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

Concept Flow - Blocks, chains, and hashing

Create Block Data

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Calculate Hash of Block

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Add Hash to Block

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Link Block to Previous Block's Hash

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Add Block to Chain

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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.