Why security prevents financial loss in Blockchain / Solidity - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a transaction is valid before processing.

Blockchain / Solidity

if transaction.[1]():
    process(transaction)

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

Asign

Bis_valid

Csend

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods that perform actions instead of checking validity.

The is_valid() method checks if the transaction meets all security rules before processing.

2fill in blank

medium

Complete the code to verify the digital signature of a transaction.

Blockchain / Solidity

if verify_signature(transaction.[1], public_key):
    approve(transaction)

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

Ahash

Bamount

Ctimestamp

Dsignature

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to verify the hash or amount instead of the signature.

The signature is used to verify that the transaction was authorized by the owner.

3fill in blank

hard

Fix the error in the code that prevents unauthorized spending.

Blockchain / Solidity

if balance >= transaction.[1]:
    deduct(balance, transaction.amount)

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

Aamount

Bfee

Csignature

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Checking fee or signature instead of the amount.

The code must check if the balance is enough for the amount to prevent overspending.

4fill in blank

hard

Fill both blanks to create a secure transaction record with hash and timestamp.

Blockchain / Solidity

transaction_record = {
    'hash': [1],
    'timestamp': [2]
}

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

Acompute_hash(transaction)

Bget_time()

Ccurrent_time()

Dtransaction_data

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong functions for hash or timestamp.

The compute_hash(transaction) creates a unique fingerprint, and current_time() records when it happened.

5fill in blank

hard

Fill all three blanks to filter and store only secure transactions.

Blockchain / Solidity

secure_transactions = {
    [1]: [2] for [3] in transactions
    if verify_signature([3], public_key)
}

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

Atx_id

Btx_data

Ctx

Dtransaction

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing variable names or using wrong keys and values.

We use tx_id as the key, tx_data as the value, and loop over each tx in transactions.

Practice

1. Why is security important in blockchain to prevent financial loss?

easy

A. It increases the size of the blockchain.

B. It stops unauthorized people from accessing your digital money.

C. It makes transactions slower.

D. It allows anyone to change transaction history.

Solution

1. Step 1: Understand unauthorized access risk

   Unauthorized access means someone can take your money without permission.

2. Step 2: Recognize security's role

   Security prevents unauthorized access, protecting your digital assets.

3. Final Answer:

   It stops unauthorized people from accessing your digital money. -> Option B

4. Quick Check:

   Security prevents unauthorized access = C [OK]

Hint: Security blocks unauthorized access to your money [OK]

Common Mistakes:

• Thinking security slows down transactions
• Believing blockchain size affects security
• Assuming anyone can change history

2. Which of the following is the correct way to verify a transaction in blockchain code?

easy

A. if transaction.verify(): process()

B. if verify(transaction): process()

C. verify if transaction process()

D. process() if verify transaction

Solution

1. Step 1: Identify correct method call syntax

   In modern blockchain code, transactions have a verify() method called with dot notation.

2. Step 2: Check each option's syntax

   if transaction.verify(): process() uses correct method call: transaction.verify(). Others have syntax errors.

3. Final Answer:

   if transaction.verify(): process() -> Option A

4. Quick Check:

   Method call syntax = B [OK]

Hint: Use dot notation for method calls like transaction.verify() [OK]

Common Mistakes:

• Writing method calls without parentheses
• Using wrong order of keywords
• Missing dot between object and method

3. What will the following code output if the transaction is invalid?

transaction = {'valid': False}
def verify(tx):
    return tx['valid']

if verify(transaction):
    print('Transaction accepted')
else:
    print('Transaction rejected')

medium

A. Error: KeyError

B. Transaction accepted

C. Transaction rejected

D. No output

Solution

1. Step 1: Check transaction validity

   The transaction dictionary has 'valid' set to False.

2. Step 2: Evaluate verify function and if condition

   verify(transaction) returns False, so else branch runs printing 'Transaction rejected'.

3. Final Answer:

   Transaction rejected -> Option C

4. Quick Check:

   False validity means rejection = A [OK]

Hint: False validity leads to rejection message [OK]

Common Mistakes:

• Assuming True by default
• Confusing dictionary keys
• Expecting no output on False

4. Find the error in this blockchain transaction verification code:

def verify(tx):
    if tx['amount'] > 0
        return True
    else:
        return False

medium

A. Incorrect function name

B. Wrong comparison operator

C. Missing return statement

D. Missing colon after if condition

Solution

1. Step 1: Check syntax of if statement

   The if line lacks a colon at the end, which is required in Python syntax.

2. Step 2: Verify other parts

   Comparison operator and return statements are correct; function name is valid.

3. Final Answer:

   Missing colon after if condition -> Option D

4. Quick Check:

   Python if needs colon = A [OK]

Hint: Always put colon after if condition in Python [OK]

Common Mistakes:

• Forgetting colon after if
• Changing comparison operator unnecessarily
• Omitting return statements

5. You want to prevent double spending in your blockchain app. Which security practice helps most to avoid financial loss?

hard

A. Verify each transaction is unique and not spent before

B. Store transactions only on one computer

C. Ignore transaction verification for speed

D. Allow multiple transactions with same ID

Solution

1. Step 1: Understand double spending risk

   Double spending means using the same money twice, causing financial loss.

2. Step 2: Identify security practice to prevent it

   Verifying each transaction is unique and not spent before stops double spending.

3. Final Answer:

   Verify each transaction is unique and not spent before -> Option A

4. Quick Check:

   Unique transaction verification prevents loss = D [OK]

Hint: Check transaction uniqueness to stop double spending [OK]

Common Mistakes:

• Allowing duplicate transaction IDs
• Skipping verification for speed
• Relying on single computer storage