【放码过来】谈双重支付

注：本文没有讲述什么是双花问题，以及各种共识协定是如何防范双花的，只是从代码角度让之前对双重支付有一定了解的同学有更具体化的认识。当然，也是自己在看书的时候，为达到知行合一的简单实践。

先从维基百科上面摘录关于双重支付的资料。链接。大家简单回顾下。

双重支付（又称一币多付、双花攻击[1]，double-spending）是一种数字货币失败模式的构想，即同一个数字token可以被花用两次以上。不像具有实体的符号货币如硬币，电子文件可被复制，所以花用这个行为并不会从原持有者身上移除拥有的状态，也就是”创建”已支付但未移除的货币，加上属于收款者的已支付的同金额货币，或是使收款者凭空多出多重支付的金额，犹如伪钞般，造成通货膨胀而导致货币贬值，从而不再让人信任并愿意持有及流通。[2][3]防止双重支付需要其他的措施。

代码可从此链接访问

其中可以设置在交易完成之后，经过了多少次确认之后，evil nodes开始双重支付。最后使用一个图来表示，有正确交易的链的增长情况和包含不正确交易的链的增长情况。

恶意节点们没有51%算力时，失败的双重支付

恶意节点们超过51%算力时，成功的双重支付

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import uuid
import random
import matplotlib.pyplot as pltclass Node:def __init__(self, name, latest_block, is_good=True):self.name = nameself.latest_block = latest_blockself.is_good = is_gooddef create_block(self, wait_time=False):new_block = Noneif wait_time is True :new_block = Block(uuid.uuid1(), self.latest_block, True)elif self.is_good is not True or self.latest_block.in_good_chain is not True:new_block = Block(uuid.uuid1(), self.latest_block, False)else:new_block = Block(uuid.uuid1(), self.latest_block, True)return new_blockdef update_latest_block(self, block):self.latest_block = blockclass Block:def __init__(self, id, pre_block, in_good_chain=True):self.id = idself.pre_block = pre_blockself.in_good_chain = in_good_chainclass System:def __init__(self):self.genesis_block = System.create_genesis_block()self.latest_node = self.genesis_block@staticmethoddef create_genesis_block():genesis_block = Block('0', None)return genesis_block@staticmethoddef boardcast_latest_block(latest_blocks, nodes):max_length = 0max_length_block = Nonefor b in latest_blocks:l = System.length_of_chain(b)if l > max_length:max_length_block = bmax_length = lfor n in nodes:n.update_latest_block(max_length_block)def latest_node(self):return self.latest_node@staticmethoddef length_of_chain(block):length = 1while block.pre_block is not None:length += 1block = block.pre_blockreturn length# infact, each trans will be boardcast, and nodes will create blocks,# then a random node will success create a block than othersdef select_node(self, nodes):import time# time.sleep(0.5)index =  random.randrange(0, len(nodes)-1)return nodes[index]if __name__ == '__main__':bitcoin = System()# init good nodes and evil nodestotal_node_num = 100good_node_num = 49evil_node_num = total_node_num - good_node_numname_index = 1good_nodes = []evil_nodes = []total_nodes = []for i in range(good_node_num):g = Node(str(name_index), bitcoin.latest_node)name_index += 1good_nodes.append(g)total_nodes.append(g)for j in range(evil_node_num):e = Node(str(name_index), bitcoin.latest_node, False)name_index += 1evil_nodes.append(e)total_nodes.append(e)random.shuffle(total_nodes)# the evil node will backup a node for fork-attackingfork_block = bitcoin.latest_nodegood_latest_block = bitcoin.latest_nodeevil_latest_block = bitcoin.latest_nodeattack_delay_block_num = 6# before attack the good nodes and evil nodes will work for the right thingsfor i in range(attack_delay_block_num):the_node = bitcoin.select_node(total_nodes)latest_block = the_node.create_block(wait_time=True)latest_blocks = [latest_block, ]System.boardcast_latest_block(latest_blocks, nodes=total_nodes)good_latest_block = latest_block# now, the evil nodes start to attackevil_latest_blocks = [fork_block]System.boardcast_latest_block(evil_latest_blocks, evil_nodes)choose_good_node_times = 0choose_evil_node_times = 0good_chain_length_records = []evil_chain_length_records = []index_records = []max_times = 500b_first_show = Trueplt.figure()plt.xlim(0, max_times)plt.ylim(0, max_times / 2)for i in range(max_times):the_node = bitcoin.select_node(total_nodes)if the_node.is_good is False:choose_evil_node_times += 1print("Choose a evil node. good/evil ratio is {}".format(choose_good_node_times / choose_evil_node_times))else:choose_good_node_times += 1latest_block = the_node.create_block()if latest_block.in_good_chain is False:evil_latest_block = latest_block# evil nodes only admit the chain contains evil blockSystem.boardcast_latest_block([evil_latest_block, ], evil_nodes)else:good_latest_block = latest_blockSystem.boardcast_latest_block([good_latest_block, evil_latest_block], good_nodes)good_chain_length = System.length_of_chain(good_latest_block)evil_chain_length = System.length_of_chain(evil_latest_block)good_chain_length_records.append(good_chain_length)evil_chain_length_records.append(evil_chain_length)index_records.append(i)print("Iteration number:\t{}".format(i))print("good chain length is {}".format(good_chain_length))print("evil chain length is {}".format(evil_chain_length))if good_chain_length > evil_chain_length:print("Evil can never prevail over good")else:print("While the priest climbs a foot, the devil climbs ten")if i % 10 == 0:plt.plot(index_records, good_chain_length_records, "b", label="length of good chain")plt.plot(index_records, evil_chain_length_records, "r", label="length of evil chain")if b_first_show is True:b_first_show = Falseplt.legend()plt.pause(0.5)

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