Public vs private blockchains - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Which statement best describes a public blockchain?

Choose the statement that correctly describes a public blockchain.

A It is controlled by a single organization with restricted access.

B Anyone can join and participate without permission.

C Only invited members can join and validate transactions.

D It uses a private network with no public access.

💡 Hint

Think about who can access and participate in a public blockchain.

A public blockchain is open to everyone. Anyone can join, validate transactions, and participate without needing permission.

🧠 Conceptual

intermediate

What is a key feature of a private blockchain?

Identify the key feature that distinguishes a private blockchain from a public one.

A It is maintained by a decentralized group of unknown validators.

B It allows anonymous participation from anyone worldwide.

C It uses proof-of-work mining to secure the network.

D It restricts access to a selected group of participants.

💡 Hint

Consider who controls access in a private blockchain.

Private blockchains restrict access to a specific group, usually controlled by an organization or consortium.

❓ Predict Output

advanced

What is the output of this blockchain node access check?

Given the code below simulating node access in a blockchain network, what will be printed?

Blockchain / Solidity

class BlockchainNode:
    def __init__(self, is_public):
        self.is_public = is_public
    def can_join(self, user):
        if self.is_public:
            return True
        else:
            return user in ['Alice', 'Bob']

node_public = BlockchainNode(True)
node_private = BlockchainNode(False)

print(node_public.can_join('Eve'))
print(node_private.can_join('Eve'))

A True\nTrue

B False\nTrue

C True\nFalse

D False\nFalse

💡 Hint

Check how the can_join method works for public and private nodes.

The public node allows anyone to join, so 'Eve' returns True. The private node only allows 'Alice' and 'Bob', so 'Eve' returns False.

❓ Predict Output

advanced

What error does this private blockchain access code raise?

Analyze the code below and identify the error it produces when run.

Blockchain / Solidity

class PrivateBlockchain:
    def __init__(self, members):
        self.members = members
    def add_member(self, member):
        if member not in self.members:
            self.members.append(member)
    def validate_transaction(self, user):
        if user not in self.members:
            raise PermissionError('User not authorized')
        return 'Transaction validated'

chain = PrivateBlockchain(['Alice', 'Bob'])
print(chain.validate_transaction('Eve'))

A PermissionError: User not authorized

B AttributeError: 'PrivateBlockchain' object has no attribute 'members'

C TypeError: 'in' requires string as left operand

D No error, prints 'Transaction validated'

💡 Hint

Look at the validate_transaction method and the user passed.

The user 'Eve' is not in the members list, so the method raises a PermissionError with the message 'User not authorized'.

🧠 Conceptual

expert

Which blockchain type best fits a company needing fast, private transactions with controlled access?

Choose the blockchain type that best suits a company requiring fast transaction processing, privacy, and controlled participant access.

A Private blockchain with restricted access and permissioned nodes.

B Public blockchain with open participation and proof-of-work consensus.

C Public blockchain with anonymous nodes and no access control.

D Hybrid blockchain with no restrictions on transaction visibility.

💡 Hint

Consider privacy and access control needs for a company.

A private blockchain with permissioned nodes offers controlled access and privacy, making it suitable for companies needing fast and private transactions.

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