# Frontend - fhevmjs lib

This document provides an overview of the `fhevmjs` library, detailing its initialization, instance creation, input handling, encryption, and re-encryption processes.

[fhevmjs](https://github.com/zama-ai/fhevmjs/) is designed to assist in creating encrypted inputs and retrieving re-encryption data off-chain through a gateway. The library works with any fhEVM and fhEVM Coprocessors.

## Init (browser)

If you are using `fhevmjs` in a web application, you need to initialize it before creating an instance. To do this, you should call `initFhevm` and wait for the promise to resolve.

```javascript
import { FhevmInstance, createInstance } from "fhevmjs/node";

initFhevm().then(() => {
  const instance = await createInstance({
    network: window.ethereum,
    gatewayUrl: "https://gateway.sepolia.zama.ai",
    kmsContractAddress: "0x9D6891A6240D6130c54ae243d8005063D05fE14b",
    aclContractAddress: "0xFee8407e2f5e3Ee68ad77cAE98c434e637f516e5",
  });
});
```

## Create instance

This function returns an instance of fhevmjs, which accepts an object containing:

- `kmsContractAddress`: the address of the KMSVerifier contract;
- `aclContractAddress`: the address of the ACL contract;
- `networkUrl` or `network`: the URL or Eip1193 object provided by `window.ethereum` - used to fetch chainId and KMS nodes' public key
- `gatewayUrl`: the URL of the gateway - used to retrieve the public key, ZKPoK public parameters and send inputs and get reencryption
- `chainId` (optional): the chainId of the network
- `publicKey` (optional): if the public key has been fetched separately or stored in cache, you can provide it
- `publicParams` (optional): if the public params has been fetched separately or stored in cache, you can provide it

```javascript
import { createInstance } from "fhevmjs";

const instance = await createInstance({
  networkUrl: "https://eth-sepolia.public.blastapi.io",
  gatewayUrl: "https://gateway.sepolia.zama.ai",
  kmsContractAddress: "0x9D6891A6240D6130c54ae243d8005063D05fE14b",
  aclContractAddress: "0xFee8407e2f5e3Ee68ad77cAE98c434e637f516e5",
});
```

Using `window.ethereum` object:

```javascript
import { FhevmInstance, createInstance } from "fhevmjs/bundle";

const instance = await createInstance({
  network: window.ethereum,
  gatewayUrl: "https://gateway.sepolia.zama.ai",
  kmsContractAddress: "0x9D6891A6240D6130c54ae243d8005063D05fE14b",
  aclContractAddress: "0xFee8407e2f5e3Ee68ad77cAE98c434e637f516e5",
});
```

## Input

This method creates an encrypted input and returns an input object. It requires both the user address and the contract address to ensure the encrypted input isn't reused inappropriately in a different context. An input can include **multiple values of various types**, resulting in a single ciphertext that packs these values.

```javascript
const userAddress = "0xa5e1defb98EFe38EBb2D958CEe052410247F4c80";
const contractAddress = "0xfCefe53c7012a075b8a711df391100d9c431c468";

const input = instance.createEncryptedInput(contractAddress, userAddress);
```

### input.addBool, input.add8, ...

Input object has different method to add values:

- `addBool`
- `add4`
- `add8`
- `add16`
- `add32`
- `add64`
- `add128`
- `add256`
- `addBytes64`
- `addBytes128`
- `addBytes256`
- `addAddress`

```javascript
const input = instance.createEncryptedInput(contractAddress, userAddress);

input.addBool(true);
input.add16(239);
input.addAddress("0xa5e1defb98EFe38EBb2D958CEe052410247F4c80");
input.addBool(true);
```

### input.encrypt and input.send

These methods process values and return the necessary data for use on the blockchain. The `encrypt` method encrypts these values and provides parameters for use. The `send` method encrypts, dispatches the ciphertext and proof to the coprocessor, and returns the required parameters.

```javascript
input.addBool(true);
input.addBool(true);
input.add8(4);
const inputs = await input.encrypt(); // or input.send() if using a coprocessor

contract.myExample(
  "0xa5e1defb98EFe38EBb2D958CEe052410247F4c80",
  inputs.handles[0],
  32,
  inputs.handles[1],
  inputs.handles[2],
  true,
  inputs.inputProof,
);
```

## Re-encryption

### Keypair

A keypair consists of a private key and a public key, both generated by the dApp. These keys are used to reencrypt a blockchain ciphertext, allowing it to be securely transferred to user-specific keypairs.

```javascript
// Generate the private and public key, used for the reencryption
const { publicKey, privateKey } = instance.generateKeypair();
```

Verifying that the public key used in the reencryption process belongs to the user requires the user to sign the public key linked to a specific contract address. This signature allows any ciphertext allowed for the user and the contract can be reencrypted using the signed public key. To streamline user interaction during the signature process, we utilize the EIP712 standard as the object to be signed.

```javascript
// Create an EIP712 object for the user to sign.
const eip712 = instance.createEIP712(publicKey, CONTRACT_ADDRESS);
```

This `eip712` can be signed using `eth_signTypedData_v4` for example in a browser:

```javascript
const params = [USER_ADDRESS, JSON.stringify(eip712)];
const signature = await window.ethereum.request({ method: "eth_signTypedData_v4", params });
```

Note: it is recommended to store the keypair and the signature in the user's browser to avoid re-requesting signature on every user connection.

### Re-encryption

Reencrypt method will use the `gatewayUrl` to get the reencryption of a ciphertext and decrypt it.

```javascript
const handle = await erc20.balanceOf(userAddress); // returns the handle of hte ciphertext as a uint256 (bigint)
const myBalance = await instance.reencrypt(handle, privateKey, publicKey, signature, contractAddress, userAddress);
```
