EIP712Storage.sol

// SPDX-License-Identifier: MIT
// By 0xAA 
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";

contract EIP712Storage {
    using ECDSA for bytes32;

    bytes32 private constant EIP712DOMAIN_TYPEHASH = keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)");
    bytes32 private constant STORAGE_TYPEHASH = keccak256("Storage(address spender,uint256 number)");
    bytes32 private DOMAIN_SEPARATOR;
    uint256 number;
    address owner;

    constructor(){
        DOMAIN_SEPARATOR = keccak256(abi.encode(
            EIP712DOMAIN_TYPEHASH, // type hash
            keccak256(bytes("EIP712Storage")), // name
            keccak256(bytes("1")), // version
            block.chainid, // chain id
            address(this) // contract address
        ));
        owner = msg.sender;
    }

    /**
     * @dev Store value in variable
     */
    function permitStore(uint256 _num, bytes memory _signature) public {
        // 检查签名长度，65是标准r,s,v签名的长度
        require(_signature.length == 65, "invalid signature length");
        bytes32 r;
        bytes32 s;
        uint8 v;
        // 目前只能用assembly (内联汇编)来从签名中获得r,s,v的值
        assembly {
            /*
            前32 bytes存储签名的长度 (动态数组存储规则)
            add(sig, 32) = sig的指针 + 32
            等效为略过signature的前32 bytes
            mload(p) 载入从内存地址p起始的接下来32 bytes数据
            */
            // 读取长度数据后的32 bytes
            r := mload(add(_signature, 0x20))
            // 读取之后的32 bytes
            s := mload(add(_signature, 0x40))
            // 读取最后一个byte
            v := byte(0, mload(add(_signature, 0x60)))
        }

        // 获取签名消息hash
        bytes32 digest = keccak256(abi.encodePacked(
            "\x19\x01",
            DOMAIN_SEPARATOR,
            keccak256(abi.encode(STORAGE_TYPEHASH, msg.sender, _num))
        )); 
        
        address signer = digest.recover(v, r, s); // 恢复签名者
        require(signer == owner, "EIP712Storage: Invalid signature"); // 检查签名

        // 修改状态变量
        number = _num;
    }

    /**
     * @dev Return value 
     * @return value of 'number'
     */
    function retrieve() public view returns (uint256){
        return number;
    }    
}