An introduction to the different types of cross bridge solutions


Inter-chain bridges

To achieve interoperability across a heterogeneous blockchain, several techniques have been developed. Here we will present three technical solutions for cross-chain transactions, analyze their characteristics and discuss their potential risks. These three solutions are notary, hash time lock, and relay model.

The model of the notary

The most intuitive approach to achieving cross-chain interoperability is to appoint a trusted third party, such as a notary, to coordinate cross-chain operations by being responsible for verifying and forwarding cross-chain transactions. chains. Two types of notaries can be used; a single-signature notary or a multiple-signature notary.

A single sign-on notary, also known as a centralized notary, collects transaction data from the source chain, validates it, and initiates execution of the transaction on the target chain. It is a simple model with high processing speed. However, its downside is its vulnerability to single node failure or misbehavior.

With a multi-signature notary, a cross-chain request, initiated by ‘Account A’ in ‘Blockchain 1’ must be successfully verified by the majority of notaries. Once verified, the signatures are posted to the corresponding transaction to be executed on ‘Blockchain 2’. In order to tolerate Byzantine faults, cross-chain transactions can only be processed if more than two-thirds of the notaries reach a consensus and sign the transaction.

Hash time lock

Blockchain 2.0 provided, for the first time, a trusted decentralized execution environment for smart contracts that automate asset management under trustless conditions. Smart contracts are automated protocols controlled by code, providing read and write functions as user interfaces and triggering specific operations based on user transactions, which operate in a trustless environment.

The hash time lock model is a cross-chain technology deployed by smart contracts. The specific process is as follows:

1. The initiator of the cross-chain transaction chooses a confidential random number S, then calculates the hash value h=Hash(S) of S and sends it to the responder of the cross-chain transaction

2. The initiator and responder lock their asset into the smart contract, with the intention of trading with each other on their respective blockchains. Both contracts are locked using the key h, while the key to unlock the assets is the random number S. The assets are locked in the smart contract for a duration denoted by T1 and T2 respectively, where T1 must be greater than T2. Only the initiator and the responder can unlock the respective assets using the random number S.

3. The initiator releases the contract assets by announcing S within the time period T2. If the time exceeds the duration set by the initiator, the contract automatically returns the assets to the responder.

The hash time locking model effectively solves the trust problem inherent in the cross-chain process. As long as the initiator maintains the secrecy of the random number S and the time window, (T1 – T2) provides enough time for the responder to unlock the assets, both parties can complete a cross-chain transaction in a decentralized manner.

The relay model

The relay model for cross-chain transactions is an abstraction of cross-chain operations where data verification in the cross-chain process is abstracted into a consensus problem at the relay layer. A blockchain with improved scalability is developed on this layer of abstraction and a third blockchain, the relay chain, then emerges for interoperability.

In the relay model, a series of relay nodes are deployed in each blockchain network, which are responsible for monitoring and synchronizing that blockchain’s transaction data with the relay chain. Relay chain consensus nodes verify the validity of cross-chain transactions and trigger the execution of corresponding transactions.

The figure and the steps listed show a typical relay model cross-chain transaction

  1. User initiates a cross-chain transaction request in the source chain

  2. The relay node monitors and synchronizes transaction data with the relay chain

  3. The relay chain consensus node verifies the validity of the transaction

  4. The consensus node constructs the corresponding transaction

  5. A super-majority of consensus nodes signs the transaction, forming a set of signatures

  6. Relay node monitors transactions and signatures

  7. The relay node transports the transaction to the target chain

The consensus mechanism of a relay chain determines the performance and security of the cross-chain service. Classical Byzantine fault-tolerant algorithms, such as PBFT, are able to achieve high throughput provided the supermajority of nodes are correct. Improved versions of Byzantine fault-tolerant algorithms, such as HotStuff, further reduce communication complexity and support a larger scale of nodes participating in consensus.

Relay chains are technically difficult to implement, as they require high levels of engineering complexity, however, they have advantages. Relay chains with smart contracts can form a cross-chain service network, with one relay chain communicating information between multiple blockchains, expanding the scope of value transfer. Poly Network is an example of a successful implementation of the cross-chain relay model.

The relay model achieves an abstraction layer for interoperability, allowing data to be securely exchanged and validated between heterogeneous blockchains. In the case of Poly Network, a relay chain was built for more efficient data exchange and validation. Beyond that, there is potential for indigenous economies to flourish within Poly Network’s ecosystem thanks to its comprehensive blockchain infrastructure.

Interoperability in Crypto

The three aforementioned cross-chain bridging solutions were developed to provide interoperability between heterogeneous blockchains, which provides many advantages. These solutions have been implemented and proven to work. In addition to this, new iterations and continuous improvements are made to these models.

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