Blocks, chains, and hashing in Blockchain / Solidity

Introduction

Blocks, chains, and hashing help keep digital information safe and connected. They make sure data can't be changed without everyone knowing.

When you want to store transactions securely like in digital money.

When you need to keep records that everyone can trust without a middleman.

When you want to track items in a supply chain so no one can cheat.

When you want to create a secure digital identity system.

When you want to build apps where data must be permanent and tamper-proof.

Syntax

Blockchain / Solidity

Block {
  index: number
  timestamp: string
  data: any
  previousHash: string
  hash: string
}

Chain = array of Blocks

hash = hashFunction(blockData)

A block holds data and links to the previous block using a hash.

The chain is a list of blocks connected by hashes.

Examples

This is the first block, called the genesis block. It has no previous block, so previousHash is '0'.

Blockchain / Solidity

block = {
  index: 1,
  timestamp: '2024-06-01T12:00:00Z',
  data: 'First block data',
  previousHash: '0',
  hash: 'abc123'
}

This block links to the first block by using its hash in previousHash.

Blockchain / Solidity

block = {
  index: 2,
  timestamp: '2024-06-01T12:05:00Z',
  data: 'Second block data',
  previousHash: 'abc123',
  hash: 'def456'
}

Sample Program

This program creates a simple blockchain with a genesis block and two more blocks. Each block stores data and links to the previous block using a hash. The hash is created by combining the block's details and running it through a secure hash function.

Blockchain / Solidity

import hashlib
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.hash = self.calculate_hash()

    def calculate_hash(self):
        block_string = f'{self.index}{self.timestamp}{self.data}{self.previous_hash}'
        return hashlib.sha256(block_string.encode()).hexdigest()

class Blockchain:
    def __init__(self):
        self.chain = [self.create_genesis_block()]

    def create_genesis_block(self):
        return Block(0, time.strftime('%Y-%m-%d %H:%M:%S'), 'Genesis Block', '0')

    def get_latest_block(self):
        return self.chain[-1]

    def add_block(self, data):
        latest_block = self.get_latest_block()
        new_index = latest_block.index + 1
        new_timestamp = time.strftime('%Y-%m-%d %H:%M:%S')
        new_block = Block(new_index, new_timestamp, data, latest_block.hash)
        self.chain.append(new_block)

# Create blockchain and add blocks
my_blockchain = Blockchain()
my_blockchain.add_block('First real block data')
my_blockchain.add_block('Second real block data')

# Print the blockchain
for block in my_blockchain.chain:
    print(f'Block {block.index}:')
    print(f'  Timestamp: {block.timestamp}')
    print(f'  Data: {block.data}')
    print(f'  Previous Hash: {block.previous_hash}')
    print(f'  Hash: {block.hash}\n')

Important Notes

Hashing makes sure each block is unique and any change breaks the chain.

Each block points to the previous one, so the chain is linked like a real chain.

Changing one block means changing all after it, which is very hard to do secretly.

Summary

Blocks hold data and connect using hashes.

Chains link blocks so data stays safe and unchangeable.

Hashing creates a unique fingerprint for each block.

Practice

1. What is the main purpose of a hash in a blockchain block?

easy

A. To create a unique fingerprint of the block's data

B. To store the block's timestamp

C. To list all transactions in the block

D. To connect the block to the internet

Solution

1. Step 1: Understand what a hash does

   A hash is a unique code generated from the block's data, like a fingerprint.

2. Step 2: Identify the purpose of the hash in blockchain

   The hash ensures the block's data cannot be changed without detection.

3. Final Answer:

   To create a unique fingerprint of the block's data -> Option A

4. Quick Check:

   Hash = Unique fingerprint [OK]

Hint: Hash = unique code for block data [OK]

Common Mistakes:

• Confusing hash with timestamp
• Thinking hash stores transactions
• Believing hash connects to internet

2. Which of the following is the correct way to represent a block's hash in code?

easy

A. hash = block_data + SHA256

B. hash = SHA256(block_data)

C. hash = SHA256(block_data + hash)

D. hash = block_data.hash()

Solution

1. Step 1: Recognize the correct hash function usage

   SHA256 is a common hash function applied to block data to get the hash.

2. Step 2: Check each option for correct syntax

   hash = SHA256(block_data) correctly applies SHA256 to block_data. Others misuse the function or syntax.

3. Final Answer:

   hash = SHA256(block_data) -> Option B

4. Quick Check:

   Correct hash function call = hash = SHA256(block_data) [OK]

Hint: Hash function called with data inside parentheses [OK]

Common Mistakes:

• Adding hash function as string instead of calling it
• Using incorrect syntax for hash function
• Confusing method call with variable

3. Consider this simplified Python code for creating a blockchain block hash:

import hashlib
block_data = 'Block 1 data'
hash_object = hashlib.sha256(block_data.encode())
block_hash = hash_object.hexdigest()
print(block_hash)

What will be the output type of block_hash?

medium

A. integer

B. bytes

C. string

D. list

Solution

1. Step 1: Understand hashlib.sha256 and hexdigest()

   hashlib.sha256 returns a hash object; hexdigest() converts it to a hex string.

2. Step 2: Identify the output type of hexdigest()

   hexdigest() returns a string representing the hash in hexadecimal form.

3. Final Answer:

   string -> Option C

4. Quick Check:

   hexdigest() output = string [OK]

Hint: hexdigest() returns string, digest() returns bytes [OK]

Common Mistakes:

• Thinking hexdigest() returns bytes
• Confusing output with integer or list
• Ignoring .encode() usage

4. The following code tries to link blocks in a chain by storing the previous block's hash. What is the error?

class Block:
    def __init__(self, data, prev_hash):
        self.data = data
        self.prev_hash = prev_hash

block1 = Block('First block', None)
block2 = Block('Second block', block1.hash)

medium

A. block1.hash is not defined before use

B. prev_hash should be a list, not a single value

C. None cannot be used as prev_hash

D. block2 data should be a number, not string

Solution

1. Step 1: Check block1 attributes

   block1 has data and prev_hash but no attribute named 'hash'.

2. Step 2: Identify usage of undefined attribute

   block2 tries to use block1.hash which does not exist, causing an error.

3. Final Answer:

   block1.hash is not defined before use -> Option A

4. Quick Check:

   Accessing undefined attribute = block1.hash is not defined before use [OK]

Hint: Check if attribute exists before using it [OK]

Common Mistakes:

• Assuming hash attribute exists by default
• Confusing prev_hash type requirements
• Thinking data type causes error

5. You want to create a simple blockchain where each block stores its own hash and the previous block's hash. Which code snippet correctly computes and links the hashes for two blocks?

hard

A. import hashlib

class Block:
def __init__(self, data, prev_hash):
self.data = data
self.prev_hash = prev_hash
self.hash = hashlib.sha256(data.encode()).hexdigest()

block1 = Block('Block 1', '0')
block2 = Block('Block 2', block1.hash)

B. import hashlib

class Block:
def __init__(self, data):
self.data = data
self.hash = hashlib.sha256(data.encode()).hexdigest()

block1 = Block('Block 1')
block2 = Block('Block 2')
block2.prev_hash = block1.hash

C. class Block:
def __init__(self, data, prev_hash):
self.data = data
self.prev_hash = prev_hash
self.hash = data + prev_hash

block1 = Block('Block 1', '0')
block2 = Block('Block 2', block1.hash)

D. import hashlib

class Block:
def __init__(self, data, prev_hash):
self.data = data
self.prev_hash = prev_hash
self.hash = hashlib.sha256((data + prev_hash).encode()).hexdigest()

block1 = Block('Block 1', '0')
block2 = Block('Block 2', block1.hash)

Solution

1. Step 1: Check hash calculation includes previous hash

   The correct implementation computes the hash from both data and prev_hash: self.hash = hashlib.sha256((data + prev_hash).encode()).hexdigest(), ensuring secure linking.

2. Step 2: Verify other options

   B: prev_hash set after hash computation (hash ignores prev_hash); C: no hashing, just concatenation; D: hash ignores prev_hash.

3. Final Answer:

   self.hash = hashlib.sha256((data + prev_hash).encode()).hexdigest() -> Option D

4. Quick Check:

   Hash(data + prev_hash) [OK]

Hint: Hash must include previous hash to link blocks [OK]

Common Mistakes:

• Not including prev_hash in hash calculation
• Assigning prev_hash after block creation
• Using plain concatenation without hashing