extend its branches beyond its own walled garden, and that’s important because however vibrant it is, Polkadot needs to help its users engage with the other cool stuff out there.

Blockchains at the Layer 0 or Layer 1 level exist in isolated environments with distinct rules and consensus mechanisms↗. This isolation prohibits native communication between them, and assets cannot freely traverse these boundaries. Bridges break down the boundaries by opening up:

Cross-Chain Asset Transfer: Empowering users to shift assets across different blockchain networks effortlessly.

Diverse Dapp Access: Dapps can harness the unique strengths of various blockchains, enhancing their capabilities and design space for innovation.

User Accessibility: Bridges enable users to explore new platforms and leverage the benefits of diverse blockchain ecosystems.

A vital milestone: Hello BEEFY

You may have seen references to BEEFY↗ on Polkadot-centric social media accounts, and wondered what exactly it is.

BEEFY stands for Bridge Efficiency Enabling Finality Yielder. Earlier this week it was enabled on Polkadot↗, triggering ripples of excitement across the ecosystem.

This protocol supports efficient bridging between Polkadot’s central Relay Chain and segregated “foreign” blockchains, such as Ethereum, which were not built with the Polkadot interchain operability in mind.

BEEFY allows participants of the remote network to verify finality proofs created by Polkadot’s Relay Chain validators. In other words: clients in the Ethereum network should be able to verify that the Polkadot network is at a specific state.

Storing all the information necessary to verify the state of the remote chain triggers prohibitive costs. BEEFY addresses these concerns by providing a more lightweight and efficient finality solution.

BEEFY operates on top of a pre-existing protocol called GRANDPA, utilizing a consensus extension and a light client protocol. This provides a more agile solution: smaller consensus justifications and efficient communication between nodes.

It enhances the efficiency and reliability of cross-chain communication, both within the Polkadot ecosystem by providing a lightweight finality solution compatible with a variety of target chains.

A big landmark: Bridging to Kusama

The Bridge Hub system parachain has operated on Polkadot since 2023. The Bridge Hub↗ has all the required bridge pallets in its runtime to enable trustless bridging with other networks. It draws its security entirely from the Relay Chain and governance directly from the DOT token.

Significantly, it has already achieved a notable landmark by serving as the launchpad for the Polkadot <> Kusama bridge, the first trustless bridge for the ecosystem, with one of the most secure designs in the space.

This particular bridge represents a major stride forward in the realm of blockchain technology, offering a vivid illustration of what true interoperability looks like in practice. It brings to life the vision of a seamlessly interconnected blockchain ecosystem.

Showcasing how any type of on-chain asset can be fluidly moved and managed between the Polkadot and Kusama networks, it will help enhance liquidity and opens new avenues for decentralized applications.

The significance of the Polkadot <> Kusama bridge goes beyond its current capabilities. It sets a precedent for future blockchain developments, highlighting the potential for more complex and integrated blockchain applications. The challenges and considerations faced during its development provided valuable insights for users and developers, contributing to the ongoing discourse on scalability, security, and user experience in blockchain technology.

The Bridge Hub also has all the technical features in place to allow developers to explore a myriad of other bridging options with Polkadot as the source (or origin).

Snowbridge: A trustless path to Ethereum

Snowbridge↗, a long-awaited initiative brought by the Snowfork team, with assistance from Parity Technologies, is another important bridging upgrade for the Polkadot ecosystem. Currently all Ethereum assets in the Polkadot ecosystem flow through bridges that rely on third-party signatures - which is still standard procedure across Web3. These include bridging or governance signatures from private keys, tokenholders or validators that do not match either Polkadot or Ethereum’s validator set.

All parachains and dapps in the Polkadot ecosystem where these assets flow now have their trust model handicapped by these third parties, rather than taking their full security from the Polkadot Relay Chain - and this negatively impacts Polkadot’s overall scope. 

Snowbridge’s model is trustless, relying only on signatures from first-party Polkadot and Ethereum validators and transparent, permissionless and auditable on-chain logic. Just like the Polkadot <> Kusama bridge, it runs on Bridge Hub. Snowbridge’s imminent launch represents a major opportunity for growth for Polkadot, unlocking a fresh wave of interactions with Ethereum.

Hyperbridge: An alternative way forward

Hyperbridge (short for hyper-scalable bridge) is a technological advancement developed by Polytope Labs. Rather than being specifically built as a Polkadot <> Ethereum bridge, it is crafted to enable secure and state-proof-based interoperability across all blockchains and all consensus protocols. It does this with its own chain that performs cryptography verification on behalf of other blockchains.

Polytope Labs maintains a secure connection between blockchains which requires verification of consensus, consensus faults, state proofs, and state transitions.But this is a complex set of processes that is prone to centralization risks and exploits on account of human error.

Polytope’s solution is to develop a coprocessor model ↗made possible by Polkadot’s architecture. The computational load is taken off-chain with only the cryptographic proofs of the execution brought on-chain. Bottlenecks are avoided by sharing the workload across multiple parachains using Polkadot as the computation layer. Hyperbridge’s ambitious plans include Polkadot becoming a data availability layer for Ethereum.

Conclusion: Polkadot’s bridge-built future

Blockchain Bridges are essential lifelines in the complex web of interconnected blockchain networks. Polkadot is committed to exploring these in a way that allows bridging to be implemented in a trustless and decentralized manner, while retaining Relay Chain-level security and scalability.

To this end, it has already opened up the Polkadot <> Kusama bridge, and with Snowbridge and Hyperbridge potentially following on swiftly, there is much for both the existing community and future adopters to look forward to. It is fair to say that Polkadot has adopted a truly pioneering and revolutionary approach to bridging in Web3 that is likely to stand the test of time.