Cơ bản về Blockchain và Cryptocurrency

# Simple Blockchain Implementation
import hashlib
import json
import time

class Block:
    def __init__(self, index, timestamp, data, previous_hash):
        self.index = index
        self.timestamp = timestamp
        self.data = data
        self.previous_hash = previous_hash
        self.nonce = 0
        self.hash = self.calculate_hash()
    
    def calculate_hash(self):
        block_string = json.dumps({
            "index": self.index,
            "timestamp": self.timestamp,
            "data": self.data,
            "previous_hash": self.previous_hash,
            "nonce": self.nonce
        }, sort_keys=True)
        return hashlib.sha256(block_string.encode()).hexdigest()
    
    def mine_block(self, difficulty):
        target = "0" * difficulty
        while self.hash[:difficulty] != target:
            self.nonce += 1
            self.hash = self.calculate_hash()
        print(f"Block mined: {self.hash}")

class Blockchain:
    def __init__(self):
        self.chain = [self.create_genesis_block()]
        self.difficulty = 2
    
    def create_genesis_block(self):
        return Block(0, time.time(), "Genesis Block", "0")
    
    def get_latest_block(self):
        return self.chain[-1]
    
    def add_block(self, new_block):
        new_block.previous_hash = self.get_latest_block().hash
        new_block.mine_block(self.difficulty)
        self.chain.append(new_block)
    
    def is_chain_valid(self):
        for i in range(1, len(self.chain)):
            current_block = self.chain[i]
            previous_block = self.chain[i-1]
            
            if current_block.hash != current_block.calculate_hash():
                return False
            
            if current_block.previous_hash != previous_block.hash:
                return False
        
        return True

# Test blockchain
my_blockchain = Blockchain()
my_blockchain.add_block(Block(1, time.time(), "Transaction 1", ""))
my_blockchain.add_block(Block(2, time.time(), "Transaction 2", ""))

print("Blockchain valid:", my_blockchain.is_chain_valid())