Rollups (Optimistic vs ZK) in Blockchain / Solidity - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Optimistic Rollup Fraud Proof Simulation

Consider the following simplified Python code simulating an optimistic rollup fraud proof check. What is the output when the code runs?

Blockchain / Solidity

def optimistic_rollup_check(state, new_state):
    # Fraud proof triggers if new_state is less than state
    if new_state < state:
        return "Fraud detected"
    else:
        return "State accepted"

print(optimistic_rollup_check(100, 90))

A State accepted

B Fraud detected

C TypeError

D SyntaxError

💡 Hint

Think about the condition that triggers fraud detection in optimistic rollups.

The function checks if the new state is less than the current state. Since 90 < 100, it returns 'Fraud detected'.

🧠 Conceptual

intermediate

Difference in Verification Time Between Rollups

Which statement correctly describes the verification time difference between Optimistic Rollups and ZK Rollups?

A Optimistic Rollups use zero-knowledge proofs for instant verification

B Optimistic Rollups verify transactions instantly, ZK Rollups delay verification

C Both rollups verify transactions instantly with no delay

D ZK Rollups verify transactions instantly using proofs, Optimistic Rollups delay verification due to fraud proofs

💡 Hint

Recall how each rollup type handles transaction verification.

ZK Rollups generate cryptographic proofs that verify transactions instantly, while Optimistic Rollups assume transactions are valid and only verify if a fraud proof is submitted, causing delay.

🔧 Debug

advanced

Identify the Error in ZK Rollup Proof Generation Code

What error will the following Python code raise when simulating a ZK Rollup proof generation?

Blockchain / Solidity

def generate_zk_proof(data):
    # Simulate proof generation
    proof = data / 0
    return proof

print(generate_zk_proof(10))

A ZeroDivisionError

B No error, outputs 0

C ValueError

D TypeError

💡 Hint

Look at the operation dividing by zero.

Dividing any number by zero in Python raises a ZeroDivisionError.

📝 Syntax

advanced

Syntax Error in Optimistic Rollup Challenge Period Code

Which option contains the correct syntax to define a function that returns the challenge period length in days?

Blockchain / Solidity

def challenge_period():
    return 7

A

def challenge_period()
    return 7

B

def challenge_period():
return 7

C

def challenge_period():
    return 7

D

def challenge_period:
    return 7

💡 Hint

Check for colons and indentation in function definitions.

Option C correctly includes the colon after the function name and indents the return statement properly.

🚀 Application

expert

Calculate Final State After Batch Processing in ZK Rollup

Given the following Python code simulating batch state updates in a ZK Rollup, what is the final state printed?

Blockchain / Solidity

def zk_rollup_batch(states):
    final_state = 0
    for s in states:
        final_state += s * 2
    return final_state

batch_states = [1, 3, 5]
print(zk_rollup_batch(batch_states))

A 18

B 9

C 15

D 30

💡 Hint

Multiply each state by 2 and add them all up.

The function doubles each state and sums them: (1*2)+(3*2)+(5*2) = 2+6+10 = 18.

Practice

1.

What is the main difference between Optimistic Rollups and ZK Rollups?

easy

A. Optimistic Rollups assume transactions are valid until challenged; ZK Rollups use proofs to verify immediately.

B. Optimistic Rollups use zero-knowledge proofs; ZK Rollups wait for challenges.

C. Both rollups require a waiting period before finalizing transactions.

D. ZK Rollups do not move any work off the main blockchain.

Solution

1. Step 1: Understand Optimistic Rollups behavior

   Optimistic Rollups trust transactions are valid initially and allow a challenge period to dispute invalid ones.

2. Step 2: Understand ZK Rollups behavior

   ZK Rollups generate cryptographic proofs that transactions are valid immediately, so no waiting period is needed.

3. Final Answer:

   Optimistic Rollups assume transactions are valid until challenged; ZK Rollups use proofs to verify immediately. -> Option A

4. Quick Check:

   Trust first vs proof first [OK]

Hint: Optimistic trusts first, ZK proves first instantly [OK]

Common Mistakes:

• Confusing which rollup uses proofs immediately
• Thinking both rollups have the same waiting period
• Assuming ZK Rollups do not move work off-chain

2.

Which of the following is the correct syntax to describe a ZK Rollup in a blockchain smart contract comment?

// This rollup uses ______ to verify transactions instantly

easy

A. manual challenges

B. a waiting period

C. optimistic assumptions

D. zero-knowledge proofs

Solution

1. Step 1: Identify ZK Rollup verification method

   ZK Rollups use zero-knowledge proofs to verify transactions instantly.

2. Step 2: Match the correct phrase in the comment

   The comment should mention "zero-knowledge proofs" to describe ZK Rollups.

3. Final Answer:

   zero-knowledge proofs -> Option D

4. Quick Check:

   ZK Rollups = zero-knowledge proofs [OK]

Hint: ZK means zero-knowledge proofs, not waiting [OK]

Common Mistakes:

• Choosing 'waiting period' which applies to Optimistic Rollups
• Confusing optimistic assumptions with ZK proofs
• Selecting manual challenges which are for Optimistic Rollups

3.

Consider this pseudocode for an Optimistic Rollup transaction verification:

function verifyTransaction(tx) {
  assume tx is valid
  wait 7 days for challenge
  if no challenge then finalize tx
  else revert tx
}

What will happen if a fraudulent transaction is submitted and no one challenges it?

medium

A. The system will generate a zero-knowledge proof to verify it.

B. The transaction will be rejected immediately.

C. The fraudulent transaction will be finalized after 7 days.

D. The transaction will be finalized instantly without waiting.

Solution

1. Step 1: Analyze the verification logic

   The function assumes transactions are valid and waits 7 days for any challenge.

2. Step 2: Consider no challenge scenario

   If no challenge occurs within 7 days, the transaction is finalized regardless of validity.

3. Final Answer:

   The fraudulent transaction will be finalized after 7 days. -> Option C

4. Quick Check:

   Optimistic waits then finalizes if no challenge [OK]

Hint: No challenge means finalize after wait in Optimistic Rollups [OK]

Common Mistakes:

• Thinking fraudulent tx is rejected immediately
• Assuming zero-knowledge proofs are used here
• Believing finalization is instant without waiting

4.

Identify the bug in this ZK Rollup pseudocode snippet:

function verifyZKProof(proof) {
  if (proof.isValid) {
    finalizeTransaction()
  } else {
    wait 7 days for challenge
  }
}

What is the main issue?

medium

A. finalizeTransaction() should be called only after waiting.

B. ZK Rollups should not wait for challenges; proof validity is immediate.

C. The proof.isValid check is missing a negation operator.

D. The function should always wait 7 days regardless of proof validity.

Solution

1. Step 1: Understand ZK Rollup verification

   ZK Rollups use immediate proof verification and do not require waiting periods.

2. Step 2: Analyze the code logic

   The code incorrectly waits 7 days if proof is invalid, which contradicts ZK Rollup design.

3. Final Answer:

   ZK Rollups should not wait for challenges; proof validity is immediate. -> Option B

4. Quick Check:

   ZK Rollups = instant proof, no wait [OK]

Hint: ZK proofs mean no waiting, immediate finalize [OK]

Common Mistakes:

• Thinking waiting is needed for ZK Rollups
• Misreading proof validity condition
• Assuming finalizeTransaction requires delay

5.

You want to design a rollup system that minimizes user waiting time but can handle complex computations off-chain. Which rollup type should you choose and why?

hard

A. ZK Rollup, because it provides immediate proof and faster finality.

B. Optimistic Rollup, because it finalizes instantly without proofs.

C. Optimistic Rollup, because it uses zero-knowledge proofs for speed.

D. ZK Rollup, because it waits for challenges before finalizing.

Solution

1. Step 1: Identify rollup goals

   The goal is to minimize waiting time and handle complex off-chain computations.

2. Step 2: Compare rollup features

   ZK Rollups provide immediate validity proofs, enabling faster finality without waiting periods, suitable for complex computations.

3. Final Answer:

   ZK Rollup, because it provides immediate proof and faster finality. -> Option A

4. Quick Check:

   Minimize wait + complex work = ZK Rollup [OK]

Hint: ZK Rollups = fast finality with proofs, best for complex tasks [OK]

Common Mistakes:

• Choosing Optimistic Rollup for instant finality incorrectly
• Confusing which rollup uses zero-knowledge proofs
• Thinking ZK Rollups wait for challenges