[Vocdoni] Script to generate the zkAddress and circuit inputs with the correct encoding on TypeScript.

main.ts

import { ZkAddress } from "./zkAddress.js";
import { CensusGenProofZk } from "./zkCensus.js";

(async function() {
    // Create a ZkAddress instance from a provided string
    const accountPrivateKey = "6430ab787ad5130942369901498a118fade013ebab5450efbfb6acac66d8fb88";
    const electionId = "c5d2460186f760d51371516148fd334b4199052f01538553aa9a020200000000";
    const censusRoot = "21f20a61be6bb9415b777367989313a2640109990d187e397fa74256361f0e11";
    
    const zkAddr = await ZkAddress.FromString(accountPrivateKey);
    const zkProof = await CensusGenProofZk(zkAddr, censusRoot, electionId);
    console.log(zkProof);
})()

package.json

{
  "scripts": {
    "build": "tsc",
    "start": "node main.js"
  },
  "type": "module",
  "dependencies": {
    "blake-hash": "^2.0.0",
    "circomlibjs": "^0.1.7",
    "ffjavascript": "^0.2.57",
    "snarkjs": "^0.5.0"
  },
  "devDependencies": {
    "@types/node": "^18.13.0"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "es2020",
    /* Modules */
    "module": "ESNext",
    "moduleResolution": "node",
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,

    /* Type Checking */
    "strict": false,
    "skipLibCheck": true
  }
}

utils.ts

import * as crypto from "crypto";

export function littleEndianToNBytes(num : bigint, nBytes : number): bigint {
    const strNum = BigInt(num).toString(2);
    const nBits = nBytes * 8;
    
    // little endian
    const m = strNum.length - nBits;
    return BigInt("0b"+strNum.slice(m));
}

export function arboHash(input : string): string[] {
    const inputBuff = Buffer.from(input, "hex");
    const inputHash = crypto.createHash("sha256").update(inputBuff).digest();
    
    return [
        BigInt("0x" + inputHash.subarray(0, 16).reverse().toString("hex")).toString(),
        BigInt("0x" + inputHash.subarray(16, 32).reverse().toString("hex")).toString()
    ];
}

export function bigIntToStr(bi : bigint): string {
    const hex = bi.toString(16);
    return (hex.length % 2 != 0) ? "0" + hex : hex;
}

export function strToBigInt(str : string): bigint {
    const strBuff = Buffer.from(str, "hex");
    return BigInt("0x"+ strBuff.reverse().toString("hex"));
}

zkAddress.ts

import { buildPoseidon, buildEddsa, buildBabyjub } from "circomlibjs";
import { Scalar, utils } from "ffjavascript";
import createBlakeHash from "blake-hash";

import { arboHash, littleEndianToNBytes } from "./utils.js";

export const ZkAddressDefaultLenght : number = 20;

export class ZkAddress {
    private _privateKey : bigint;
    private _publicKey : bigint;
    private _address : bigint;

    private _bbjbjb : any;
    private _poseidon : any;
    private _eddsa : any;

    public get privateKey(): string {
        return this._privateKey.toString();
    }

    public get publicKey(): string {
        return this._publicKey.toString();
    }

    public static async FromString(accountPrivateKey : string) : Promise<ZkAddress> {
        const self = new ZkAddress()
        self._bbjbjb = await buildBabyjub();
        self._poseidon = await buildPoseidon();
        self._eddsa = await buildEddsa();

        self.calcPrivateKey(accountPrivateKey);
        self.calcPublicKey();
        self.calcAddress();

        return self;
    }

    public getNullifier(electionId : string) : string {
        const electionHash = arboHash(electionId);

        const nullifierHash = this._poseidon([
            this._privateKey,
            electionHash[0],
            electionHash[1]
        ]);

        return this._poseidon.F.toObject(nullifierHash).toString();
    }

    public toString(): string {
        const hexAddr = this._address.toString(16);
        const buffAddr = Buffer.from(hexAddr, "hex").reverse();
        return buffAddr.toString("hex");
    }

    private calcPrivateKey(seed : string) {
        // Get buffSeed from hexadecimal string
        const buffSeed = Buffer.from(seed, "hex");
        // Instance the EDDSA curve with the blake512 hash of the accPrivKey (trim it at 32 bytes)
        const privateKeyHash = this._eddsa.pruneBuffer(createBlakeHash("blake512").update(buffSeed).digest().slice(0, 32));

        // Return the Little Endian BigInt representation of the hash, shifted 3 bytes to the right
        this._privateKey = Scalar.shr(utils.leBuff2int(privateKeyHash), 3);
    }

    private calcPublicKey() {
        const A = this._bbjbjb.mulPointEscalar(this._bbjbjb.Base8, this._privateKey.toString());
        const rawPubKey = this._poseidon([A[0], A[1]]);

        this._publicKey = BigInt(utils.stringifyFElements(this._poseidon.F, rawPubKey));
    }

    private calcAddress() {
        const rawAddress = littleEndianToNBytes(this._publicKey, ZkAddressDefaultLenght);
        this._address = BigInt(utils.stringifyFElements(this._poseidon.F, rawAddress));
    }
}

zkCensus.ts

import { verify } from "crypto";
import { arboHash, bigIntToStr, strToBigInt } from "./utils.js";
import { ZkAddress } from "./zkAddress.js";
import * as snarkjs from "snarkjs";

export async function CensusGenProofZk(zkAddr : ZkAddress, censusRoot : string, electionId : string) : Promise<any> {
    // TODO: make a http request to /censuses/{censusRoot}/type to check if the 
    // current census is a zkweighted one.
    
    // TODO: make a http request to /censuses/{censusRoot}/proof/{zkAddr} to get
    // the correct formated siblings and vote weight.
    const mockCensusSiblings = ["580248495380568564123114759404848148595751514101110571080218449954471889552","633773650715998492339991508741183573324294111862264796224228057688802848801","4989687640539066742397958643613126020089632025064585620245735039080884277747","3863931244154987782138626341091989752721559524607603934673141845608721144566","4688361268219000735189072482478199217683515606859266889918935760406393255791","13548759886643907861771470850568634627625204202812868538547476494298706407763","2714600331130374821301177119574534122901226309274343707259485383900427657102","11263980701448890785172384726839875656945350242204041401041770089765910047533","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0","0"];
    const mockWeight = 10n
    
    const strCensusRoot = strToBigInt(censusRoot).toString();
    const circuitInputs = {
        censusRoot: strCensusRoot,
        censusSiblings: mockCensusSiblings,
        weight: mockWeight.toString(),
        privateKey: zkAddr.privateKey,
        voteHash: arboHash(bigIntToStr(mockWeight)),
        processId: arboHash(electionId),
        nullifier: zkAddr.getNullifier(electionId)
    }

    // This line assumes that the circuit wasm and proving key artifacts are 
    // downloaded and stored into the same folder that this file.
    const { proof, publicSignals } = await snarkjs.groth16.fullProve(circuitInputs, "./circuit.wasm", "./proving_key.zkey");

    return {
        proof, 
        publicSignals,
        weight: mockWeight,
        nullifier: zkAddr.getNullifier(electionId)
    }
}