## Introduction to RLN

A great introduction to RLN was written by Rasul [here](https://mirror.xyz/privacy-scaling-explorations.eth/iCLmH1JVb7fDqp6Mms2NR001m2_n5OOSHsLF2QrxDnQ), discussing the capabilities of RLN. Here's a graph to understand the dependency tree of RLN, and it's downstream users -


## Where RLN *should* be used

RLN should be used in the following scenarios -
- You operate in a privacy-constrained environment, i.e you wouldn't want to reveal information about yourself (IP Address, KYC).
- The application doesn't care about a specific user, rather, that a message has been sent from someone within a set of users they care about.

## Where RLN *shouldn't* be used

RLN shouldn't be used in the following scenario -
- Applications which employ traditional DoS protection mechanisms like IP address tracking, and user KYC

## How can I use RLN today?

### 1. Define your application's registration and slashing criteria

By using `RlnBase.sol`, as described above, you have full overriding capabilities over the registration process.

This could mean, you accept an arbitrary amount of erc20, check if the user has a soul bound token, or any other mechanism, with which you wish to verify that a given user *can* join your membership group.
This example will just check if the provided idCommitment starts with `0xdead`.

We will also not require a membership deposit.

Our slashing model will not use a proof, and only the owner can slash the users.

### 2. Start a new project

Use any of the smart contract tooling, foundry/hardhat/truffle to scaffold your project, and install [rln-contract](https://github.com/vacp2p/rln-contract) as a dependency. We will use [Foundry](https://github.com/foundry-rs/foundry). Installation instructions are provided [here](https://book.getfoundry.sh/getting-started/installation).

1. Start a new project

	forge init hello_rln_app && cd hello_rln_app

2. Install the `rln-contract` dependency

	forge install vacp2p/rln-contract

3. Install the `Openzeppelin-contracts` dependency

	forge install Openzeppelin/openzeppelin-contracts

### 3. Create the contract
1. Create a new contract in `src/` called `HelloRln.sol`

    // SPDX-License-Identifier: MIT
    pragma solidity 0.8.15;

    import "rln-contract/RlnBase.sol";
    import {Ownable} from "openzeppelin-contracts/contracts/access/Ownable.sol";

    contract HelloRln is Ownable, RlnBase {
        bytes2 public constant start = 0xdead;
        uint256 private membershipDeposit = 0;
        uint256 private depth = 20;

        constructor(address _poseidonHasher) Ownable() RlnBase(membershipDeposit, depth, _poseidonHasher, address(0)) {}

        function _validateRegistration(uint256 idCommitment) internal pure override {
            if (bytes2(bytes32(idCommitment)) != start) revert FailedValidation();
        }

        function _validateSlash(uint256 idCommitment, address payable receiver, uint256[8] calldata proof)
            internal
            view
            override
            onlyOwner
        {}
    }

2. Compile

	forge build

3. You can write tests in the `test` directory to test out this logic. A full example can be found [here](https://github.com/vacp2p/hello_rln_app)

### 4. Deploy the contract
1. [Deploy the contract](https://book.getfoundry.sh/forge/deploying), and obtain the address.

### 5. Use the new rln app with nwaku
1. Follow the [rln-relay guide](https://github.com/waku-org/nwaku/blob/bb9d231bdca1deb1955cf2c3a083b4a107d87076/docs/operators/how-to/run-with-rln.md), and replace the address with the newly deployed contract
2. Done! you are now running rln-relay, with your app-specific validation enabled 🚀

OR

### 5. Use the new rln app with your own p2p system
1. Implement rln-relay for your application, using [zerokit's public/ffi api](https://github.com/vacp2p/zerokit)

## Attribution

Most of this work would not be possible without the efforts of the [PSE group](https://pse.dev/) and past contributors at [Vac](https://vac.dev)

## References

1. [circom-rln](https://github.com/Rate-Limiting-Nullifier/circom-rln)
2. [pmtree](https://github.com/Rate-Limiting-Nullifier/pmtree)
3. [RLN Spec](https://rfc.vac.dev/spec/32)
4. [RLN-Relay Spec](https://rfc.vac.dev/spec/17)
5. [Zerokit](https://github.com/vacp2p/zerokit)