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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a rollup type variable.

Blockchain / Solidity

let rollupType = '[1]';

Drag options to blanks, or click blank then click option'

AOptimistic

BZK

CSidechain

DSharding

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Optimistic with ZK rollups.

Using unrelated terms like Sidechain or Sharding.

The variable rollupType should be set to Optimistic to represent an Optimistic Rollup.

2fill in blank

medium

Complete the code to check if a rollup uses zero-knowledge proofs.

Blockchain / Solidity

if (rollupType === '[1]') {
  console.log('This rollup uses zero-knowledge proofs.');
}

Drag options to blanks, or click blank then click option'

AZK

BOptimistic

CPlasma

DStateChannel

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for Optimistic instead of ZK.

Using unrelated rollup types.

The condition should check for ZK to identify zero-knowledge proof rollups.

3fill in blank

hard

Fix the error in the function that returns the rollup's verification method.

Blockchain / Solidity

function getVerificationMethod(type) {
  return type === '[1]' ? 'Fraud Proofs' : 'SNARKs';
}

Drag options to blanks, or click blank then click option'

ARollup

BOptimistic

CZK

DPlasma

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for ZK instead of Optimistic.

Using generic terms like Rollup.

The function should check if the type is Optimistic to return 'Fraud Proofs'.

4fill in blank

hard

Fill both blanks to create a dictionary mapping rollup types to their verification methods.

Blockchain / Solidity

const verificationMethods = {
  Optimistic: '[1]',
  ZK: '[2]'
};

Drag options to blanks, or click blank then click option'

AFraud Proofs

BSNARKs

CPlasma

DSidechain

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the verification methods.

Using unrelated terms like Plasma or Sidechain.

Optimistic rollups use Fraud Proofs, and ZK rollups use SNARKs for verification.

5fill in blank

hard

Fill all three blanks to filter rollups that use fraud proofs and map their names.

Blockchain / Solidity

const rollups = [
  { name: 'Optimism', type: '[1]' },
  { name: 'Arbitrum', type: 'Optimistic' },
  { name: 'zkSync', type: 'ZK' }
];

const fraudProofRollups = rollups
  .filter(r => r.type === '[2]')
  .map(r => r.[3]);

Drag options to blanks, or click blank then click option'

AOptimistic

Cname

Dtype

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ZK' instead of 'Optimistic' for filtering.

Mapping the wrong property like 'type'.

Both blanks 1 and 2 should be Optimistic to filter fraud proof rollups, and name is used to map their names.

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