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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