This article will introduce how to use blockchain to trade .

【 This article forbids any form of full text paste reprint , This article is from  zacky31  The essay of 】

The goal is ：

stay Last article in , We've created a trusted blockchain . But at present, the chain created contains less useful information . today , I'm going to replace the previous with trading information data Content , I'm going to create a simple cryptocurrency , called “noobcoin”.

Premise ：

  • I have learned the basic knowledge of blockchain
  • be used GSON and bounceycastle 

Let's get started

In cryptocurrencies , The ownership of money will be transferred by way of transaction , People involved in the transaction will have an address , Used to send and receive transaction amounts .

Upper figure , It reflects a transaction process .

First , We create a Wallet class , It is used to store public key and private key .

import java.security.PrivateKey;

import java.security.PublicKey;
public class Wallet {

public PrivateKey privateKey;

public PublicKey publicKey;
}

that , What are the public key and private key used for ？

Actually , The public key is the address we need for money . In the course of each transaction can be shared with the trading party . Our private key is used to encrypt every transaction , This ensures that people with private keys can trade . The private key can only be said to know ！ meanwhile , The public key is passed to the trading party along with the transaction , Can be used to verify the signature , Determine whether the data has been tampered with .

Next , You need to generate key value pairs of public key and private key . I'm going to use elliptic curve cryptography to generate . Let's revise it Wallet Class .

import java.security.*;

import java.security.spec.ECGenParameterSpec;
public class Wallet {

public PrivateKey privateKey;

public PublicKey publicKey;
public Wallet() {

generateKeyPair();

}
private void generateKeyPair() {

try {

Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider());

KeyPairGenerator keyGen = KeyPairGenerator.getInstance("ECDSA", "BC");

SecureRandom random = SecureRandom.getInstance("SHA1PRNG");

ECGenParameterSpec ecSpec = new ECGenParameterSpec("prime192v1");

keyGen.initialize(ecSpec, random);

KeyPair keyPair = keyGen.generateKeyPair();
privateKey = keyPair.getPrivate();

publicKey = keyPair.getPublic();

} catch (Exception e) {

throw new RuntimeException(e);

}

}
public static void main(String[] args) {

Wallet wallet = new Wallet();

System.out.println(wallet.publicKey);

System.out.println(wallet.privateKey);

}

}

up to now , We've created the wallet , Think about how to design a deal .

In every transaction , You need to carry some data ：

  • The payer's public key
  • The payee's public key
  • Transaction amount
  • Input , Previous trading references , Prove that the payer has the funds to carry out the transaction
  • Output , Display the relevant address received in this transaction
  • Encrypted signature , It is used to prove that the data has not been tampered in the process of transaction

Then go and create a Transaction Class .

import java.security.PublicKey;

import java.util.ArrayList;
public class Transcation {

public String transcationId;

public PublicKey sender;

public PublicKey reciepient;

public float value;

public byte[] signature;
public ArrayList<TranscationInput> inputs = new ArrayList<TranscationInput>();

public ArrayList<TranscationOutput> outputs = new ArrayList<TranscationOutput>();
private static int sequence = 0;
public Transcation(PublicKey from, PublicKey to, float value, ArrayList<TranscationInput> inputs) {

this.sender = from;

this.reciepient = to;

this.value = value;

this.inputs = inputs;

}
private String calulateHash() {

sequence++;

return StringUtil.applySha256(

StringUtil.getStringFromKey(sender) +

StringUtil.getStringFromKey(reciepient) +

Float.toString(value) + sequence

);

}
}

So far, , The program will report an error , For a while , I'm going to create  TransactionInput class and  TransactionOutput class . meanwhile , The transaction class needs to include methods for generating and verifying signatures and methods for verifying transactions . however , All of these have to wait a moment .

Consider the purpose of the signature and how it works ？

Signature plays two important roles in blockchain ： First , The signature guarantees the payer's consumption ; secondly , Preventing others from tampering with transactions that have taken place . This is using the private key to encrypt data , Public key to verify its reliability .

Let's start doing these things . Before that , stay StringUtils Class , Add two methods .applyECDSASig()  Used to generate signatures ,verifyECDSASig()   To verify the signature .

 public static byte[] applyECDSASig(PrivateKey privateKey, String input) {

Signature dsa;

byte[] output = new byte[0];

try {

dsa = Signature.getInstance("ECDSA", "BS");

dsa.initSign(privateKey);

byte[] strByte = input.getBytes();

dsa.update(strByte);

byte[] realSig = dsa.sign();

output = realSig;

} catch (Exception e) {

throw new RuntimeException(e);

}

return output;

}
public static boolean verifyECDSASig(PublicKey publicKey,String data,byte[] signature){

try{

Signature ecdsaVerify = Signature.getInstance("ECDSA","BC");

ecdsaVerify.initVerify(publicKey);

ecdsaVerify.update(data.getBytes());

return ecdsaVerify.verify(signature);

}catch (Exception e){

throw new RuntimeException(e);

}

}
public static String getStringFromKey(Key key){

return Base64.getEncoder().encodeToString(key.getEncoded());

}

next , Just use the method in the tool class , stay Transaction Class generateSignature()  and verifiySignatur() Method .

 public void generateSignature(PrivateKey privateKey) {

String data = StringUtil.getStringFromKey(sender) + StringUtil.getStringFromKey(reciepient);

signature = StringUtil.applyECDSASig(privateKey, data);

}
public boolean verfiySignature() {

String data = StringUtil.getStringFromKey(sender) + StringUtil.getStringFromKey(reciepient);

return StringUtil.verifyECDSASig(sender, data, signature);

}

It's more than half done , Go first Chain Class to test it .

import org.bouncycastle.jce.provider.BouncyCastleProvider;
import java.security.Security;

import java.util.ArrayList;
public class Chain {

public static ArrayList<Block> blockChain = new ArrayList<>();

public static int difficulty = 5;

public static Wallet walletA;

public static Wallet walletB;
public static void main(String[] args) {

Security.addProvider(new BouncyCastleProvider());

walletA = new Wallet();

walletB = new Wallet();
System.out.println("Private and Public keys:");

System.out.println(StringUtil.getStringFromKey(walletA.privateKey));

System.out.println(StringUtil.getStringFromKey(walletA.publicKey));
Transcation transcation = new Transcation(walletA.publicKey, walletB.publicKey, 5, null);

transcation.generateSignature(walletA.privateKey);
System.out.println("Is signature verified");

System.out.println(transcation.verfiySignature());

}

}

We can see , Successfully generated Private key and Public key , And be able to verify the authenticity .

Consider such a scenario , When you have a bitcoin , You have to have received a bitcoin before , Bitcoin's ledger doesn't add a bitcoin to your account or subtract a bitcoin from the consumer , The consumer can only point to a bitcoin that he or she has previously accepted , So a transaction output is created to show the address a bitcoin sent to you （ The input of the transaction points to the output of the previous transaction ）.

Starting from this point , We'll follow the instructions in bitcoin , All unused transaction outputs are called UTXO.

So create a TransactionInput class .

public class TransactionInput {

public String transactionOutputId;

public TransactionOutput UTXO;
public TransactionInput(String transactionOutputId) {

this.transactionOutputId = transactionOutputId;

}

}

as well as  TransactionOutputs class .  The output of the transaction will show the final amount sent to each party from the transaction , So it is referenced in the new transaction and not entered , As proof of the amount you can send .

import java.security.PublicKey;
public class TransactionOutput {

public String id;

public PublicKey reciepient;

public float value;

public String parentTransactionId;
public TransactionOutput(PublicKey reciepient, float value, String parentTransactionId) {

this.reciepient = reciepient;

this.value = value;

this.parentTransactionId = parentTransactionId;

this.id = StringUtil.applySha256(StringUtil.getStringFromKey(reciepient)+Float.toString(value)+parentTransactionId);

}
public boolean isMine(PublicKey publicKey) {

return (publicKey == reciepient);

}

}

Blocks in the chain will receive a lot of transaction information , So the blockchain will be very, very long , It takes a long time to process a new transaction because we have to look for and check its input , To get around this, we save an extra set called used transactions as available inputs , So adding a set to the main function is called UTXO.

Now revise it Transaction class . Add ways to process transactions .

public boolean processTransaction() {
if(verifySignature() == false) {

System.out.println("#Transaction Signature failed to verify");

return false;

}
for(TransactionInput i : inputs) {

i.UTXO = Chain.UTXOs.get(i.transactionOutputId);

}
if(getInputsValue() < Chain.minimumTransaction) {

System.out.println("Transaction Inputs too small: " + getInputsValue());

System.out.println("Please enter the amount greater than " + Chain.minimumTransaction);

return false;

}
float leftOver = getInputsValue() - value;

transactionId = calulateHash();

outputs.add(new TransactionOutput( this.reciepient, value,transactionId));

outputs.add(new TransactionOutput( this.sender, leftOver,transactionId));
for(TransactionOutput o : outputs) {

Chain.UTXOs.put(o.id , o);

}
for(TransactionInput i : inputs) {

if(i.UTXO == null) continue;

Chain.UTXOs.remove(i.UTXO.id);

}
return true;

}
public float getInputsValue() {

float total = 0;

for(TransactionInput i : inputs) {

if(i.UTXO == null) continue;

total += i.UTXO.value;

}

return total;

}

In this method, we perform some checks to make sure the transaction is valid , We collect input to produce output , most important of all , At the end of the feeding , We abandoned the input in our UTXO list , This means that an available transaction output must be input before it can be used , So the input value must be fully used , So payers have to change their own amount status . This is very similar to our actual situation , When you give 5 Yuan and then spent 1 element , I'll find you 4 element .

Last , Update Wallet , Add methods to get balance, etc . You are free to add some other features to your wallet , For example, keep a history of your transactions and so on .

public float getBalance() {

float total = 0;

for (Map.Entry<String, TransactionOutput> item: Chain.UTXOs.entrySet()){

TransactionOutput UTXO = item.getValue();

if(UTXO.isMine(publicKey)) {

UTXOs.put(UTXO.id,UTXO);

total += UTXO.value ;

}

}

return total;

}
public Transaction sendFunds(PublicKey _recipient,float value ) {

if(getBalance() < value) {

System.out.println("#Not Enough funds to send transaction. Transaction Discarded.");

return null;

}

ArrayList<TransactionInput> inputs = new ArrayList<TransactionInput>();
float total = 0;

for (Map.Entry<String, TransactionOutput> item: UTXOs.entrySet()){

TransactionOutput UTXO = item.getValue();

total += UTXO.value;

inputs.add(new TransactionInput(UTXO.id));

if(total > value) break;

}
Transaction newTransaction = new Transaction(publicKey, _recipient , value, inputs);

newTransaction.generateSignature(privateKey);
for(TransactionInput input: inputs){

UTXOs.remove(input.transactionOutputId);

}
return newTransaction;

}

Now? , There's already a trading system , It needs to be put in the blockchain . We replace some useless information in the block with a list of transactions , But in a single block there may be 1000 A deal , This leads to a lot of hash Calculation , Don't worry, we're using trading here merkle  Trees , You'll see later . Let's add a help method to create  merkleroot  stay StringUtils Class 

public static String getMerkleRoot(ArrayList<Transaction> transactions) {

int count = transactions.size();

ArrayList<String> previousTreeLayer = new ArrayList<>();

for (Transaction transaction : transactions) {

previousTreeLayer.add(transaction.transactionId);

}

ArrayList<String> treeLayer = previousTreeLayer;

while (count > 1) {

treeLayer = new ArrayList<>();

for (int i = 1; i < previousTreeLayer.size(); i++) {

treeLayer.add(applySha256(previousTreeLayer.get(i - 1) + previousTreeLayer.get(i)));

}

count = treeLayer.size();

previousTreeLayer = treeLayer;

}

String merkleRoot = (treeLayer.size() == 1) ? treeLayer.get(0) : "";

return merkleRoot;

}
public static String getDificultyString(int difficulty) {

return new String(new char[difficulty]).replace('\0', '0');

}

Block The class code is as follows ：

import java.util.ArrayList;

import java.util.Date;
public class Block {
public String hash;

public String previousHash;

public String merkleRoot;

public String data;

public ArrayList<Transaction> transactions = new ArrayList<>();

public long timeStamp;

public int nonce;
public Block(String previousHash) {

this.previousHash = previousHash;

this.timeStamp = new Date().getTime();

this.hash = calculateHash();

}
public Block(String data, String previousHash) {

this.data = data;

this.previousHash = previousHash;

this.timeStamp = new Date().getTime();

this.hash = calculateHash();

}
public String calculateHash() {

String calculatedhash = StringUtil.applySha256(

previousHash +

Long.toString(timeStamp) +

Integer.toString(nonce) +

merkleRoot

);

return calculatedhash;

}
public void mineBlock(int difficulty) {

merkleRoot = StringUtil.getMerkleRoot(transactions);

String target = StringUtil.getDificultyString(difficulty);

while (!hash.substring(0, difficulty).equals(target)) {

nonce++;

hash = calculateHash();

}

System.out.println("Block Mined!!! : " + hash);

}
public boolean addTransaction(Transaction transaction) {

if (transaction == null) return false;

if ((previousHash != "0")) {

if ((transaction.processTransaction() != true)) {

System.out.println("Transaction failed to process. Discarded.");

return false;

}

}

transactions.add(transaction);

System.out.println("Transaction Successfully added to Block");

return true;

}
}

A complete test .

We've tested sending money from the wallet before , Then modify the blockchain to check its effectiveness . But first, let's create some new currencies , There are many ways to create a new currency , Take the blockchain in bitcoin as an example ： A successful miner will get a reward . But here we just want to release money in the genesis block . It's like bitcoin , So we modify our main function to achieve the following purpose .

  • Genesis block releases 100 A currency for walletA
  • Modify the blockchain to verify the validity of the account
  • Test to see if everything is running

Chain class .

import org.bouncycastle.jce.provider.BouncyCastleProvider;
import java.security.Security;

import java.util.ArrayList;

import java.util.HashMap;
public class Chain {

public static ArrayList<Block> blockchain = new ArrayList<>();

public static HashMap<String, TransactionOutput> UTXOs = new HashMap<>();

public static int difficulty = 3;

public static float minimumTransaction = 0.1f;

public static Wallet walletA;

public static Wallet walletB;

public static Transaction genesisTransaction;
public static void main(String[] args) {

Security.addProvider(new BouncyCastleProvider());

walletA = new Wallet();

walletB = new Wallet();
Wallet coinbase = new Wallet();
genesisTransaction = new Transaction(coinbase.publicKey, walletA.publicKey, 100f, null);

genesisTransaction.generateSignature(coinbase.privateKey);

genesisTransaction.transactionId = "0";

genesisTransaction.outputs.add(new TransactionOutput(genesisTransaction.reciepient, genesisTransaction.value, genesisTransaction.transactionId));

UTXOs.put(genesisTransaction.outputs.get(0).id, genesisTransaction.outputs.get(0));
System.out.println("Creating and Mining Genesis block... ");

Block genesis = new Block("0");

genesis.addTransaction(genesisTransaction);

addBlock(genesis);
//testing

Block block1 = new Block(genesis.hash);

System.out.println("\nWalletA's balance is: " + walletA.getBalance());

System.out.println("\nWalletA is Attempting to send funds (40) to WalletB...");

block1.addTransaction(walletA.sendFunds(walletB.publicKey, 40f));

addBlock(block1);

System.out.println("\nWalletA's balance is: " + walletA.getBalance());

System.out.println("WalletB's balance is: " + walletB.getBalance());
Block block2 = new Block(block1.hash);

System.out.println("\nWalletA Attempting to send more funds (1000) than it has...");

block2.addTransaction(walletA.sendFunds(walletB.publicKey, 1000f));

addBlock(block2);

System.out.println("\nWalletA's balance is: " + walletA.getBalance());

System.out.println("WalletB's balance is: " + walletB.getBalance());
Block block3 = new Block(block2.hash);

System.out.println("\nWalletB is Attempting to send funds (20) to WalletA...");

block3.addTransaction(walletB.sendFunds(walletA.publicKey, 20));

System.out.println("\nWalletA's balance is: " + walletA.getBalance());

System.out.println("WalletB's balance is: " + walletB.getBalance());
isChainValid();

}
public static void addBlock(Block newBlock) {

newBlock.mineBlock(difficulty);

blockchain.add(newBlock);

}
public static Boolean isChainValid() {

Block currentBlock;

Block previousBlock;

String hashTarget = new String(new char[difficulty]).replace('\0', '0');

HashMap<String, TransactionOutput> tempUTXOs = new HashMap<>();

tempUTXOs.put(genesisTransaction.outputs.get(0).id, genesisTransaction.outputs.get(0));
for (int i = 1; i < blockchain.size(); i++) {
currentBlock = blockchain.get(i);

previousBlock = blockchain.get(i - 1);

if (!currentBlock.hash.equals(currentBlock.calculateHash())) {

System.out.println("#Current Hashes not equal");

return false;

}

if (!previousBlock.hash.equals(currentBlock.previousHash)) {

System.out.println("#Previous Hashes not equal");

return false;

}

if (!currentBlock.hash.substring(0, difficulty).equals(hashTarget)) {

System.out.println("#This block hasn't been mined");

return false;

}
TransactionOutput tempOutput;

for (int t = 0; t < currentBlock.transactions.size(); t++) {

Transaction currentTransaction = currentBlock.transactions.get(t);
if (!currentTransaction.verifySignature()) {

System.out.println("#Signature on Transaction(" + t + ") is Invalid");

return false;

}

if (currentTransaction.getInputsValue() != currentTransaction.getOutputsValue()) {

System.out.println("#Inputs are note equal to outputs on Transaction(" + t + ")");

return false;

}
for (TransactionInput input : currentTransaction.inputs) {

tempOutput = tempUTXOs.get(input.transactionOutputId);
if (tempOutput == null) {

System.out.println("#Referenced input on Transaction(" + t + ") is Missing");

return false;

}
if (input.UTXO.value != tempOutput.value) {

System.out.println("#Referenced input Transaction(" + t + ") value is Invalid");

return false;

}
tempUTXOs.remove(input.transactionOutputId);

}
for (TransactionOutput output : currentTransaction.outputs) {

tempUTXOs.put(output.id, output);

}
if (currentTransaction.outputs.get(0).reciepient != currentTransaction.reciepient) {

System.out.println("#Transaction(" + t + ") output reciepient is not who it should be");

return false;

}

if (currentTransaction.outputs.get(1).reciepient != currentTransaction.sender) {

System.out.println("#Transaction(" + t + ") output 'change' is not sender.");

return false;

}
}
}

System.out.println("Blockchain is valid");

return true;

}

}

Running results .

Now the wallet can send the amount safely in the blockchain , Only when the wallet has the amount can it be sent to others . That means you have your own cryptocurrency .

To sum up , We have implemented... In the blockchain ：

  • Allow all users to create wallets
  • utilize ECDSA Create a public and private key for the wallet
  • Through digital signature algorithm to prove ownership, so you can transfer funds safely
  • Finally, all users are allowed to add transactions in the blockchain

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