初识区块链——用JS构建你自己的区块链
前言
区块链太复杂，那我们就讲点简单的。用JS来构建你自己的区块链系统，寥寥几行代码就可以说明区块链的底层数据结构、POW挖矿思想和交易过程等。当然了，真实的场景远远远比这复杂。本文的目的仅限于让大家初步了解、初步认识区块链。
文章内容主要参考视频：Building a blockchain with Javascript (https://www.youtube.com/playlist?list=PLzvRQMJ9HDiTqZmbtFisdXFxul5k0F-Q4)
感谢原作者，本文在原视频基础上做了修改补充，并加入了个人理解。
认识区块链
区块链顾名思义是由区块连接而成的链，因此最基本的数据结构是Block。每个Block都含有timestamp、data、hash、previousHash等信息。其中data用来存储数据，previousHash是前一个区块的hash值。示意如下：
hash是对区块信息的摘要存储，hash的好处是任意长度的信息经过hash都可以映射成固定长度的字符串，如可用sha256：
calculateHash() {
return SHA256(this.previousHash + this.timestamp + JSON.stringify(this.data)).toString();
}
Block的数据结构
Block的最基本数据结构如下：
class Block {
constructor(timestamp, data, previousHash = \'\') {
this.timestamp = timestamp;
this.data = data;
this.previousHash = previousHash;
//对hash的计算必须放在最后，保证所有数据赋值正确后再计算
this.hash = this.calculateHash();
}
calculateHash() {
return SHA256(this.previousHash + this.timestamp + JSON.stringify(this.data)).toString();
}
}
BlockChain的数据结构
多个Block链接而成BlockChain，显然可用用数组或链表来表示，如：
class BlockChain {
constructor() {
this.chain = [];
}
}
创世区块
正所谓万物始于一，区块链的第一个区块总是需要人为来手动创建，这个区块的previousHash为空，如：
createGenesisBlock() {
return new Block("2018-11-11 00:00:00", "Genesis block of simple chain", "");
}
区块链的构造方法也应改为：
class BlockChain {
constructor() {
this.chain = [this.createGenesisBlock()];
}
}
添加区块
每新加一个区块，必须保证与原有区块链连接起来，即：
class BlockChain {
getLatestBlock() {
return this.chain[this.chain.length - 1];
}
addBlock(newBlock) {
//新区块的前一个hash值是现有区块链的最后一个区块的hash值；
newBlock.previousHash = this.getLatestBlock().hash;
//重新计算新区块的hash值（因为指定了previousHash)；
newBlock.hash = newBlock.calculateHash();
//把新区块加入到链中；
this.chain.push(newBlock);
}
...
}
校验区块链
区块链数据结构的核心是保证前后链接、无法篡改，但是如果有人真的篡改了某个区块，我们该如何校验发现呢？最笨也是最自然是想法就是遍历所有情况，逐一校验，如：
isChainValid() {
//遍历所有区块
for (let i = 1; i < this.chain.length; i++) {
const currentBlock = this.chain[i];
const previousBlock = this.chain[i - 1];
//重新计算当前区块的hash值，若发现hash值对不上，说明该区块有数据被篡改，hash值未重新计算
if (currentBlock.hash !== currentBlock.calculateHash()) {
console.error("hash not equal: " + JSON.stringify(currentBlock));
return false;
}
//判断当前区块的previousHash是否真的等于前一个区块的hash，若不等，说明前一个区块被篡改，虽然hash值被重新计算正确，但是后续区块的hash值并未重新计算，导致整个链断裂
if (currentBlock.previousHash !== previousBlock.calculateHash) {
console.error("previous hash not right: " + JSON.stringify(currentBlock));
return false;
}
}
return true;
}
Just run it
跑起来看看，即：
let simpleChain = new BlockChain();
simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));
simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));
console.log(JSON.stringify(simpleChain, null, 4));
console.log("is the chain valid? " + simpleChain.isChainValid());
结果如下：
ali-186590cc4a7f:simple-chain shanyao$ node main_1.js
{
"chain": [
{
"timestamp": "2018-11-11 00:00:00",
"data": "Genesis block of simple chain",
"previousHash": "",
"hash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89"
},
{
"timestamp": "2018-11-11 00:00:01",
"data": {
"amount": 10
},
"previousHash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89",
"hash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529"
},
{
"timestamp": "2018-11-11 00:00:02",
"data": {
"amount": 20
},
"previousHash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529",
"hash": "274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34"
}
]
}
is the chain valid? true
注意看其中的previousHash与hash，确实是当前区块的previousHash指向前一个区块的hash。
篡改下试试
都说区块链不可篡改，是真的吗？让我们篡改第2个区块试试，如：
let simpleChain = new BlockChain();
simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));
simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));
console.log("is the chain valid? " + simpleChain.isChainValid());
//将第2个区块的数据，由10改为15
simpleChain.chain[1].data = {amount: 15};
console.log("is the chain still valid? " + simpleChain.isChainValid());
console.log(JSON.stringify(simpleChain, null, 4));
结果如下：
ali-186590cc4a7f:simple-chain shanyao$ node main_1.js
is the chain valid? true
hash not equal: {"timestamp":"2018-11-11 00:00:01","data":{"amount":15},"previousHash":"fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89","hash":"150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529"}
is the chain still valid? false
{
"chain": [
{
"timestamp": "2018-11-11 00:00:00",
"data": "Genesis block of simple chain",
"previousHash": "",
"hash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89"
},
{
"timestamp": "2018-11-11 00:00:01",
"data": {
"amount": 15
},
"previousHash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89",
"hash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529"
},
{
"timestamp": "2018-11-11 00:00:02",
"data": {
"amount": 20
},
"previousHash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529",
"hash": "274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34"
}
]
}
显然，篡改了数据之后，hash值并未重新计算，导致该区块的hash值对不上。
再篡改下试试
那么，如果我们聪明点，篡改后把hash值也重新计算会如何？
let simpleChain = new BlockChain();
simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));
simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));
console.log("is the chain valid? " + simpleChain.isChainValid());
//篡改后重新计算hash值
simpleChain.chain[1].data = {amount: 15};
simpleChain.chain[1].hash = simpleChain.chain[1].calculateHash();
console.log("is the chain still valid? " + simpleChain.isChainValid());
console.log(JSON.stringify(simpleChain, null, 4));
结果如下：
ali-186590cc4a7f:simple-chain shanyao$ node main_1.js
is the chain valid? true
previous hash not right: {"timestamp":"2018-11-11 00:00:02","data":{"amount":20},"previousHash":"150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529","hash":"274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34"}
is the chain still valid? false
{
"chain": [
{
"timestamp": "2018-11-11 00:00:00",
"data": "Genesis block of simple chain",
"previousHash": "",
"hash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89"
},
{
"timestamp": "2018-11-11 00:00:01",
"data": {
"amount": 15
},
"previousHash": "fd56967ff621a4090ff71ce88fdd456547d1c92d2e93766b7e8791f7a5f91f89",
"hash": "74d139274fb692495b7c805dd5822faa0c5b5e6058b6beef96e87e18ab83a6b1"
},
{
"timestamp": "2018-11-11 00:00:02",
"data": {
"amount": 20
},
"previousHash": "150b196268a0152e9f0e719ac131a722472a809f49bd507965029a78c7400529",
"hash": "274a7a13ed20118e8cb745654934a7e24a4d59333ba17dfbf5d4cfe0fa8a6e34"
}
]
}
显然，第3个区块的previousHash并未指向第2个区块的hash。
是真的无法篡改吗
其实并不是，如果我们再聪明一点，把后续区块的hash值也重新计算一下，不就OK了吗？ 确实如此，如：
let simpleChain = new BlockChain();
simpleChain.addBlock(new Block("2018-11-11 00:00:01", {amount: 10}));
simpleChain.addBlock(new Block("2018-11-11 00:00:02", {amount: 20}));
console.log("is the chain valid? " + simpleChain.isChainValid());
//篡改第2个区块
simpleChain.chain[1].data = {amount: 15};
simpleChain.chain[1].hash = simpleChain.chain[1].calculateHash();
//并把第3个区块也重新计算
simpleChain.chain[2].previousHash = simpleChain.chain[1].hash;
simpleChain.chain[2].hash = simpleChain.chain[2].calculateHash();
console.log("is the chain still valid? " + simpleChain.isChainValid());
console.log(JSON.stringify(simpleChain, null, 4
原文链接
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