区块链学习笔记之2022ACTF—bet2loss

// SPDX-License-Identifier: MITpragma solidity ^0.8.0;contract BetToken {    /* owner */    address owner;    /* token related */    mapping(address => uint256) public balances;    /* random related */    uint256 nonce;    uint256 cost;    uint256 lasttime;    mapping(address => bool) public airdroprecord;    mapping(address => uint256) public logger;    constructor() {owner = msg.sender;        balances[msg.sender] = 100000;        nonce = 0;        cost = 10;        lasttime = block.timestamp;    }    function seal(address to, uint256 amount) public {require(msg.sender == owner, "you are not owner");        balances[to] += amount;    }    function checkWin(address candidate) public {require(msg.sender == owner, "you are not owner");        require(candidate != owner, "you are cheating");        require(balances[candidate] > 2000, "you still not win");        balances[owner] += balances[candidate];        balances[candidate] = 0;    }    function transferTo(address to, uint256 amount) public pure {        require(amount == 0, "this function is not impelmented yet");    }    function airdrop() public {        require(            airdroprecord[msg.sender] == false,            "you already got your airdop"        );        airdroprecord[msg.sender] = true;        balances[msg.sender] += 30;    }    function bet(uint256 value, uint256 mod) public {        address _addr = msg.sender;        // make sure pseudo-random is strong        require(lasttime != block.timestamp);        require(mod >= 2 && mod <= 12);        require(logger[msg.sender] <= 20);        logger[msg.sender] += 1;        require(balances[msg.sender] >= cost);        // watchout, the sender need to approve such first        balances[msg.sender] -= cost;        // limit        value = value % mod;        // not contract        uint32 size;        assembly {            size := extcodesize(_addr)        }        require(size == 0);        // rnd gen        uint256 rand = uint256(            keccak256(                abi.encodePacked(                    nonce,                    block.timestamp,                    block.difficulty,                    msg.sender                )            )        ) % mod;        nonce += 1;        lasttime = block.timestamp;        // for one, max to win 12 * 12 - 10 == 134        // if 20 times all right, will win 2680        if (value == rand) {            balances[msg.sender] += cost * mod;        }    }}