前言
难得在短暂的假期结束前还有能参加的CTF比赛，趁机打一打，题目质量也很合适。这次出乎意料的排在了前面一些的位置，明明没有写出几道题，属于是运气问题。
Blockchain
VNloan
这题出乎意料地拿到了一血，我在工作量证明上就卡了47分钟，连题都看不到- -
这题我也不知道是不是预期解，感觉我的解法太过简单，可能和出题人最初的想法不一致。根据题目信息我猜测是考察闪电贷攻击，但根据flag的内容又发现考的好像是重入攻击，但我自己写的时候，完全就没有用到任何的攻击类型，就达成条件了，也有可能是出题人在编写合约代码时出现了一些纰漏，总之既然题目已经公开，也只能按当时题目的样子去解了。
按比赛时情况，我们先分析一下题目中的两个合约代码。
//Setup.sol
pragma solidity 0.4.26;
import "./VNETH.sol";
contract Setup {
VNETH public vneth;
bool public Solved = false;
constructor() public payable {
vneth = new VNETH();
}
function checksuccess() public {
if (vneth.balanceOf(msg.sender) >= 5000) Solved = true;
}
function isSolved() public view returns (bool) {
if (Solved == true) {
return true;
}
return false;
}
}
//VNETH.sol
pragma solidity 0.4.26;
contract VNETH {
address public owner;
string public name = "VN ETHER";
string public symbol = "VNeth";
uint8 public decimals = 18;
bool public isLoan=false;
event Approval(address indexed src, address indexed guy, uint wad);
event Transfer(address indexed src, address indexed dst, uint wad);
event Deposit(address indexed dst, uint wad);
event Withdrawal(address indexed src, uint wad);
mapping (address => uint) public balanceOf;
mapping (address => mapping (address => uint)) public allowance;
constructor()public{
owner=msg.sender;
balanceOf[owner]=1e18 ether;
balanceOf[address(this)]=1e18 ether;
}
function() external payable {
deposit();
}
function deposit() public payable {
balanceOf[msg.sender] += msg.value;
emit Deposit(msg.sender, msg.value);
}
function withdraw(uint wad) public {
require(balanceOf[msg.sender] >= wad);
balanceOf[msg.sender] -= wad;
(msg.sender).transfer(wad);
emit Withdrawal(msg.sender, wad);
}
function totalSupply() public view returns (uint) {
return address(this).balance;
}
function approve(address guy, uint wad) public returns (bool) {
allowance[msg.sender][guy] = wad;
emit Approval(msg.sender, guy, wad);
return true;
}
function transfer(address dst, uint wad) public returns (bool) {
return transferFrom(msg.sender, dst, wad);
}
function fakeflashloan(uint256 value,address target,bytes memory data) public{
require(isLoan==false&&value>=0&&value<=1000);
balanceOf[address(this)]-=value;
balanceOf[target]+=value;
address(target).call(data);
isLoan=true;
require(balanceOf[target]>=value);
balanceOf[address(this)]+=value;
balanceOf[target]-=value;
isLoan=false;
}
function transferFrom(address src, address dst, uint wad)
public
returns (bool)
{
require(balanceOf[src] >= wad);
if (src != msg.sender && allowance[src][msg.sender] != 2**256-1) {
require(allowance[src][msg.sender] >= wad);
allowance[src][msg.sender] -= wad;
}
balanceOf[src] -= wad;
balanceOf[dst] += wad;
emit Transfer(src, dst, wad);
return true;
}
}
可以发现当我们的代币余额大于等于5000 wei时，即可达成解出条件，且vneth的属性是public的，可以直接读取到合约地址。
当时代码审计到这里我就不往下看了，因为已经可以快速达成解出条件，只需要往vneth合约转账5000 wei的test ETH，我们的代币余额就已经达成要求，所以当时我是一脸懵，这道题考察的是什么呢，也许是面向新手的链上交互操作吧，只要懂得如何与链交互就能解出题目。
下面是我的半自动脚本，事先需要请求水龙头向Option 1得到的deployer地址转账一些test ETH作为手续费，然后部署题目合约。
同时欢迎使用我的 Poseidon 库，Poseidon_Blockchain可以让你轻松地与链上交互(主流的区块链题目大都与以太坊有关)，即使你刚刚入门。
出题人环境部署得很好，禁用了大部分链上函数，使得没有办法扫链获得其他人的交易信息，也因此Poseidon_Blockchain需要进行一些代码行的注释暂停使用后才能运行。注释掉第68、69行，将第158、178行的gasPrice修改为5*10**9(即5 Gwei)。
from Poseidon_Blockchain import *
from loguru import logger
import requests
import solcx
# deployer:0x4c7A8Fbffe8585aCB736FBc1a885Df2083f48F79
# token:v4.local.YLxt2XkscX0zOuSjwGprwVy2k_kKhTq-EJIRolVu-kMjr7khCqejWz9raWvWKrp15YyP4_o5yuVYw5iKq7deMsjZxlQxoj1BgbGu1Ut-UOj1HCuywiEBa0UjXuBHyeCGgB2Ub-nY5zjdkwA2fNgiCUwdXy0Qcf5UYmRPx-TQ-CHhqA
# contract:0x3d8fB1A9aaBFEfd8cf5910202d05E664CcBd02D5
# VNCTF{I_will_us3_th3_R3ENtracyGu4rd-nexT_t1m3}
logger.add('log/VNloan_{time}.log', rotation='00:00')
# 安装并切换到对应的solidity版本
SolidityVersion = solcx.install_solc('0.4.26')
solcx.set_solc_version(SolidityVersion)
logger.success(f"Solidity Version:{SolidityVersion}")
# 连接至链 生成新账户 调用水龙头获取test ETH
chain = Chain("http://81.68.163.153:8545")
account = Account(chain, Account.CreateNewAccount()[1])
faucetHash = requests.post("http://81.68.163.153:8080/api/claim", data={"address": account.Address}).text
chain.Net.eth.wait_for_transaction_receipt(faucetHash, timeout=180)
chain.GetBalanceByAddress(account.Address)
# 实例化合约 获取vneth合约地址 向其转账5000wei
contractAddress = Web3.toChecksumAddress("0x3d8fB1A9aaBFEfd8cf5910202d05E664CcBd02D5")
abi, bytecode = SolidityToAbiAndBytecode("Setup.sol", "Setup")
contract = chain.Net.eth.contract(address=contractAddress, abi=abi)
vnethAddress = contract.functions.vneth().call()
account.SendTransactionToChain(vnethAddress, "0x", 5000)
abi, bytecode = SolidityToAbiAndBytecode("VNETH.sol", "VNETH")
vneth = chain.Net.eth.contract(address=vnethAddress, abi=abi)
logger.info(f"{vneth.functions.balanceOf(account.Address).call()}")
# 调用checksuccess修改状态变量 达成解出条件
transactionData = contract.functions.checksuccess().buildTransaction()
account.SendTransactionToChain(transactionData["to"], transactionData["data"])
logger.info(f"{contract.functions.Solved().call()}")
logger.info(f"{contract.functions.isSolved().call()}")
logger.success("Execution completed.")
运行日志如下：
2022-02-12 19:58:57.565 | SUCCESS | __main__:<module>:15 - Solidity Version:0.4.26
2022-02-12 19:58:57.659 | SUCCESS | Poseidon_Blockchain:__init__:65 -
[ConnectToChain]Successfully connected to [http://81.68.163.153:8545].
2022-02-12 19:58:57.670 | SUCCESS | Poseidon_Blockchain:CreateNewAccount:200 -
[NewAccount]
[Address]0xCDD53a6422696fbc09dc8D1d480378A93f3d605e
[PrivateKey]
2022-02-12 19:58:57.674 | SUCCESS | Poseidon_Blockchain:__init__:146 -
[ImportAccount]Successfully import account.
[Address]0xCDD53a6422696fbc09dc8D1d480378A93f3d605e
2022-02-12 19:59:02.487 | INFO | Poseidon_Blockchain:GetBalanceByAddress:116 -
[Balance][0xCDD53a6422696fbc09dc8D1d480378A93f3d605e]
[1000000000000000000 Wei]<=>[1 Ether]
2022-02-12 19:59:02.638 | INFO | Poseidon_Blockchain:SolidityToAbiAndBytecode:212 -
[CompileContract]
[Abi][{'constant': False, 'inputs': [], 'name': 'checksuccess', 'outputs': [], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'isSolved', 'outputs': [{'name': '', 'type': 'bool'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'Solved', 'outputs': [{'name': '', 'type': 'bool'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'vneth', 'outputs': [{'name': '', 'type': 'address'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'inputs': [], 'payable': True, 'stateMutability': 'payable', 'type': 'constructor'}]
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2022-02-12 19:59:02.846 | INFO | Poseidon_Blockchain:SendTransactionToChain:167 -
[SendTransaction]
[TransactionHash]0x354cb6b3307284ed4879c296bed69b45826aab9fb3641f0340ddc2ffbe4cd530
[Txn]{'from': '0xCDD53a6422696fbc09dc8D1d480378A93f3d605e', 'to': '0x7E7a74909Cab35a0885c9DC55062Ae27b8258297', 'gasPrice': 5000000000, 'gas': 3000000, 'nonce': 0, 'value': 5000, 'data': '0x', 'chainId': 1211}
2022-02-12 19:59:17.475 | SUCCESS | Poseidon_Blockchain:SendTransactionToChain:169 -
[ConfirmTransaction]
[TransactionHash]0x354cb6b3307284ed4879c296bed69b45826aab9fb3641f0340ddc2ffbe4cd530
[TransactionReceipt]AttributeDict({'blockHash': HexBytes('0x6db5d0683e2ec503e4379ec9e00af413831919375b79ff87097e68e29ddeee92'), 'blockNumber': 690658, 'contractAddress': None, 'cumulativeGasUsed': 44702, 'effectiveGasPrice': 5000000000, 'from': '0xCDD53a6422696fbc09dc8D1d480378A93f3d605e', 'gasUsed': 44702, 'logs': [AttributeDict({'address': '0x7E7a74909Cab35a0885c9DC55062Ae27b8258297', 'topics': [HexBytes('0xe1fffcc4923d04b559f4d29a8bfc6cda04eb5b0d3c460751c2402c5c5cc9109c'), HexBytes('0x000000000000000000000000cdd53a6422696fbc09dc8d1d480378a93f3d605e')], 'data': '0x0000000000000000000000000000000000000000000000000000000000001388', 'blockNumber': 690658, 'transactionHash': HexBytes('0x354cb6b3307284ed4879c296bed69b45826aab9fb3641f0340ddc2ffbe4cd530'), 'transactionIndex': 0, 'blockHash': HexBytes('0x6db5d0683e2ec503e4379ec9e00af413831919375b79ff87097e68e29ddeee92'), 'logIndex': 0, 'removed': False})], 'logsBloom': HexBytes('0x00000000000000000010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000008000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001000000000000000000000000000000000000000000000000000000040000000000000000002000000000000000000000000000000000000000000000000000000000000000000000000000000000080000000000040000000000000000000000000000400000000000000000'), 'status': 1, 'to': '0x7E7a74909Cab35a0885c9DC55062Ae27b8258297', 'transactionHash': HexBytes('0x354cb6b3307284ed4879c296bed69b45826aab9fb3641f0340ddc2ffbe4cd530'), 'transactionIndex': 0, 'type': '0x0'})
2022-02-12 19:59:17.534 | INFO | Poseidon_Blockchain:SolidityToAbiAndBytecode:212 -
[CompileContract]
[Abi][{'constant': True, 'inputs': [], 'name': 'name', 'outputs': [{'name': '', 'type': 'string'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': False, 'inputs': [{'name': 'guy', 'type': 'address'}, {'name': 'wad', 'type': 'uint256'}], 'name': 'approve', 'outputs': [{'name': '', 'type': 'bool'}], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'totalSupply', 'outputs': [{'name': '', 'type': 'uint256'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': False, 'inputs': [{'name': 'src', 'type': 'address'}, {'name': 'dst', 'type': 'address'}, {'name': 'wad', 'type': 'uint256'}], 'name': 'transferFrom', 'outputs': [{'name': '', 'type': 'bool'}], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'function'}, {'constant': False, 'inputs': [{'name': 'wad', 'type': 'uint256'}], 'name': 'withdraw', 'outputs': [], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'decimals', 'outputs': [{'name': '', 'type': 'uint8'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': True, 'inputs': [{'name': '', 'type': 'address'}], 'name': 'balanceOf', 'outputs': [{'name': '', 'type': 'uint256'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'owner', 'outputs': [{'name': '', 'type': 'address'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'isLoan', 'outputs': [{'name': '', 'type': 'bool'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': True, 'inputs': [], 'name': 'symbol', 'outputs': [{'name': '', 'type': 'string'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': False, 'inputs': [{'name': 'dst', 'type': 'address'}, {'name': 'wad', 'type': 'uint256'}], 'name': 'transfer', 'outputs': [{'name': '', 'type': 'bool'}], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'function'}, {'constant': False, 'inputs': [], 'name': 'deposit', 'outputs': [], 'payable': True, 'stateMutability': 'payable', 'type': 'function'}, {'constant': True, 'inputs': [{'name': '', 'type': 'address'}, {'name': '', 'type': 'address'}], 'name': 'allowance', 'outputs': [{'name': '', 'type': 'uint256'}], 'payable': False, 'stateMutability': 'view', 'type': 'function'}, {'constant': False, 'inputs': [{'name': 'value', 'type': 'uint256'}, {'name': 'target', 'type': 'address'}, {'name': 'data', 'type': 'bytes'}], 'name': 'fakeflashloan', 'outputs': [], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'function'}, {'inputs': [], 'payable': False, 'stateMutability': 'nonpayable', 'type': 'constructor'}, {'payable': True, 'stateMutability': 'payable', 'type': 'fallback'}, {'anonymous': False, 'inputs': [{'indexed': True, 'name': 'src', 'type': 'address'}, {'indexed': True, 'name': 'guy', 'type': 'address'}, {'indexed': False, 'name': 'wad', 'type': 'uint256'}], 'name': 'Approval', 'type': 'event'}, {'anonymous': False, 'inputs': [{'indexed': True, 'name': 'src', 'type': 'address'}, {'indexed': True, 'name': 'dst', 'type': 'address'}, {'indexed': False, 'name': 'wad', 'type': 'uint256'}], 'name': 'Transfer', 'type': 'event'}, {'anonymous': False, 'inputs': [{'indexed': True, 'name': 'dst', 'type': 'address'}, {'indexed': False, 'name': 'wad', 'type': 'uint256'}], 'name': 'Deposit', 'type': 'event'}, {'anonymous': False, 'inputs': [{'indexed': True, 'name': 'src', 'type': 'address'}, {'indexed': False, 'name': 'wad', 'type': 'uint256'}], 'name': 'Withdrawal', 'type': 'event'}]
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2022-02-12 19:59:17.596 | INFO | __main__:<module>:30 - 5000
2022-02-12 19:59:17.891 | INFO | Poseidon_Blockchain:SendTransactionToChain:167 -
[SendTransaction]
[TransactionHash]0xb00663cc2e9240c63aea8227046f4f508b1f1e5bdf406136ecc6ee5eead1170a
[Txn]{'from': '0xCDD53a6422696fbc09dc8D1d480378A93f3d605e', 'to': '0x3d8fB1A9aaBFEfd8cf5910202d05E664CcBd02D5', 'gasPrice': 5000000000, 'gas': 3000000, 'nonce': 1, 'value': 0, 'data': '0x094cc113', 'chainId': 1211}
2022-02-12 19:59:32.352 | SUCCESS | Poseidon_Blockchain:SendTransactionToChain:169 -
[ConfirmTransaction]
[TransactionHash]0xb00663cc2e9240c63aea8227046f4f508b1f1e5bdf406136ecc6ee5eead1170a
[TransactionReceipt]AttributeDict({'blockHash': HexBytes('0xa000e1f4ae1f8ce1ea493c12cf25a84946d9c4aa767b2cd02e17dad0b746679c'), 'blockNumber': 690659, 'contractAddress': None, 'cumulativeGasUsed': 29035, 'effectiveGasPrice': 5000000000, 'from': '0xCDD53a6422696fbc09dc8D1d480378A93f3d605e', 'gasUsed': 29035, 'logs': [], 'logsBloom': HexBytes('0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'), 'status': 1, 'to': '0x3d8fB1A9aaBFEfd8cf5910202d05E664CcBd02D5', 'transactionHash': HexBytes('0xb00663cc2e9240c63aea8227046f4f508b1f1e5bdf406136ecc6ee5eead1170a'), 'transactionIndex': 0, 'type': '0x0'})
2022-02-12 19:59:32.402 | INFO | __main__:<module>:34 - True
2022-02-12 19:59:32.452 | INFO | __main__:<module>:35 - True
2022-02-12 19:59:32.452 | SUCCESS | __main__:<module>:37 - Execution completed.
到服务器提交结果，获得flag。
（赛后向出题人询问了一下，确实是题目代码上写漏了，原本应该是代币余额大于5000 ether才满足条件，考察利用闪电贷内的重入攻击实现余额转换，但由于Setup合约代码第14行上少写了ether，导致合约使用了基本单位wei来计量，所以直接转账给VNETH合约5000 wei的test ETH就达成了条件。当时也是急着做其他题，这题做出来后就没管了。）
如果按原考察意图解法的话我觉得应该是调用VNETH合约的fakeflashloan()进行闪电贷，并call自己的攻击合约，攻击合约内构造持续不断地再调用闪电贷，直到代币余额达到5000 ether(即5000*10**18 wei)，之后攻击合约不再调用VNETH的fakeflashloan，而是调用Setup合约的checksuccess()，达成解出条件，之后放出执行权，回到VNETH合约，完成闪电贷攻击流程。思路大概是这样，实现起来并不复杂，只需编写攻击合约代为操作即可，时间关系我就不复现正确解法了，理解具体流程即可。关于闪电贷攻击如果想了解更多可以浏览一下这篇文章。
Crypto
ezmath
首先浏览一遍server.py，寻找关键信息。
从此处代码得知，我们需要求出第x个满足条件(2^n-1)%15=0的n，并成功提交777次至服务器，才可获得flag。
形式有点像数论领域的欧拉函数，使用简单的脚本寻找一下数学规律。
for i in range(1, 101):
if (2**i - 1) % 15 == 0:
print(i, end=" ")
# 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100可以发现，第x个满足条件的n等于4*x，还是比较容易求解的。
剩下的就是与服务器交互的问题了，使用下面的一把梭脚本直接获取flag。
import hashlib
from pwn import *
from pwnlib.util.iters import mbruteforce
def ProofOfWork_SHA256(Url, Port, Length, Begin, SendAfter):
Connection = remote(Url, Port)
Connection.recvuntil(Begin)
s = Connection.recvuntil(") == ", drop=True).decode().strip()
hash = Connection.recvuntil("\n", drop=True).decode().strip()
Charset = string.printable
Proof = mbruteforce(lambda x: hashlib.sha256((x + s).encode()).hexdigest() == hash, Charset, Length, method='fixed')
Connection.sendlineafter(SendAfter, Proof)
return Connection
Connection = ProofOfWork_SHA256("node4.buuoj.cn", 29570, 4, "sha256(XXXX+", "XXXX :")
for i in range(777):
Connection.recvuntil("plz give me the ")
num = Connection.recvuntil("th (n)", drop=True).decode().strip()
answer = str(int(num) * 4)
Connection.sendlineafter("is 15):", answer)
Connection.interactive()
Misc
仔细找找
放大图片后发现明显的像素点，尝试将其提取出来合成图片。
在合成过程中发现特殊像素点的间距是不固定的，需要写一段测试脚本将其间距求出，之后再运行完整的图片合成脚本。
（这里使用到了Poseidon_Normal里的RGBToImage()函数）
from PIL import Image
from Poseidon_Normal import *
img = Image.open('flag.png')
img = img.convert('RGB')
xf = [24, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50,
49, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 49, 50, 50, 49, 50, 50, 49, 50, 49, 50, 50]
yf = [15, 31, 31, 31, 31, 31, 31, 31, 31, 32, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 32, 31, 31, 31, 31, 31, 31, 31,
31, 31, 31, 31, 31, 31, 31, 31, 31, 32, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 32, 31, 31, 31, 31, 31, 31, 31, 31]
'''
for i in range(2205):
if img.getpixel((i, 16)) == (255, 255, 255):
xf.append(i)
xf.reverse()
for i in range(len(xf) - 1):
xf[i] -= xf[i + 1]
xf.reverse()
print(xf)
for i in range(2205):
if img.getpixel((25, i)) == (255, 255, 255):
yf.append(i)
yf.reverse()
for i in range(len(yf) - 1):
yf[i] -= yf[i + 1]
yf.reverse()
print(yf)
'''
data = []
x = 0
y = 0
xi = 0
yj = 0
for i in xf:
xi += i
x += 1
yj = 0
y = 0
for j in yf:
yj += j
y += 1
r, g, b = img.getpixel((xi, yj))
data.append((r, g, b))
RGBToImage(data, x, y)
运行脚本后得到flag图片。
Web
GameV4.0
签到题，找到疑似flag的信息，Base64解码之后再Url解码，即可得到flag。当然想玩游戏的话也是可以的。
crazy · 2023-08-26 10:18
这里使用到了Poseidon_Normal里的RGBToImage()函数
大佬，这个好像不存在了啊
B1ue1nWh1te · 2023-09-01 13:28 作者
是的，新版已经删去了。