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Registering Verification Key

In this tutorial, we will be exploring how you can register a verification key on zkVerify. By registering a verification key you can save a lot of transaction fees while sending proofs for verification. After you register a vkey, now you can use the vkHash instead for vkey while sending proofs for verification.

note

Registration of the vkey is required once per circuit. You can skip this part if you are just building for a hackathon project or POC.

To follow this tutorial, make sure you have succesfully completed this tutorial. We will be editing the files which you have already used in the previous tutorials.

First, create a new file named register.js in which we will write the logic to register our vkey on zkVerify. Open register.js in your IDE and import the following components from zkVerify JS and dotenv :

import { zkVerifySession, Library, CurveType, ZkVerifyEvents } from "zkverifyjs";
import dotenv from 'dotenv';
dotenv.config();

We would also need to import the required verification key which we have already generated in previous tutorials. Use the following code snippets :

import fs from "fs";
const key = JSON.parse(fs.readFileSync("./data/main.groth16.vkey.json"));
info

Next we will be writing the core logic to send proofs to zkVerify for verification. All the following code snippets should be inserted within async main function.

async function main(){
  // Required code
}

main();

Once you have all the requirements imported, we will start by instantiating a session with our Volta testnet with an account(This account should have $tVFY to pay for transactions).

const session = await zkVerifySession.start().Volta().withAccount(process.env.SEED_PHRASE);

Next we will call the registerVerificationKey() function with the session we created previously. We will also listen to the Finalized events for our transaction and store the vkHash in vkey.json file.

const {regevent} = await session.registerVerificationKey().groth16({library: Library.snarkjs, curve: CurveType.bn128}).execute(key);

regevent.on(ZkVerifyEvents.Finalized, (eventData) => {
    console.log('Registration finalized:', eventData);
    fs.writeFileSync("vkey.json", JSON.stringify({"hash": eventData.statementHash}, null, 2));
    return eventData.statementHash
});

Now you can run this script using the command node register.js to start the registration process. Once registered you can find a new file called vkey.json with your registered verification key in the following format

{
  "vkey": "0x828c736b33ab492251a8b275468a29ce06e98fc833c0c7f0bc7f6272b300c05b"
}

Next we will edit the existing index.js file to use the registered vkHash instead of directly using the vkey for verification. First we will import the vkey.json file and then use the withRegisteredVk() property while calling the verify() function. Also, we will pass the new registered vkHash to the verify() function.

Paste the following code snippet at the start of the main function.

const vkey = JSON.parse(fs.readFileSync("./vkey.json")) //Importing the registered vkhash

Next replace the existing verify() function with the folllwing code snippets :

const {events} = await session.verify()
    .groth16({library: Library.snarkjs, curve: CurveType.bn128})
    .withRegisteredVk()
    .execute({proofData: {
        vk: vkey.hash,
        proof: proof,
        publicSignals: publicInputs
    }, domainId: 0});

Now you can run this script using the command node index.js. After running the script a new file named aggregation.json would have been created, which has the all the details required to verify the aggregation on the target chain. You would find something like the following:

{
  "root": "0xef4752160e8d7ccbc254a87f71256990f2fcd8173e15a592f7ccc7e130aa5ab0",
  "proof": [
    "0x40fbf21f1990ef8d1425d12ec550176fe848a7c63f0c59f7a48101e51c9aceee",
    "0x0be311c3643fb3fcd2b59bf4cfd02bdef943caf78f92d94a080659468c38fef9",
    "0x2117831ac2000ccdbb51f5deef96d215961ca42920a9196259e8b6e91b9fef53"
  ],
  "numberOfLeaves": 8,
  "leafIndex": 0,
  "leaf": "0xc5a8389b231522aad8360d940eb3ce275f0446bba1a9bd188b31d1c7dd37f136",
  "domainId": 0,
  "aggregationId": 137
}