Blocks, chains, and hashing in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Blocks, chains, and hashing

Create Block Data

↓

Calculate Hash of Block

↓

Add Hash to Block

↓

Link Block to Previous Block's Hash

↓

Add Block to Chain

↓

Repeat for Next Block

Blocks hold data and a hash. Each block links to the previous block's hash, forming a chain.

Execution Sample

Blockchain / Solidity

block1 = {"data": "First block", "prev_hash": "0"}
block1["hash"] = hash(str(block1))

block2 = {"data": "Second block", "prev_hash": block1["hash"]}
block2["hash"] = hash(str(block2))

Creates two blocks, each with data and a hash linking to the previous block's hash.

Execution Table

Step	Action	Block Data	Previous Hash	Calculated Hash	Chain State
1	Create block1 data	{"data": "First block", "prev_hash": "0"}	"0"	N/A	[]
2	Calculate block1 hash	{"data": "First block", "prev_hash": "0"}	"0"	hash1	[]
3	Add hash to block1	{"data": "First block", "prev_hash": "0", "hash": "hash1"}	"0"	hash1	[]
4	Add block1 to chain	{"data": "First block", "prev_hash": "0", "hash": "hash1"}	"0"	hash1	[block1]
5	Create block2 data with prev_hash	{"data": "Second block", "prev_hash": "hash1"}	"hash1"	N/A	[block1]
6	Calculate block2 hash	{"data": "Second block", "prev_hash": "hash1"}	"hash1"	hash2	[block1]
7	Add hash to block2	{"data": "Second block", "prev_hash": "hash1", "hash": "hash2"}	"hash1"	hash2	[block1]
8	Add block2 to chain	{"data": "Second block", "prev_hash": "hash1", "hash": "hash2"}	"hash1"	hash2	[block1, block2]

💡 All blocks created and linked; chain contains two blocks.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8
block1	{}	{"data": "First block", "prev_hash": "0", "hash": "hash1"}	{"data": "First block", "prev_hash": "0", "hash": "hash1"}	{"data": "First block", "prev_hash": "0", "hash": "hash1"}	{"data": "First block", "prev_hash": "0", "hash": "hash1"}
block2	{}	{}	{}	{"data": "Second block", "prev_hash": "hash1", "hash": "hash2"}	{"data": "Second block", "prev_hash": "hash1", "hash": "hash2"}
chain	[]	[]	[block1]	[block1]	[block1, block2]

Key Moments - 2 Insights

Why does each block include the previous block's hash?

Including the previous block's hash links blocks together, making the chain secure and tamper-evident, as shown in steps 5 and 6 where block2 uses block1's hash.

What happens if the data in a block changes after hashing?

If block data changes, its hash changes too, breaking the chain link. This is why the hash is calculated after setting data, as in steps 2 and 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the previous hash value used when creating block2?

A "hash1"

B "0"

C "hash2"

D null

💡 Hint

Check Step 5 in the execution table where block2's previous hash is set.

At which step is block1 added to the chain?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Chain State' column in the execution table to see when block1 appears.

If block1's data changed after step 4, what would happen to block2's previous hash?

A It would remain the same

B It would automatically update

C It would become invalid

D It would be null

💡 Hint

Refer to the key moment about data change affecting hashes and chain integrity.

Concept Snapshot

Blocks store data and a hash.
Each block includes the previous block's hash.
Hashes link blocks into a secure chain.
Changing data changes the hash, breaking the chain.
This creates a tamper-evident blockchain.

Full Transcript

This visual trace shows how blocks are created with data and a previous hash. Each block's hash is calculated from its data and previous hash, then added to the block. Blocks are linked by including the previous block's hash, forming a chain. The chain grows as new blocks are added. Changing block data changes its hash, breaking the chain link and ensuring security.

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