Blocks, chains, and hashing in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this simple block hash calculation?

Consider a block with data 'Block1' and previous hash '0000'. The hash function returns the length of the combined string. What is the output of the hash calculation?

Blockchain / Solidity

data = 'Block1'
previous_hash = '0000'
block_string = data + previous_hash
block_hash = len(block_string)
print(block_hash)

A 11

B 10

C 9

D 12

💡 Hint

Count the characters in 'Block1' plus '0000'.

The string 'Block1' has 6 characters and '0000' has 4 characters, combined length is 10. But the code concatenates them, so total length is 10.

🧠 Conceptual

intermediate

Which property of a cryptographic hash ensures blockchain security?

Which property of a cryptographic hash function makes it hard to change a block without changing its hash?

A Fast computation

B Deterministic output

C Collision resistance

D Fixed output length

💡 Hint

Think about how hard it is to find two inputs with the same hash.

Collision resistance means it is very hard to find two different inputs producing the same hash, so changing block data changes the hash.

🔧 Debug

advanced

Why does this blockchain chain break?

This code tries to link blocks by storing previous hashes. Why does the chain verification fail?

Blockchain / Solidity

blocks = [
  {'data': 'Block1', 'prev_hash': '0000', 'hash': 'abc123'},
  {'data': 'Block2', 'prev_hash': 'abc123', 'hash': 'def456'},
  {'data': 'Block3', 'prev_hash': 'xyz789', 'hash': 'ghi012'}
]

for i in range(1, len(blocks)):
    if blocks[i]['prev_hash'] != blocks[i-1]['hash']:
        print(f"Chain broken at block {i+1}")
        break
else:
    print("Chain is valid")

A Because block 3's prev_hash does not match block 2's hash

B Because block 2's hash is incorrect

C Because block 1's prev_hash is not '0000'

D Because the loop does not check the first block

💡 Hint

Check the prev_hash of block 3 and the hash of block 2.

Block 3's prev_hash is 'xyz789' but block 2's hash is 'def456', so the chain breaks at block 3.

📝 Syntax

advanced

Which code snippet correctly creates a block hash using SHA-256?

Select the code that correctly computes a SHA-256 hash of a block's data and previous hash in Python.

A

import hashlib
block_string = data + prev_hash
block_hash = hashlib.sha256(block_string.encode()).hexdigest()

B

import hashlib
block_hash = hashlib.sha256(data + prev_hash).hexdigest()

C

import hashlib
block_hash = hashlib.sha256(block_string.encode).hexdigest()

D

import hashlib
block_hash = hashlib.sha256(block_string.encode()).digest()

💡 Hint

Remember to encode the string before hashing and use hexdigest() for readable output.

Option A correctly encodes the string and uses hexdigest() to get the hash as a hex string.

🚀 Application

expert

How many blocks are in the valid chain after this code runs?

Given this blockchain list, how many blocks form a valid chain starting from the first block?

Blockchain / Solidity

blockchain = [
  {'data': 'Genesis', 'prev_hash': '0', 'hash': 'a1'},
  {'data': 'Block1', 'prev_hash': 'a1', 'hash': 'b2'},
  {'data': 'Block2', 'prev_hash': 'b2', 'hash': 'c3'},
  {'data': 'Block3', 'prev_hash': 'x9', 'hash': 'd4'},
  {'data': 'Block4', 'prev_hash': 'd4', 'hash': 'e5'}
]

valid_count = 1
for i in range(1, len(blockchain)):
    if blockchain[i]['prev_hash'] == blockchain[i-1]['hash']:
        valid_count += 1
    else:
        break
print(valid_count)

A 2

B 4

C 5

D 3

💡 Hint

Check where the prev_hash does not match the previous block's hash.

The chain breaks at Block3 because its prev_hash 'x9' does not match Block2's hash 'c3', so only 3 blocks are valid.

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