INTRODUCTION

Public connectors are solutions that use a separate blockchain as a bridge to facilitate the exchange of data, assets, and messages between different blockchain networks without the need for intermediaries. Public connectors can be further divided into two subtypes: side chains and chain relays, and notary schemes. Here, I will discuss some of the examples and features of each subtype, and provide some use cases and benefits of public connectors.

Side chains and chain relays

Side chains and chain relays are solutions that use a secondary blockchain that is connected to a primary blockchain, and acts as a proxy or a relay for cross-chain communication and transactions. Side chains and chain relays can use different consensus mechanisms and security models, such as federated, delegated, or merged mining. Some examples of side chains and chain relays are:

- Liquid: Liquid is a side chain that is connected to the Bitcoin blockchain, and acts as a proxy for cross-chain transfers of Bitcoin and other assets. Liquid uses a federated consensus mechanism, where a set of trusted nodes validate and confirm transactions. Liquid also has a native token called L-BTC, which is pegged to Bitcoin and can be swapped at any time¹.

- Plasma: Plasma is a side chain that is connected to the Ethereum blockchain, and acts as a relay for cross-chain transactions of Ethereum and other tokens. Plasma uses a delegated consensus mechanism, where a single node or a group of nodes validate and confirm transactions. Plasma also has a native token called PLASMA, which is used for fees and rewards on the network.

- Polkadot: Polkadot is a network of interoperable blockchains that use the Cross-Chain Message Passing (XCMP) protocol, which is a method for cross-chain communication and transactions. Polkadot allows users to create and connect their own specialized blockchains, called parachains, and access a variety of decentralized applications and services across different blockchains. Polkadot also has a native token called DOT, which is used for governance, staking, and bonding on the network.

The main features of side chains and chain relays are:

- They enable fast and cheap cross-chain transactions, as they use a separate blockchain that can have higher throughput and lower fees than the primary blockchain.

- They enable flexible and customizable cross-chain transactions, as they can support different types of assets and functionalities that the primary blockchain may not have.

- They rely on the security and trust of the primary blockchain, as they inherit the consensus and validation rules of the primary blockchain, or use a subset of trusted nodes to secure the network.

Notary schemes

Notary schemes are solutions that use a set of trusted or semi-trusted nodes that act as validators or witnesses for cross-chain communication and transactions. Notary schemes can use different methods and techniques, such as threshold signatures, multisignatures, or zero-knowledge proofs. Some examples of notary schemes are:

- BTC Relay: BTC Relay is a notary scheme that uses a smart contract on the Ethereum blockchain to verify and relay Bitcoin transactions. BTC Relay uses a threshold signature method, where a quorum of nodes sign and verify Bitcoin transactions using a secret sharing scheme. BTC Relay also has a native token called BTCR, which is used for fees and rewards on the network.

- Interledger: Interledger is a notary scheme that uses a set of connectors that act as validators or witnesses for cross-chain payments of any currency or token. Interledger uses a hash time-locked contract (HTLC) method, where each connector locks a portion of the payment until the final recipient confirms the receipt. Interledger also has a native protocol called ILP, which is used for routing and messaging.

- Ren: Ren is a notary scheme that uses a network of nodes, called darknodes, that run a secure multiparty computation (SMPC) algorithm, which is a technique for cross-chain verification and validation. Ren allows users to transfer their cryptocurrencies and tokens across different blockchains without converting them or using centralized exchanges. Ren also has a native token called REN, which is used for fees and rewards on the network.

The main features of notary schemes are:

- They enable secure and trustless cross-chain transactions, as they use cryptographic protocols and smart contracts to ensure the accuracy and authenticity of the data and assets that are transferred across different blockchains.

- They enable interoperable and scalable cross-chain transactions, as they can support any type of currency or token, and handle large volumes of transactions without congesting the network.

- They rely on the incentives and reputation of the nodes, as they use economic and social mechanisms to encourage and reward the nodes for their honest and reliable behavior, and penalize or exclude the nodes for their malicious or faulty behavior.