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.