Introducing Horizon: An Ethereum-Harmony Cross-chain Bridge

Sahil Dewan
Nov 10, 2020 · 5 min read

(This blog is originally published on Oct 19, 2020 and now reposted here.)

ho·ri·zon /həˈrīzən/: the line at which the earth’s surface and the sky appear to meet.

Harmony has launched Horizon, the first version of its Ethereum-Harmony cross-chain bridge, on the mainnet. This is the foundation of many upcoming initiatives designed to bolster a vibrant crosschain ecosystem on Harmony that will allow users and developers to smoothly interact with multiple chains. When you feel like you have reached the natural limitations of a blockchain, that is where we will meet you — at the Horizon.

Horizon opens up the gates for assets to flow freely between Harmony and Ethereum blockchains and will enable users to bridge any ERC20 token, as well as LINK and BUSD. Horizon will be released in phases with the next phase being fully trustless and permissionless.

Go to to start using.

Welcome BUSD and LINK on Harmony

Horizon enables the secure bridging of two important assets to our chain: BUSD and LINK. As the first official stablecoin on Harmony, BUSD will pave the way for many of our ecosystem projects to utilize cheap and fast value transfer. With LINK and Chainlink Oracle deployed on mainnet, Harmony ecosystem projects will be able to provide ONE-ETH atomic loans and many more use cases by using trustless oracle.

Technical overview

A bridge is a technology that connects two blockchains by using two flows with the ability to verify crosschain transactions:

i) transfer assets flow: assets are locked on Ethereum and equivalent amount are minted on Harmony

ii) redeem assets flow: minted assets on Harmony are burned and the equivalent amount are unlocked on Ethereum

We optimized for four principles in bridge our design:

  1. Trustless: the bridge is as safe as the blockchains that it is connected to, with no additional security assumptions

In order to address the critical need for a bridge in the short term, we will follow a multi-phase approach. This initial version of the bridge is focused on providing a seamless and low-cost solution for bringing assets to Harmony in a trusted yet secure manner. The final phase of the bridge will be fully trustless and decentralized.

Ethereum-to-Harmony (transfer assets) flow:

  • A user requests the bridge to transfer his/her ERC20s and provides the Harmony address to receive 1:1 HRC20s on Harmony, by authorizing the bridge to lock their ERC20 assets.

Harmony-to-Ethereum (redeem assets) flow:

  • User requests the bridge to take back HRC20s and provides an Ethereum address to receive 1:1 ERC20s in Ethereum, by authorizing the bridge to burn those HRC20s.

Some key properties:

  • The ERC20 to HRC20 mapping is established by the token manager smart contract on Harmony that keeps track of all the bridged tokens (users can verify that the tokens they receive are indeed from the bridged token contracts)

Next up: Grantees, Hackathon and Trustless Horizon

Many of our grantees and partners have been waiting for the Ethereum-Harmony bridge to launch. In the next few weeks, we expect a surge of bridge transactions and bridged assets on Harmony. A few of these projects to keep an eye on: Unifi Protocol, Swoop Crosschain DEX,, Blits, and more.

Being developer-friendly is one of our top priorities and soon we will release the bridge APIs and supporting services to enable any developer or application to directly utilize Horizon. This will allow any application to swap assets and use them within the application on a single interface.

We are also working towards the trustless and decentralized version of Horizon. A white paper on our novel design is expected to be released soon. It will describe our innovations towards gas-efficient light clients, trustless cross-chain block relayers, and efficient provers and verifiers, while keeping our end goal of: trustless, decentralized, cost effective, and seamless cross-chain bridge.


To scale trust for billions of people and create a…

Medium is an open platform where 170 million readers come to find insightful and dynamic thinking. Here, expert and undiscovered voices alike dive into the heart of any topic and bring new ideas to the surface. Learn more

Follow the writers, publications, and topics that matter to you, and you’ll see them on your homepage and in your inbox. Explore

If you have a story to tell, knowledge to share, or a perspective to offer — welcome home. It’s easy and free to post your thinking on any topic. Write on Medium

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store