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Hop
Hop is a protocol that allows tokens to be sent from rollups and their corresponding layer-1 networks to L2 solutions on other blockchains quickly and trustlessly.

Hop: An Overview

Hop is a protocol that allows tokens to be sent from rollups and their corresponding layer-1 networks to L2 solutions on other blockchains quickly and trustlessly. It consists of a scalable rollup-to-rollup general token bridge for Ethereum’s layer-2 ecosystem that enables users to send tokens from one rollup to another. Hop is one of the most widely-used bridging solutions for the Ethereum network and has had over 13000 total users thus far.

In order to enable users to transfer their assets from one network to another seamlessly, Hop uses the following mechanisms:

  • The Creation of “h” Tokens - To move assets economically and quickly, Hop creates an intermediary asset called an h-token. The h-tokens are cross-network bridge tokens that account for the funds being moved across chains.

When using Hop, the end-user does not have to deal with h-tokens directly; instead, they must deal with the canonical token (or native token) of the corresponding rollup, e.g. hTokens of DAI and ETH would be hDAI and hETH.

  • Automated Market Makers - Using AMMs, Hop swaps between the h-tokens and their corresponding assets on the layer-2 networks associated with the swap.

This two-pronged approach allows users to swap between two layer-2 canonical tokens like canonical ETH or canonical DAI using the h-tokens as an intermediary asset.

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Source: Hop Protocol

Like most bridges, Hop helps increase the throughput of the Ethereum network by moving computation and some of the data storage off-chain. The siloed nature of Layer-2 networks has fragmented the DeFi space, but Hop offers a way to reconnect many of these solutions through the creation and utilization of bridges between every layer-2 network in the Ethereum ecosystem. Thus, while the native bridges only connect to Ethereum, Hop connects all the layer-2 solutions together. This enables users to “hop” between different layer-2 solutions or even withdraw their funds to the Ethereum mainnet almost instantly and at a fraction of the cost.

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How Does Hop Work?

Hop is a multi-network bridge that can be applied to any EVM-compatible chain. A transfer on Hop involves the following:

  • A cross-chain Hop Bridge

  • Hop Bonders

  • The Automated Market Maker

Moreover, a Hop Transfer includes the following information:

  • Destination Chain ID - The ID of the rollup or layer-1 destination chain

  • Recipient - The receiving address on the destination chain

  • Amount - The quantity of the token being transferred

  • Additional Information - Additional info may be included in the transfer for convenience. For example, the transfer may indicate a relayer fee to allow a transaction relayer to withdraw the Transfer on behalf of the user at its destination.

The next section will explain some of the key components of Hop architecture and how they are involved in completing a transaction using the Hop protocol.

The Hop Bridge

The Hop Bridge is a multi-chain network that connects L2 solutions on different ERC-20 blockchains. For instance, Hop has its own Optimism Bridge, Arbitrum Bridge, and Polygon Bridge.

Why do we need Hop Bridges?

Suppose we use the native token offered by each Layer-2 solution. In that case, the transfer of assets between chains will be subject to long exit periods since the assets being moved represent the original asset that only exists on the specific layer-2 solution. For instance, If users want to move ETH to Arbitrum using the native Arbitrum bridge, they will receive a canonical token aka Arbitrum Ethereum, which will be the original representation of ETH on the Arbitrum network. However, as this canonical token only exists on the Arbirtrum network, the user will not be able to convert and use it on any other Layer-2 solution instantly.

As a result, if the user then wishes to bridge the token to another L2, the transfer will be subject to long exit periods. For example, if a user wishes to convert their Arbitrum Ether into Optimism Ether, they would have to withdraw their canonical token back to the Ethereum mainnet, convert the mainnet ETH to Optimism ETH via the native Optimism bridge. This process is lengthy, and the user will have to wait for seven days for their tokens to unlock.

Converting a layer-2 token into the asset it represents on L1 is expensive and time-consuming, which creates a problem when users want to bridge from one L2 token to another. Without hop, the L2 token would have to be converted to a token on the mainnet before using a bridge to change to an asset on another blockchain. Then, the asset would have to be converted into its representation on the rollup on the destination chain.

Instead of utilizing bridges between native tokens to accomplish this task, the Hop architecture supports different “Hop Bridges” for each layer-2 solution. This allows Hop to issue Hop tokens (h-tokens) on each of its supported networks, eliminating the long exit times required by rollups. For instance, if a user wants to transfer 4 ETH from a rollup to Arbitrum using Hop, Hop would create an equivalent amount of hETH on Arbitrum. This enables the user to mint 4 ETH on the destination rollup on Arbitrum, and Hop burns the 4 ETH on the original rollup.

Here’s an example: Alice deposits 4 ETH into the L1 Bridge contract and receives 4 Hop ETH from the L2 Bridge contract.

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Source: Hop Exchange Whitepaper

The Bonder Enables Cross-Chain Swaps

Hop uses Bonders In order to facilitate these instant transfers. For a small fee, the Bonder fronts liquidity on the destination chain. This liquidity is returned when the transfer propagates through layer-1 as part of a “Transfer Root” representing a bundle of transactions.

As Bonders run a verifier node on each rollup and possess the functionality to verify transactions as they are being made, they are assured of getting their funds back. Furthermore, given the assurance that they will eventually receive their funds back, the Bonder provides upfront liquidity on the destination chain. Finally, the Bonder's locked funds are restored when the big batch of transactions on the mainnet is eventually confirmed.

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Thanks to the upfront liquidity provided by Bonders, Hop Tokens can be exchanged swiftly and affordably between all supported networks.

For instance, a user wants to withdraw hETH for ETH. As Bonders run a verifier node, they know in advance that this particular withdrawal transaction has been executed by the user and will have to be settled soon on the mainnet. Knowing this, the Bonder sends its locked up hETH on the destination chain to the user’s wallet address. As a result, the user receives the funds instantly, and when the transaction batch is settled, the Bonder receives its collateral and a small fee for their services.

The AMM

Since third parties on different rollups are unlikely to adopt htokens, Hop protocol needs a mechanism that can convert the htokens into the original or native tokens of the rollup. To do so, Hop Protocol deploys an Automated Market Maker (AMM), which enables swaps between each Hop Bridge Token and its canonical version. For example, Hop ETH or hETH - Canonical L2 ETH.

For instance, if a user wants to send ETH to Arbitrum from the Ethereum mainnet network, Hop first issues hETH to the user on Arbitrum and using the AMM, it will then swap it against Arbitrum ETH. While this might seem like different steps on different chains, it all looks like one transaction to the end-user.

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In addition to enabling the swaps, AMMs also offer a pricing mechanism for liquidity on different rollups. Moreover, it acts as an incentivization mechanism for Arbitrageurs as it enables them to rebalance liquidity in response to market movements.

The AMMs have two key users in the form of:

  • Arbitrageurs - Arbitrageur is a user that buys a token on one exchange and sells it on a different exchange for a profit when there is slippage in the price of a token. Arbitrageurs in Hop shift between "h" tokens and canonical tokens on one Hop rollup AMM and profitably trade the token on a different rollup. Because liquidity is rebalanced between AMMs, the price eventually stabilizes.

  • Liquidity Providers - Similar to how liquidity providers operate on other AMMs like Unisawp, anyone can become an LP in a Hop pool and earn fees as rewards for swaps. Moreover, as liquidity on Hop AMM is provided in the form of the same underlying asset (hETH, Arbitrum ETH, etc.) that can essentially be redeemed for the exact same amount from the mainnet, there is a very low risk of impermanent loss for a liquidity provider.

Bridge Design: Architecture

The Hop Bridge is a general token bridge. General token bridges like Hop are provided by a third party and enable users to bridge ERC-20 tokens in a generic way.

We can evaluate Hop’s architecture and design as follows:

  • Security - User funds can never be lost or stolen as the security is equal to that of the underlying rollup it supports. The worst-case scenario for the security of funds is when the Bonders go offline. Even then, the users will only face a delay equal to the exit time of the rollup. Moreover, Hop has undergone audits by Solidified and Monoceros Alpha.

  • Speed - As Hop uses a two-pronged approach of creating a cross-network token and using AMMs, it is able to execute transactions quickly.

  • Connectivity - Hop supports a wide range of destination chains, and hence it offers good connectivity.

  • Capital Efficiency - Hop can enable significant amounts of economic throughput and this is highly capital efficient. Moreover, the transaction costs to transfer assets are also relatively low.

  • Statefulness - Hop is limited in its ability to transfer specific assets, more complex state, and currently only supports 5 different assets.

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Security

Given the current architecture, a Hop bridge is only as robust as the weakest supported rollup.

Incentives

To provide upfront liquidity on the destination chain, the Bonders receive a small fee for every transaction they help facilitate.

Moreover, the liquidity providers on the Hop AMM receive a small fee from each swap as a reward for contributing passive liquidity to the AMM’s liquidity pool.

Risks

Interacting with the Hop Bridge has the following risks:

  • Centralized Bonder Network - As Hop only allows a few whitelisted entities to become a Bonder in the initial phases, Bonders risk not facilitating transactions. However, no Bonder can steal funds or censor transactions.

  • All Bonders Go Offline - All Bonders going offline is perhaps the worst-case scenario that a user can experience while interacting with Hop. In such a situation, no transfers will be bonded and as a result, will get delayed to the normal exit time of the L2s. However, it’s important to note that even here, the transfer will eventually go through but with a delay.

  • Smart Contract Risks - While have been audited by and, there is a counterparty risk involved.

  • Technology Risks - Given the nature of Hop’s operations, it is still susceptible to technology risks.

  • Systematic Risk - The Hop Bridge’s reliance on rollups for security makes the participants susceptible to systematic risks. Although regular users crossing the Hop bridge are only exposed for a brief period of time, liquidity providers for AMMs and some arbitrageurs are constantly exposed to this risk.

Supported Chains & Assets

Hop supports the following chains:

  • Ethereum

  • Arbitrum

  • Optimism

  • Polygon

  • Gnosis

Hop supports the following assets:

  • DAI

  • ETH

  • MATIC

  • USDC

  • USDT

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