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Contract managing "allowance signature" for easy off-chain authorization

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# SignedAllowance

This repository contains a minimal contract and an example of how to use "off-chain signing" to manage things like allowlists, tier-minting, early birds, etc... in NFT sale processes, without needing to save the actual lists on-chain.

Easier to manage and create than Merkle Trees, off-chain signed allowances are gas efficient ways to create allowlists.

The principle is simple and is based on the fact that each ethereum address can sign messages and that the resulting unique signature can then be verified easily in solidity.

This way, it is possible to sign as many messages(/allowances) as we want off-chain with an "allowanceSigner" wallet specifically created for this purpose and just verify on-chain that this message has actually been signed by it.

This way you only need to save on-chain the "allowanceSigner" public address instead of the full list of allowances.

This also allows to update the list of allowances anytime, since it's now off-chain and is only verified on-chain.

## Installation

`npm install --save-dev @0xdievardump/signed-allowances`

`pnpm add -D @0xdievardump/signed-allowances`

## Features

The `contracts/SignedAllowances.sol` contract contains a few functions that you can use to verify and / or use an allowance.

### validateSignature(address account, uint256 nonce, bytes memeory signature) public

Allows to:

1. validate `signature` has been created by `allowancesSigner` with the given parameters
2. verify that this signature has not already been marked as used

### \_useAllowance(address account, uint256 nonce, bytes calldata signature) internal

Allows to verify & mark an allowance as used.
Once used, calling this function again with the same parameters will revert

### \_setAllowancesSigner(address newSigner) internal

Allows to set the allowance signer. Usually used in the constructor of your contract and / or in a public setter with the `onlyOwner` modifier

## Usage

Install the package

`npm install --save-dev @0xdievardump/signed-allowances`

Extend SignedAllowance

```solidity

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.9;

import {SignedAllowance} from '@0xdievardump/contracts/SignedAllowance.sol';

contract MyMinter is SignedAllowance {
    constructor(address newAllowanceSigner) {
        _setAllowanceSigner(newAllowanceSigner);
    }

function mint(uint256 nonce, bytes32 memory signature)  public {
        // use the allowance
        _useAllowance(msg.sender, nonce, signature);
        // do your mint here...
        nftContract.mintTo(msg.sender);
    }
}
```

See `contracts/mocks/` and [the Example section](#examples) to see an extensive usage with allowlist, tiers, batch mints etc...

### Creating a list

You can use the script in `scripts/sign-allowances.js` to sign allowances.

Just set the env variable `CONTRACT_ADDRESS` and replace the content of `perAddress` with your own list of addresses & their allocation / nonce

You can then do a `npx hardhat run scripts/sign-allowances.js`

### Front-end usage

After you created a list, you can simply add it as a JSON file to your front-end.
When a user connects, you can verify it exists in the list, and pass its parameters when minting.

```html
<script>
    import allowlist from 'generated-allowlist.json';
    import { getMinter } from '$lib/modules/contracts';

export let account;

function checkAllowlist(account) {
        return;
    }

async function onMint() {
        await getMinter()
            .mint(allowlist[account].nonce, allowlist[account].signature)
            .then((tx) => tx.wait());
    }
</script>

{#if allowlist[account]}
<button on:click="{onMint}">Mint</button>
{:else}
<p>You are not in the allowlist.</p>
{/if}
```

### In contracts

contract allowing users to mint using a signature

in this contract users have to mint there full allowance in the same call

```solidity
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.9;

import '@openzeppelin/contracts/access/Ownable.sol';
import '@openzeppelin/contracts/token/ERC721/ERC721.sol';
import '@0xdievardump/contracts/SignedAllowance.sol';

contract MyToken is Ownable, ERC721, SignedAllowance {
    uint256 public lastTokenId;

constructor(address allowancesSigner_) ERC721('My Token', 'TKN') {
        // set the signer
        _setAllowancesSigner(allowancesSigner_);
    }

/// @notice sets allowance signer, this can be used to revoke all unused allowances already out there
    /// @param newSigner the new signer
    function setAllowancesSigner(address newSigner) external onlyOwner {
        _setAllowancesSigner(newSigner);
    }

/// @notice This function allows one mint per allowance.
    /// @param nonce the nonce
    /// @param signature the signature by the allowance wallet
    function mint(uint256 nonce, bytes memory signature) external {
        // this will throw if the allowance has already been used or is not valid
        _useAllowance(msg.sender, nonce, signature);
        _safeMint(msg.sender, ++lastTokenId);
    }

/// @notice This function allows `nonce` mint per allowance.
    /// @param nonce the nonce, which is also the number of mint allowed in one call
    /// @param signature the signature by the allowance wallet
    function mintBatch(uint256 nonce, bytes memory signature) external {
        // this will throw if the allowance has already been used or is not valid
        _useAllowance(msg.sender, nonce, signature);

// mint batch `nonce` elements to the caller
        _mintBatch(msg.sender, nonce);
    }

/// @notice allows to mint `howMany` to `account`
    /// @param account the account to mint to
    /// @param howMany how many to mint to this account
    function _mintBatch(address account, uint256 howMany) internal {
        uint256 tokenId = lastTokenId;
        for (uint256 i; i < howMany; i++) {
            _safeMint(account, ++tokenId);
        }

lastTokenId = tokenId;
    }
}
```

contract allowing users to mint using a signature

in this contract we keep a mapping of how many elements were already minted with a signature

This way users can mint their full allowance in several calls

```solidity
//SPDX-License-Identifier: MIT
pragma solidity ^0.8.9;

import '@openzeppelin/contracts/access/Ownable.sol';
import '@openzeppelin/contracts/token/ERC721/ERC721.sol';
import '@0xdievardump/contracts/SignedAllowance.sol';

// contract allowing users to mint using a signature
// in this contract users can mint there full allowance in sevral calls

contract MyToken is Ownable, ERC721, SignedAllowance {
    uint256 public lastTokenId;

mapping(bytes32 => uint256) public allowancesMinted;

constructor(address allowancesSigner_) ERC721('My Token', 'TKN') {
        // set the signer
        _setAllowancesSigner(allowancesSigner_);
    }

/// @notice This function allows `nonce` mint per allowance.
    /// @param howMany how many items the user wants to mint now
    /// @param nonce the nonce, which is also the number of mint allowed for this signature
    /// @param signature the signature by the allowance wallet
    function mintBatch(uint256 howMany, uint256 nonce, bytes memory signature) external {
        // this will throw if the signature is not the right one
        bytes32 signatureId = validateSignature(msg.sender, nonce, signature);

uint256 alreadyMinted = allowancesMinted[signatureId];

// verify we don't ask for too many
        requite(alreadyMinted + howMany <= nonce, "Too Many requested");

// increment the counter of how many were minted for this signature
        allowancesMinted[signatureId] += howMany;

// mint batch `nonce` elements to the caller
        _mintBatch(msg.sender, howMany);
    }

lastTokenId = tokenId;
    }
}
```

## Examples

### mint, mintFor, mintBatch, mintBatchFor, mintTier, mintTierFor, mintBatchTier, mintBatchTierFor

The mock contract `contracts/mocks/MyToken.sol` contains various minting function allowing:

-   Account minting one token using an "allowance signature"
-   Account minting one token for another account (paying the gas), using an "allowance signature"
-   Account minting several tokens using an "allowance signature"
-   Account minting several tokens for another account (paying the gas), using an "allowance signature"
-   Account minting one token, with Tier verification (early bird, allow list, etc...), using an "allowance signature"
-   Account minting one token for another account (paying the gas), with Tier verification (early bird, allow list, etc...), using an "allowance signature"
-   Account minting several tokens, with Tier verification (early bird, allow list, etc...), using an "allowance signature"
-   Account minting several tokens for another account (paying the gas), with Tier verification (early bird, allow list, etc...), using an "allowance signature"

The tests contain all needed function to understand how to sign a message off-chain in order to allow people to mint, with authorization check.

### Extensible

The encoding of the Tier in the nonce is just an example. But it is also possible to encode things into 64 bits instead of 128, and add a "token price" in the nonce, on top of the tier and the actual nonce.

Lots of things possible here.

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