Validating your statements

Once the proof has been fetched, we can now verify it using a custom contract on Gnosis Chain. The contract will use the HashiProver helper library to check the proof against the block header stored on Gnosis Chain.

Deploying the custom Verification Contract on Gnosis Chain

Here is a contract that deployed on Gnosis Chain can be used to read and verify the USDC total supply on Optimism:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import { HashiProver } from "https://github.com/gnosis/hashi/blob/feat/hashi-prover/packages/evm/contracts/prover/HashiProver.sol";

contract UsdcTotalSupplyReader is HashiProver {
    event UsdcTotalSupply(uint256 totalSupply);

    constructor(address shoyuBashi) HashiProver(shoyuBashi) {}

    function readTotalSupply(HashiProver.AccountAndStorageProof calldata proof) external {
        require(proof.chainId == 10, "Invalid chain id"); // Optimism
        require(proof.account == 0x0b2C639c533813f4Aa9D7837CAf62653d097Ff85, "Invalid account");
        require(proof.storageKeys.length == 1 && proof.storageKeys[0] == bytes32(uint256(0xb)), "Invalid storage key");

        // Verify the proof against the latest block header stored in Hashi
        uint256 totalSupply = uint256(bytes32(verifyForeignStorage(proof)[0]));
        emit UsdcTotalSupply(totalSupply);
    }
}

Deployment:

• HashiProver & ShoyuBashi are already deployed on a list of networks that can be called from the custom verification contract (this contract). In order to configure a custom oracle set for it is possible to deploy a new ShoyuBashi contract, inherit HashiProver contract and configure your oracle set in the ShoyuBashi contract.
• In this example, pass to the custom verification contract constructor <ShoyuBashi contract address on Gnosis Chain>

readTotalSupply explanation:

Inputs:

• proof: the HashiAPI retreived storage proof.

Logic:

• check that the proof chainID matches the origin chain chainID (Optimism in this case).

• check that the USDC contract address.

• check that storage key match the expected value.

• This contract uses HashiProver to verify on Gnosis Chain the storage proof coming from Optimism fetched earlier.

• It checks that the proof is from Optimism (chain ID 10), and that the USDC contract address and storage key match the expected values.

• The verifyForeignStorage function validates the proof against the block header stored on Gnosis Chain, which was relayed from Optimism.

• If the proof is valid, the contract emits the total supply of USDC on Optimism.

Check verifyForeignStorage implementation here.

Submitting the Proof

Once the contract is deployed on Gnosis Chain, you can submit the proof fetched from the API using the following command:

// Example of submitting the proof in Solidity (or via web3 call)
HashiProver.AccountAndStorageProof memory proof = { /* fetched proof from curl command */ };
usdcTotalSupplyReader.readTotalSupply(proof);

This will trigger the contract to verify the proof using the block header stored in Hashi from Optimism. If the proof is valid, the total supply of USDC will be emitted.