Python创建区块链

看了很久的区块链知识点，在学习前编写过此篇博客的内容，现在再次编写一次，希望可以有更多的学习心得体会

环境准备

确保安装了Python3.6+，pip，Flask，Requests,如果在mac上python的环境是3.6＋，安装Flask和Requests应使用pip3

pip3 install Flask==0.12.2 requests==2.18.4

与此同时还需要HTTTP客户端，比如Postman，cURL或其他客户端。本次实例中，使用的是postman。

构建BlockChain

Blockchain类

首先创建Blockchain类，在构造函数中创建了两个列表，一个用于存储区块链，一个用于存储交易。 以下是Blockchian类框架：

class Blockchain(object):
    def __init__(self):
        self.chain = []
        self.current_transactions = []

    def new_block(self):
        # Creates a new Block and adds it to the chain
        pass

    def new_transaction(self):
        # Adds a new transaction to the list of transactions
        pass

    @staticmethod
    def hash(block):
        # Hashes a Block
        pass

    @property
    def last_block(self):
        # Returns the last Block in the chain
        pass

Blockchain类用来管理链条，它能存储交易，加入新块等。

块结构

每个区块包含属性：索引（index），Unix时间戳（timestamp），交易列表（transactions），工作量证明，上一个区块的Hash。下边是一个区块的结构：

block = {
    'index': 1,
    'timestamp': 1506057125.900785,
    'transactions': [
        {
            'sender': "8527147fe1f5426f9dd545de4b27ee00",
            'recipient': "a77f5cdfa2934df3954a5c7c7da5df1f",
            'amount': 5,
        }
    ],
    'proof': 324984774000,
    'previous_hash': "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"
}

通过区块Hash，保障区块链的不可变性，如果攻击者破坏前面的某个区块，那么后边的所有区块Hash都会变得不正确。

交易

添加一项交易，完善new_transaction方法

class Blockchain(object):
    ...

    def new_transaction(self, sender, recipient, amount):
        """
        生成新交易信息，信息将加入到下一个待挖的区块中
        :param sender: <str> Address of the Sender
        :param recipient: <str> Address of the Recipient
        :param amount: <int> Amount
        :return: <int> The index of the Block that will hold this transaction
        """

        self.current_transactions.append({
            'sender': sender,
            'recipient': recipient,
            'amount': amount,
        })

        return self.last_block['index'] + 1

通过向列表中添加交易，返回该记录被添加到区块，下一个待挖掘的区块的索引，等下在用户提交交易时会用。

创建新块

当把Blockchain实例话后，需要创造创世区块，并且加入工作量的证明。每个区块都需要经过工作量证明，俗称挖矿。完善new_block（），new_transaction（）和hash（）方法。

import hashlib
import json
from time import time


class Blockchain(object):
    def __init__(self):
        self.current_transactions = []
        self.chain = []

        # Create the genesis block
        self.new_block(previous_hash=1, proof=100)

    def new_block(self, proof, previous_hash=None):
        """
        生成新块
        :param proof: <int> The proof given by the Proof of Work algorithm
        :param previous_hash: (Optional) <str> Hash of previous Block
        :return: <dict> New Block
        """

        block = {
            'index': len(self.chain) + 1,
            'timestamp': time(),
            'transactions': self.current_transactions,
            'proof': proof,
            'previous_hash': previous_hash or self.hash(self.chain[-1]),
        }

        # Reset the current list of transactions
        self.current_transactions = []

        self.chain.append(block)
        return block

    def new_transaction(self, sender, recipient, amount):
        """
        生成新交易信息，信息将加入到下一个待挖的区块中
        :param sender: <str> Address of the Sender
        :param recipient: <str> Address of the Recipient
        :param amount: <int> Amount
        :return: <int> The index of the Block that will hold this transaction
        """
        self.current_transactions.append({
            'sender': sender,
            'recipient': recipient,
            'amount': amount,
        })

        return self.last_block['index'] + 1

    @property
    def last_block(self):
        return self.chain[-1]

    @staticmethod
    def hash(block):
        """
        生成块的 SHA-256 hash值
        :param block: <dict> Block
        :return: <str>
        """

        # We must make sure that the Dictionary is Ordered, or we'll have inconsistent hashes
        block_string = json.dumps(block, sort_keys=True).encode()
        return hashlib.sha256(block_string).hexdigest()

实现工作量证明

新区块依赖工作量证明算法（proof of work）构造。PoW的目标是找出一个符合特定条件的数字，这个数字很难计算，但容易验证。 实现一个较简单的pow算法，规则：寻找一个数p，使得他与前一个区块的proof拼接撑的字符串的Hash值为4个0开头。

import hashlib
import json

from time import time
from uuid import uuid4


class Blockchain(object):
    ...

    def proof_of_work(self, last_proof):
        """
        简单的工作量证明:
         - 查找一个 p' 使得 hash(pp') 以4个0开头
         - p 是上一个块的证明,  p' 是当前的证明
        :param last_proof: <int>
        :return: <int>
        """

        proof = 0
        while self.valid_proof(last_proof, proof) is False:
            proof += 1

        return proof

    @staticmethod
    def valid_proof(last_proof, proof):
        """
        验证证明: 是否hash(last_proof, proof)以4个0开头?
        :param last_proof: <int> Previous Proof
        :param proof: <int> Current Proof
        :return: <bool> True if correct, False if not.
        """

        guess = f'{last_proof}{proof}'.encode()
        guess_hash = hashlib.sha256(guess).hexdigest()
        return guess_hash[:4] == "0000"

算法复杂度通过修改0的个数。0个数越多，复杂度越大。

Blockchain作为API接口

使用python flask框架，一个轻量Web应用框架，它方便将网络请求映射到python函数，现在将Blockchain运行在基于Flask web上。 我们将创建三个接口：

• /transactions/new 创建一个交易并且添加到区块
• ／mine 告诉服务器去挖掘新区块
• ／chain 返回整个区块链

创建节点

添加框架代码：

import hashlib
import json
from textwrap import dedent
from time import time
from uuid import uuid4

from flask import Flask


class Blockchain(object):
    ...


# Instantiate our Node
app = Flask(__name__)

# Generate a globally unique address for this node
node_identifier = str(uuid4()).replace('-', '')

# Instantiate the Blockchain
blockchain = Blockchain()


@app.route('/mine', methods=['GET'])
def mine():
    return "We'll mine a new Block"

@app.route('/transactions/new', methods=['POST'])
def new_transaction():
    return "We'll add a new transaction"

@app.route('/chain', methods=['GET'])
def full_chain():
    response = {
        'chain': blockchain.chain,
        'length': len(blockchain.chain),
    }
    return jsonify(response), 200

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

发送交易

发送的节点的交易数据结构如下：

{
 "sender": "my address",
 "recipient": "someone else's address",
 "amount": 5
}

基于接口添加交易，代码如下：

import hashlib
import json
from textwrap import dedent
from time import time
from uuid import uuid4

from flask import Flask, jsonify, request

...

@app.route('/transactions/new', methods=['POST'])
def new_transaction():
    values = request.get_json()

    # Check that the required fields are in the POST'ed data
    required = ['sender', 'recipient', 'amount']
    if not all(k in values for k in required):
        return 'Missing values', 400

    # Create a new Transaction
    index = blockchain.new_transaction(values['sender'], values['recipient'], values['amount'])

    response = {'message': f'Transaction will be added to Block {index}'}
    return jsonify(response), 201

挖矿

做三件事情：

• 计算工作量证明PoW
• 通过增加一个交易授予矿工（自己）一个币
• 构造新区块并将其添加到链中 ```Python import hashlib import json

from time import time from uuid import uuid4

from flask import Flask, jsonify, request

…

@app.route(‘/mine’, methods=[‘GET’]) def mine(): # We run the proof of work algorithm to get the next proof… last_block = blockchain.last_block last_proof = last_block[‘proof’] proof = blockchain.proof_of_work(last_proof)

# 给工作量证明的节点提供奖励.
# 发送者为 "0" 表明是新挖出的币
blockchain.new_transaction(
    sender="0",
    recipient=node_identifier,
    amount=1,
)

# Forge the new Block by adding it to the chain
block = blockchain.new_block(proof)

response = {
    'message': "New Block Forged",
    'index': block['index'],
    'transactions': block['transactions'],
    'proof': block['proof'],
    'previous_hash': block['previous_hash'],
}
return jsonify(response), 200

交易的接受者为服务器节点。

### 一致性（共识）
区块链系统是分布式的，既然是分布式的，那么我们究竟拿什么保证所有节点有同样的链，，如果网络上有许多节点，那么必须实现一致性算法。

### 注册节点
在实现一致性算法前，我们需要找到一种方式让一个节点知道其相邻节点。每个节点都需要保存一份包含网络中其他节点的记录。因此让我们新增几个接口：
* ／nodes／register 接受URL形式的新节点列表
* ／nodes/resolve 执行一致性算法，解决任何冲突问题，确保节点拥有链结构。

提供注册节点方法：
```Python
from urllib.parse import urlparse
...


class Blockchain(object):
    def __init__(self):
        ...
        self.nodes = set()
        ...

    def register_node(self, address):
        """
        Add a new node to the list of nodes
        :param address: <str> Address of node. Eg. 'http://192.168.0.5:5000'
        :return: None
        """

        parsed_url = urlparse(address)
        self.nodes.add(parsed_url.netloc)

实现共识算法

前边提到不同的节点拥有不同的链，为了解决问题，规定最长的，有效的链才是最终的链。长链才有效。 共识算法如下：

import requests


class Blockchain(object)
    ...

    def valid_chain(self, chain):
        """
        Determine if a given blockchain is valid
        :param chain: <list> A blockchain
        :return: <bool> True if valid, False if not
        """

        last_block = chain[0]
        current_index = 1

        while current_index < len(chain):
            block = chain[current_index]
            print(f'{last_block}')
            print(f'{block}')
            print("\n-----------\n")
            # Check that the hash of the block is correct
            if block['previous_hash'] != self.hash(last_block):
                return False

            # Check that the Proof of Work is correct
            if not self.valid_proof(last_block['proof'], block['proof']):
                return False

            last_block = block
            current_index += 1

        return True

    def resolve_conflicts(self):
        """
        共识算法解决冲突
        使用网络中最长的链.
        :return: <bool> True 如果链被取代, 否则为False
        """

        neighbours = self.nodes
        new_chain = None

        # We're only looking for chains longer than ours
        max_length = len(self.chain)

        # Grab and verify the chains from all the nodes in our network
        for node in neighbours:
            response = requests.get(f'http://{node}/chain')

            if response.status_code == 200:
                length = response.json()['length']
                chain = response.json()['chain']

                # Check if the length is longer and the chain is valid
                if length > max_length and self.valid_chain(chain):
                    max_length = length
                    new_chain = chain

        # Replace our chain if we discovered a new, valid chain longer than ours
        if new_chain:
            self.chain = new_chain
            return True

        return False

添加相应的路由，一个用来注册节点，另一个解决冲突。

@app.route('/nodes/register', methods=['POST'])
def register_nodes():
    values = request.get_json()

    nodes = values.get('nodes')
    if nodes is None:
        return "Error: Please supply a valid list of nodes", 400

    for node in nodes:
        blockchain.register_node(node)

    response = {
        'message': 'New nodes have been added',
        'total_nodes': list(blockchain.nodes),
    }
    return jsonify(response), 201


@app.route('/nodes/resolve', methods=['GET'])
def consensus():
    replaced = blockchain.resolve_conflicts()

    if replaced:
        response = {
            'message': 'Our chain was replaced',
            'new_chain': blockchain.chain
        }
    else:
        response = {
            'message': 'Our chain is authoritative',
            'chain': blockchain.chain
        }

    return jsonify(response), 200

参考文献

原文链接：https://hackernoon.com/learn-blockchains-by-building-one-117428612f46