Blocks, chains, and hashing in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a block with a timestamp.

Blockchain / Solidity

block = {
    'timestamp': [1],
    'data': 'Transaction Data'
}

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

Achain

Bhashlib.sha256()

Ctime.time()

Dblockchain

Attempts:

3 left

💡 Hint

Common Mistakes

Using a hashing function instead of a timestamp.

Using undefined variables like 'blockchain' or 'chain'.

The time.time() function returns the current time in seconds since the epoch, which is used as the timestamp for the block.

2fill in blank

medium

Complete the code to calculate the hash of a block's data.

Blockchain / Solidity

import hashlib

block_data = 'Sample Block Data'
block_hash = hashlib.sha256([1].encode()).hexdigest()

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

Ablock_data

Bblockchain

Ctimestamp

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to hash undefined variables.

Forgetting to encode the string before hashing.

We hash the string block_data by encoding it first, then applying SHA-256 hashing.

3fill in blank

hard

Fix the error in the code to link the current block to the previous block's hash.

Blockchain / Solidity

current_block = {
    'data': 'Some Data',
    'previous_hash': [1]
}

previous_block_hash = 'abc123def456'

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

Ahashlib.sha256()

Bprevious_block

Ccurrent_hash

Dprevious_block_hash

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole previous block instead of its hash.

Using undefined variables.

The previous_hash field should store the hash string of the previous block, which is previous_block_hash.

4fill in blank

hard

Fill both blanks to create a chain of blocks where each block stores the hash of the previous block.

Blockchain / Solidity

blockchain = []

first_block = {'data': 'Genesis Block', 'previous_hash': '0'}
blockchain.append(first_block)

new_block = {
    'data': 'Second Block',
    'previous_hash': [1]
}
blockchain.append(new_block)

print(blockchain[1]['previous_hash'] == [2])

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

Ablockchain[0]['previous_hash']

Bblockchain[0]['data']

Cblockchain[0]

Dblockchain[0]['hash']

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong index or field for previous hash.

Confusing data with hash.

The previous_hash of the new block should be the hash of the first block. Here, for simplicity, we link to the first block's hash. The print statement checks if the new block's previous_hash equals the first block's hash.

5fill in blank

hard

Fill the blanks to create a block with data, calculate its hash, and link it to the previous block's hash.

Blockchain / Solidity

import hashlib

previous_block = {'data': 'Block 1', 'hash': 'abc123'}

new_block = {
    'data': [1],
    'previous_hash': [2]
}

block_string = new_block['data'] + new_block['previous_hash']
new_block['hash'] = hashlib.sha256(block_string.encode()).hexdigest()

print(new_block['hash'])

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

A'Block 2'

Bprevious_block['hash']

C'abc123'

D'Block 1'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong string for data or previous hash.

Not concatenating strings properly before hashing.

The new block's data is 'Block 2'. Its previous_hash is the hash from the previous block, accessed as previous_block['hash'].

Practice

1. What is the main purpose of a hash in a blockchain block?

easy

A. To create a unique fingerprint of the block's data

B. To store the block's timestamp

C. To list all transactions in the block

D. To connect the block to the internet

Solution

1. Step 1: Understand what a hash does

   A hash is a unique code generated from the block's data, like a fingerprint.

2. Step 2: Identify the purpose of the hash in blockchain

   The hash ensures the block's data cannot be changed without detection.

3. Final Answer:

   To create a unique fingerprint of the block's data -> Option A

4. Quick Check:

   Hash = Unique fingerprint [OK]

Hint: Hash = unique code for block data [OK]

Common Mistakes:

• Confusing hash with timestamp
• Thinking hash stores transactions
• Believing hash connects to internet

2. Which of the following is the correct way to represent a block's hash in code?

easy

A. hash = block_data + SHA256

B. hash = SHA256(block_data)

C. hash = SHA256(block_data + hash)

D. hash = block_data.hash()

Solution

1. Step 1: Recognize the correct hash function usage

   SHA256 is a common hash function applied to block data to get the hash.

2. Step 2: Check each option for correct syntax

   hash = SHA256(block_data) correctly applies SHA256 to block_data. Others misuse the function or syntax.

3. Final Answer:

   hash = SHA256(block_data) -> Option B

4. Quick Check:

   Correct hash function call = hash = SHA256(block_data) [OK]

Hint: Hash function called with data inside parentheses [OK]

Common Mistakes:

• Adding hash function as string instead of calling it
• Using incorrect syntax for hash function
• Confusing method call with variable

3. Consider this simplified Python code for creating a blockchain block hash:

import hashlib
block_data = 'Block 1 data'
hash_object = hashlib.sha256(block_data.encode())
block_hash = hash_object.hexdigest()
print(block_hash)

What will be the output type of block_hash?

medium

A. integer

B. bytes

C. string

D. list

Solution

1. Step 1: Understand hashlib.sha256 and hexdigest()

   hashlib.sha256 returns a hash object; hexdigest() converts it to a hex string.

2. Step 2: Identify the output type of hexdigest()

   hexdigest() returns a string representing the hash in hexadecimal form.

3. Final Answer:

   string -> Option C

4. Quick Check:

   hexdigest() output = string [OK]

Hint: hexdigest() returns string, digest() returns bytes [OK]

Common Mistakes:

• Thinking hexdigest() returns bytes
• Confusing output with integer or list
• Ignoring .encode() usage

4. The following code tries to link blocks in a chain by storing the previous block's hash. What is the error?

class Block:
    def __init__(self, data, prev_hash):
        self.data = data
        self.prev_hash = prev_hash

block1 = Block('First block', None)
block2 = Block('Second block', block1.hash)

medium

A. block1.hash is not defined before use

B. prev_hash should be a list, not a single value

C. None cannot be used as prev_hash

D. block2 data should be a number, not string

Solution

1. Step 1: Check block1 attributes

   block1 has data and prev_hash but no attribute named 'hash'.

2. Step 2: Identify usage of undefined attribute

   block2 tries to use block1.hash which does not exist, causing an error.

3. Final Answer:

   block1.hash is not defined before use -> Option A

4. Quick Check:

   Accessing undefined attribute = block1.hash is not defined before use [OK]

Hint: Check if attribute exists before using it [OK]

Common Mistakes:

• Assuming hash attribute exists by default
• Confusing prev_hash type requirements
• Thinking data type causes error

5. You want to create a simple blockchain where each block stores its own hash and the previous block's hash. Which code snippet correctly computes and links the hashes for two blocks?

hard

A. import hashlib

class Block:
def __init__(self, data, prev_hash):
self.data = data
self.prev_hash = prev_hash
self.hash = hashlib.sha256(data.encode()).hexdigest()

block1 = Block('Block 1', '0')
block2 = Block('Block 2', block1.hash)

B. import hashlib

class Block:
def __init__(self, data):
self.data = data
self.hash = hashlib.sha256(data.encode()).hexdigest()

block1 = Block('Block 1')
block2 = Block('Block 2')
block2.prev_hash = block1.hash

C. class Block:
def __init__(self, data, prev_hash):
self.data = data
self.prev_hash = prev_hash
self.hash = data + prev_hash

block1 = Block('Block 1', '0')
block2 = Block('Block 2', block1.hash)

D. import hashlib

class Block:
def __init__(self, data, prev_hash):
self.data = data
self.prev_hash = prev_hash
self.hash = hashlib.sha256((data + prev_hash).encode()).hexdigest()

block1 = Block('Block 1', '0')
block2 = Block('Block 2', block1.hash)

Solution

1. Step 1: Check hash calculation includes previous hash

   The correct implementation computes the hash from both data and prev_hash: self.hash = hashlib.sha256((data + prev_hash).encode()).hexdigest(), ensuring secure linking.

2. Step 2: Verify other options

   B: prev_hash set after hash computation (hash ignores prev_hash); C: no hashing, just concatenation; D: hash ignores prev_hash.

3. Final Answer:

   self.hash = hashlib.sha256((data + prev_hash).encode()).hexdigest() -> Option D

4. Quick Check:

   Hash(data + prev_hash) [OK]

Hint: Hash must include previous hash to link blocks [OK]

Common Mistakes:

• Not including prev_hash in hash calculation
• Assigning prev_hash after block creation
• Using plain concatenation without hashing