1、浏览器区块链Demo
区块链的概念起源于比特币，其本质上是一个去中心化的数据库，是分布式数据存储、点对点传输（P2P）、共识机制、加密算法等计算机技术的新型应用模式（摘自百科）。
为了更好地形象地了解区块链的一些概念和结构组成，可以查看浏览器版的区块链Demo。
浏览器版区块链Demo链接
浏览器版区块链Demo
通过该Demo，可以深刻地理解区块链、创世区块、时间戳、哈希、区块链哈希计算、前继哈希、挖矿、NONCE随机数、添加有效区块、P2P网络、节点、51%攻击等相关概念（需要点英文理解能力）
2、安装命令行工具Blockchain CLI
除了浏览器，还可以通过安装一个区块链Demo的命令行工具来理解区块链结构组成。
将本篇单独分拆成一个文档的主要原因是，安装这个工具的时候，遇到了一个坑，发现安装不了。尝试了多个方法，结果都一样。该Demo有十个月左右没更新了，在Issue讨论区别人也碰到了一些错误，安装不了。后面，演示一下问题所在和最后解决的方法。
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安装前准备
Github地址：
https://github.com/seanjameshan/blockchain-cli上图来自Github作者的图片安装有个前提，就是确保已经安装了Node.js（包含npm）。
可以先通过终端检查安装情况:
wenzildeiMac:~ wenzil$ npm -v 3.10.10 wenzildeiMac:~ wenzil$ node -v v6.9.5如果没有安装Node.js，可以通过如下网址下载对应的安装包，安装比较简单：
https://nodejs.org/en/download/安装方法一：官网介绍，然后执行如下命令
# Clone this repository $ git clone https://github.com/seanseany/blockchain-cli # Go into the repository $ cd blockchain-cli # Install dependencies $ npm install # Run the app $ npm start安装方法二：通过在终端执行如下命令来安装：
npm install blockchain-cli -g -
安装问题描述
方法一跟方法二可能都会碰到安装不了的问题，问题都是相同的。以下以Git安装为例，演示下问题。wenzildeiMac:Blockchain wenzil$ git clone https://github.com/seanjameshan/blockchain-cli.git Cloning into 'blockchain-cli'... remote: Counting objects: 510, done. ^Cceiving objects: 33% (169/510), 76.01 KiB | 36.00 KiB/s wenzildeiMac:Blockchain wenzil$ cd .. wenzildeiMac:study wenzil$ git clone https://github.com/seanjameshan/blockchain-cli.git Cloning into 'blockchain-cli'... remote: Counting objects: 510, done. remote: Total 510 (delta 0), reused 0 (delta 0), pack-reused 510 Receiving objects: 100% (510/510), 3.00 MiB | 129.00 KiB/s, done. Resolving deltas: 100% (276/276), done. wenzildeiMac:study wenzil$ cd blockchain-cli wenzildeiMac:blockchain-cli wenzil$ npm install npm WARN deprecated formatio@1.2.0: This package is unmaintained. Use @sinonjs/formatio instead > fsevents@1.2.4 install /Users/wenzil/Desktop/study/blockchain-cli/node_modules/fsevents > node install [fsevents] Success: "/Users/wenzil/Desktop/study/blockchain-cli/node_modules/fsevents/lib/binding/Release/node-v48-darwin-x64/fse.node" already installed Pass --update-binary to reinstall or --build-from-source to recompile > wrtc@0.0.62 install /Users/wenzil/Desktop/study/blockchain-cli/node_modules/wrtc > node-pre-gyp install --fallback-to-build node-pre-gyp ERR! Tried to download(undefined): https://node-webrtc.s3.amazonaws.com/wrtc/v0.0.62/Release/node-v48-darwin-x64.tar.gz node-pre-gyp ERR! Pre-built binaries not found for wrtc@0.0.62 and node@6.9.5 (node-v48 ABI, unknown) (falling back to source compile with node-gyp) ACTION Downloading WebRTC libraries and headers third_party/webrtc > wrtc@0.0.62 download-webrtc-libraries-and-headers /Users/wenzil/Desktop/study/blockchain-cli/node_modules/wrtc > node scripts/download-webrtc-libraries-and-headers.js Attempting to download WebRTC libraries and headers for platform "darwin" and architecture "x64" from https://webrtc-libraries-and-headers.s3.amazonaws.com/v1/build/webrtc-50%2B49f7bd3.darwin.x64.tar.gz ^Cmake: *** [third_party/webrtc] Interrupt: 2碰到的问题：
1、可能一直卡在"node-pre-gyp install --fallback-to-build"这一步，网上也有对应的解决方案，但是好像没有效果；
2、可能一直卡在上面这一步，一直在尝试下载"...darwin.x64.tar.gz"的文件；
3、重新安装不同的Node.js版本，也是一样有问题。
一直卡着不动的话，在Mac下可以按”Control+C“中断下载，如上图后面中断了。尝试了多种方法，都是失败，一度放弃。 -
解决安装问题
在百度中，无意发现了这个网站。
http://npm.taobao.org/package/blockchain-cli
里面有个很显眼的地方，写着如下一段命令：$ cnpm install blockchain-cli于是，就开始安装cnpm
sudo npm install -g cnpm --registry=https://registry.npm.taobao.orgwenzildeiMac:~ wenzil$ cnpm install blockchain-cli -g Downloading blockchain-cli to /Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli_tmp Copying /Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli_tmp/_blockchain-cli@1.0.5@blockchain-cli to /Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli Installing blockchain-cli's dependencies to /Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli/node_modules [1/9] ascii-table@0.0.9 installed at node_modules/_ascii-table@0.0.9@ascii-table [2/9] cli-spinners@^1.0.0 installed at node_modules/_cli-spinners@1.3.1@cli-spinners [3/9] colors@^1.1.2 installed at node_modules/_colors@1.2.5@colors [4/9] crypto-js@^3.1.9-1 installed at node_modules/_crypto-js@3.1.9-1@crypto-js [5/9] cli-table2@^0.2.0 installed at node_modules/_cli-table2@0.2.0@cli-table2 [6/9] vorpal-log@^1.1.0 installed at node_modules/_vorpal-log@1.1.0@vorpal-log [7/9] peer-exchange@^2.2.0 installed at node_modules/_peer-exchange@2.2.0@peer-exchange [8/9] vorpal@^1.12.0 installed at node_modules/_vorpal@1.12.0@vorpal [9/9] wrtc@0.0.62 installed at node_modules/_wrtc@0.0.62@wrtc execute post install 1 scripts... [1/1] scripts.install wrtc@0.0.62 run "node-pre-gyp install --fallback-to-build" node-pre-gyp http GET https://node-webrtc.s3.amazonaws.com/wrtc/v0.0.62/Release/node-v48-darwin-x64.tar.gz node-pre-gyp http 200 https://node-webrtc.s3.amazonaws.com/wrtc/v0.0.62/Release/node-v48-darwin-x64.tar.gz [wrtc] Success: "/Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli/node_modules/_wrtc@0.0.62@wrtc/build/wrtc/v0.0.62/Release/node-v48-darwin-x64/wrtc.node" is installed via remote [1/1] scripts.install wrtc@0.0.62 finished in 2m Recently updated (since 2018-05-09): 3 packages (detail see file /Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli/node_modules/.recently_updates.txt) 2018-05-15 → wrtc@0.0.62 › tar-fs@1.16.2 › tar-stream@^1.1.2(1.6.1) (05:19:03) 2018-05-11 → colors@^1.1.2(1.2.5) (14:15:42) 2018-05-10 → wrtc@0.0.62 › download@5.0.3 › decompress@4.2.0 › make-dir@^1.0.0(1.3.0) (21:52:55) All packages installed (246 packages installed from npm registry, used 2m, speed 47.94kB/s, json 236(1.48MB), tarball 4.3MB) [blockchain-cli@1.0.5] link /Users/wenzil/.nvm/versions/node/v6.9.5/bin/blockchain@ -> /Users/wenzil/.nvm/versions/node/v6.9.5/lib/node_modules/blockchain-cli/main.js -
使用Blockchain CLI:
安装成功后，在终端中输入"blockchain"命令wenzildeiMac:~ wenzil$ blockchain 👋 Welcome to Blockchain CLI! Commands: help [command...] Provides help for a given command. exit Exits application. blockchain See the current state of the blockchain. mine <data> Mine a new block. Eg: mine hello! open <port> Open port to accept incoming connections. Eg: open 2727 connect <host> <port> Connect to a new peer. Eg: connect localhost 2727 peers Get the list of connected peers. discover Discover new peers from your connected peers. blockchain →然后在"blockchain →"后面输入"blockchain"或者"bc"查看创世区块的结构
blockchain → bc ┌─────────────────────────────────────────┐ │ 🏆 Genesis Block │ ├────────────────────┬────────────────────┤ │ ⏮ Previous Hash │ 0 │ ├────────────────────┼────────────────────┤ │ 📅 Timestamp │ Thu, 27 Jul 2017 │ │ │ 02:30:00 GMT │ ├────────────────────┼────────────────────┤ │ 📄 Data │ Welcome to │ │ │ Blockchain CLI! │ ├────────────────────┼────────────────────┤ │ 📛 Hash │ 0000018035a828da0… │ ├────────────────────┼────────────────────┤ │ 🔨 Nonce │ 56551 │ └────────────────────┴────────────────────┘ blockchain →然后输入"mine wenzil"，进行模拟挖矿，产生新的区块：
blockchain → mine wenzil 💶 Mining new block. ┌─────────────────────────────────────────┐ │ ⛓ Block #1 │ ├────────────────────┬────────────────────┤ │ ⏮ Previous Hash │ 0000018035a828da0… │ ├────────────────────┼────────────────────┤ │ 📅 Timestamp │ Wed, 16 May 2018 │ │ │ 08:02:12 GMT │ ├────────────────────┼────────────────────┤ │ 📄 Data │ wenzil │ ├────────────────────┼────────────────────┤ │ 📛 Hash │ 0000a632fdcbfdee6… │ ├────────────────────┼────────────────────┤ │ 🔨 Nonce │ 43137 │ └────────────────────┴────────────────────┘ 🎉 Congratulations! A new block was mined. 💎 ⬆ Sending peer latest block哈希值是唯一标识数据的固定长度的数值(十六进制64位)。
Hash是通过将Index、Previous Hash、Timestamp、Data和Nonce作为输入值来计算的。CryptoJS.SHA256(index + previousHash + timestamp + data + nonce)注：CryptoJS (crypto.js)为JavaScript提供了各种各样的加密算法，支持的算法包括MD5、AES、SHA-1、SHA-256等等
举例（blockchaindemo的例子，看最前面第一图）：
f ( index + previous hash + timestamp + data + nonce ) = hash
于是有了
f ( 0 + "0" + 1508270000000 + "Welcome to Blockchain Demo 2.0!" + 604 ) = 000dc75a315c77a1f9c98fb6247d03dd18ac52632d7dc6a9920261d8109b37cf
打开如下网址，作验证：
https://anders.com/blockchain/hash.html
哈希算法验证
是不是发现跟刚才推导出来的哈希值一模一样，很神奇的赶脚，哈哈。
上述举例中对应的JS代码如下：
// const Block = require("./Block.js");
const crypto = require("crypto");
// class Blockchain {
// constructor() { ... }
// get() { ... }
// get latestBlock() { ... }
// isValidHashDifficulty(hash) { ... }
calculateHashForBlock(block) {
const { index, previousHash, timestamp, transactions, nonce } = block;
return this.calculateHash(
index,
previousHash,
timestamp,
transactions,
nonce
);
}
calculateHash(index, previousHash, timestamp, data, nonce) {
return crypto
.createHash("sha256") // SHA256 Hash Function
.update(index + previousHash + timestamp + data + nonce)
.digest("hex");
}
// };
// module.exports = Blockchain;
四个前导0是一个有效Hash的最低要求，所需前导0的数量被称为难度，也被称为工作量证明（PoW）。验证有效哈希难度的JS代码如下：
function isValidHashDifficulty(hash, difficulty) {
for (var i = 0, b = hash.length; i < b; i ++) {
if (hash[i] !== '0') {
break;
}
}
return i >= difficulty;
}
PS：刚入坑的小白，很多不懂，还请各位大佬多赐教，谢谢！
