# Bridges

A cornerstone technology of blockchain interoperability is the blockchain bridge. Blockchain bridges are ways for two economically sovereign and technologically diverse chains to communicate with each other. Bridge designs come in a variety of flavors ranging from centralised and trusted to more decentralised and trustless. Polkadot favors the latter bridge designs for its ecosystem. However, there is nothing that blocks a development team from building and deploying the former.

While bridge designs are now getting to a place where they are sufficiently planned out, there have not been too many used heavily in production. For this reason, you can consider this page a work in progress. It will be updated as more information is determined and available.

##### Purpose of bridging

Bridges are specifically for making the Polkadot ecosystem compatible with external blockchains such as Bitcoin, Ethereum, or Tezos (among others). For information on XCM, the native interoperability technology that allows parachains to communicate trustlessly, please see the dedicated cross consensus page on the Wiki.

## Bridging Methods​

Building a bridge that is as decentralised and trustless as possible can be done through any of the following methods (ordered by suggested methodology):

• Bridge pallets - For Substrate-native chains, use a bridge pallet (e.g. Kusama \<\> Polkadot bridge, since both networks' parachains use Substrate).
• Smart contracts - If the chain is not on Substrate, you should have smart contracts on the non-Substrate chain to bridge (e.g. Ethereum mainnet will have a bridge smart contract that initiates Eth transactions based on incoming XCMP messages).
• Higher-order protocols - If your chain does not support smart contracts (e.g. Bitcoin), you should use XClaim or similar protocols to bridge.

### via Bridge Pallets​

Receiving messages on Polkadot from an external, non-parachain blockchain can be possible through a Substrate pallet. The Substrate instance can then be deployed to Polkadot either as a system-level parachain (native extension to the core Polkadot software) or as a community-operated parachain.

An example of a bridge that would strictly use bridge pallets would be a Kusama \<\> Polkadot bridge, since both use parachains based on Substrate.

For the standalone chains that will not have a parachain bridging module on Polkadot (non-Substrate), it will be necessary to deploy bridge contracts (see below).

### via Smart Contracts​

Given the generality of blockchain platforms with Turing-complete smart contract languages, it is possible to bridge Polkadot and any other smart contract capable blockchain.

Those who are already familiar with Ethereum may know of the now archived Parity Bridge and the efforts being made to connect PoA sidechains to the Ethereum mainnet. The Parity bridge is a combination of two smart contracts, one deployed on each chain, that allow for cross-chain transfers of value. As an example of usage, the initial Parity Bridge proof of concept connects two Ethereum chains, main and side. Ether deposited into the contract on main generates a balance denominated in ERC-20 tokens on side. Conversely, ERC-20 tokens deposited back into the contract on side can free up Ether on main.

##### note

To learn more on how Bitcoin and Ethereum can cooperate and collaborate Through Polkadot, check out this explainer video here

### via Higher-Order Protocols​

Higher-order protocols (like XCLAIM) can be used to bridge but should only be used when other options are not available. XCLAIM, in particular, requires any swappable asset to be backed by a collateral of higher value than the swappable assets, which adds additional overhead.

An example of a network that would be well-suited for higher-order protocols would be Bitcoin, since it does not support smart-contracts and it's not based on Substrate.

## Examples​

### Ethereum Bridge (Smart Contracts \<> Polkadot)​

As explained by Dr. Gavin Wood in a blog post from late 2019, there are three ways that the Polkadot and Substrate ecosystem can be bridged to the Ethereum ecosystem.

1. Polkadot <-> Ethereum Public Bridge.
2. Substrate <-> Parity Ethereum (Openethereum) Bridge.
3. The Substrate EVM module.

Please read the blog article for fuller descriptions of each one of these options.

### Bitcoin Bridge (XCLAIM \<> Substrate \<> Polkadot)​

The Interlay team has written a specification on a Bitcoin bridge that is based on the XCLAIM design paper. The protocol enables a two-way bridge between Polkadot and Bitcoin. It allows holders of BTC to "teleport" their assets to Polkadot as PolkaBTC, and holders of PolkaBTC to burn their assets for BTC on the Bitcoin chain.

The Bitcoin bridge, as documented in the specification, is composed of two logically different components:

• The XCLAIM component maintains all accounts that own PolkaBTC.
• The BTC-Relay is responsible for verifying the Bitcoin state when a new transaction is submitted.

For full details on how it works, please refer to the specification.

There is now a reference implementation and testnet available.