Public vs private blockchains

Introduction

Blockchains help keep data safe and shared. Public and private blockchains decide who can join and see the data.

When you want everyone to see and join the network, like for a public cryptocurrency.

When a company wants to keep data private and only allow certain people to join.

When you want fast transactions inside a trusted group, like a business team.

When you want to build a community project open to all.

When you need to control who can add or change data for security reasons.

Syntax

Blockchain / Solidity

Public Blockchain:
- Anyone can join and read data.
- Anyone can add new data (transactions).
- Examples: Bitcoin, Ethereum.

Private Blockchain:
- Only invited members can join.
- Only certain members can add data.
- Examples: Hyperledger Fabric, Corda.

Public blockchains are open and transparent.

Private blockchains are closed and controlled.

Examples

This shows who can join and use each type.

Blockchain / Solidity

Public Blockchain Example:
Anyone can download the Bitcoin software and start using it.

Private Blockchain Example:
A bank creates a blockchain only its branches can access.

Shows the control difference between public and private blockchains.

Blockchain / Solidity

Public Blockchain:
- Decentralized
- No single owner

Private Blockchain:
- Centralized control
- Owned by an organization

Sample Program

This simple program shows how public blockchains allow anyone to join and add data, while private blockchains only allow known members.

Blockchain / Solidity

class Blockchain:
    def __init__(self, is_public):
        self.is_public = is_public
        self.members = []
        self.data = []

    def join(self, member):
        if self.is_public or member in self.members:
            print(f"{member} joined the blockchain.")
            if member not in self.members:
                self.members.append(member)
        else:
            print(f"{member} cannot join this private blockchain.")

    def add_data(self, member, data):
        if self.is_public or member in self.members:
            self.data.append((member, data))
            print(f"{member} added data: {data}")
        else:
            print(f"{member} cannot add data to this private blockchain.")

# Public blockchain example
public_chain = Blockchain(is_public=True)
public_chain.join("Alice")
public_chain.add_data("Alice", "Transaction 1")

# Private blockchain example
private_chain = Blockchain(is_public=False)
private_chain.members = ["Bob", "Carol"]
private_chain.join("Bob")
private_chain.add_data("Bob", "Private Transaction")
private_chain.join("Eve")
private_chain.add_data("Eve", "Hacker Transaction")

Important Notes

Public blockchains are slower but more open.

Private blockchains are faster but less open.

Choose based on who needs access and trust level.

Summary

Public blockchains are open to everyone.

Private blockchains restrict access to trusted members.

Each type fits different needs for sharing and security.

Practice

1. Which statement best describes a public blockchain?

easy

A. Anyone can join and participate without permission.

B. Only invited members can access the network.

C. It is controlled by a single organization.

D. It uses private keys only for administrators.

Solution

1. Step 1: Understand public blockchain access

   Public blockchains are open networks where anyone can join and participate without needing permission.

2. Step 2: Compare with private blockchain features

   Private blockchains restrict access to trusted members, unlike public ones.

3. Final Answer:

   Anyone can join and participate without permission. -> Option A

4. Quick Check:

   Public blockchain = open access [OK]

Hint: Public means open to all, no permission needed [OK]

Common Mistakes:

• Confusing public with private blockchain access
• Thinking public blockchains are controlled by one entity
• Assuming private keys limit public blockchain access

2. Which of the following is the correct way to describe a private blockchain?

easy

A. It is always slower than public blockchains.

B. It allows anyone to validate transactions.

C. It restricts access to selected participants only.

D. It uses proof-of-work exclusively.

Solution

1. Step 1: Identify private blockchain access rules

   Private blockchains restrict access to selected participants, unlike public blockchains.

2. Step 2: Eliminate incorrect options

   Private blockchains do not necessarily use proof-of-work or are always slower; these are unrelated to access control.

3. Final Answer:

   It restricts access to selected participants only. -> Option C

4. Quick Check:

   Private blockchain = restricted access [OK]

Hint: Private means limited access to trusted members only [OK]

Common Mistakes:

• Assuming private blockchains allow anyone to validate
• Confusing consensus methods with access control
• Believing private blockchains are always slower

3. Consider this code snippet representing blockchain access control:

def can_access(user, blockchain_type):
    if blockchain_type == 'public':
        return True
    elif blockchain_type == 'private':
        return user in ['Alice', 'Bob']
    else:
        return False

print(can_access('Alice', 'public'))
print(can_access('Eve', 'private'))
print(can_access('Bob', 'private'))

What is the output?

medium

A. False
False
True

B. True
False
True

C. True
True
False

D. False
True
False

Solution

1. Step 1: Evaluate access for 'Alice' on public blockchain

   Public blockchain always returns True, so can_access('Alice', 'public') is True.

2. Step 2: Evaluate access for 'Eve' on private blockchain

   'Eve' is not in ['Alice', 'Bob'], so can_access('Eve', 'private') returns False.

3. Step 3: Evaluate access for 'Bob' on private blockchain

   'Bob' is in the allowed list, so can_access('Bob', 'private') returns True.

4. Final Answer:

   True False True -> Option B

5. Quick Check:

   Public always True, private checks user list [OK]

Hint: Public always True; private checks user list membership [OK]

Common Mistakes:

• Assuming private blockchain allows all users
• Mixing up True/False return values
• Ignoring the user list check for private blockchains

4. The following code is intended to check if a user can access a private blockchain. What is the error?

def check_access(user, blockchain):
    if blockchain == 'private':
        if user == ['Alice', 'Bob']:
            return True
        else:
            return False
    else:
        return True

print(check_access('Alice', 'private'))

medium

A. The user comparison uses == with a list instead of membership check.

B. The function does not return anything for public blockchains.

C. The else block is missing for private blockchain condition.

D. The print statement has incorrect syntax.

Solution

1. Step 1: Analyze user comparison in private blockchain check

   The code uses 'user == ['Alice', 'Bob']' which compares a string to a list, always False.

2. Step 2: Identify correct way to check user membership

   It should use 'user in ['Alice', 'Bob']' to check if user is in the allowed list.

3. Final Answer:

   The user comparison uses == with a list instead of membership check. -> Option A

4. Quick Check:

   Use 'in' to check list membership, not '==' [OK]

Hint: Use 'in' to check if user is in list, not '==' [OK]

Common Mistakes:

• Using '==' to compare a string with a list
• Forgetting to handle public blockchain case
• Syntax errors in print statement

5. You want to design a blockchain system for a company where only employees can add transactions, but anyone can view the ledger. Which blockchain type fits best and why?

hard

A. Private blockchain, because it hides the ledger from everyone except employees.

B. Public blockchain, because it allows everyone to add transactions.

C. Public blockchain, because it restricts access to employees only.

D. Private blockchain, because it restricts who can add transactions but allows public viewing.

Solution

1. Step 1: Identify who can add transactions

   The company wants only employees to add transactions, so write access must be restricted.

2. Step 2: Identify who can view the ledger

   Anyone can view the ledger, so read access is open to the public.

3. Step 3: Match blockchain type to requirements

   Private blockchains restrict write access to trusted members but can allow public read access, fitting the need.

4. Final Answer:

   Private blockchain, because it restricts who can add transactions but allows public viewing. -> Option D

5. Quick Check:

   Restrict writes, allow public reads = Private blockchain [OK]

Hint: Restrict who writes, allow public reads means private blockchain [OK]

Common Mistakes:

• Choosing public blockchain for restricted write access
• Thinking private blockchain hides ledger from public
• Confusing read and write permissions