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Alice, Bob, and Charlie are part of the Bitcoin network.
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There’s a blockchain with already 100 blocks in it.
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Each block contains multiple verified transactions, and every block links to the previous one via a cryptographic hash.
🔹 Step-by-Step Scenario
🧾 Step 1: A Transaction Occurs
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Alice sends 1 BTC to Bob.
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The transaction is broadcast to the network.
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Miners verify it and include it in Block 101.
Block 101 now contains:
Once this block is validated (mined), it is added to the blockchain and copied to all nodes.
🔏 Step 2: The Transaction Becomes Immutable
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Each block’s hash is based on its contents and the hash of the previous block.
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So Block 102’s hash is based on Block 101’s hash, and so on.
Now the blockchain looks like:
If someone changes anything in Block 101 — even a single letter — the hash of Block 101 will change, breaking the chain from Block 101 onward.
🔥 Step 3: What Happens If Someone Tries to Alter the Data?
Imagine Charlie is a malicious actor. He wants to change the transaction in Block 101 to say:
Consequences:
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The hash of Block 101 changes.
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This invalidates Block 102, which contains Block 101’s old hash.
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So he must now recalculate Block 102, Block 103, and every block up to the current one.
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Each block requires huge amounts of computation (Proof of Work), which he must do faster than the entire rest of the network combined.
This is practically impossible without owning more than 50% of the network’s mining power — known as a 51% attack.
✅ Result: The Data Is Immutable
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Alice’s transaction is locked into history.
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No one can alter it unless the majority of the global network agrees (consensus), which:
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Is economically infeasible
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Risks detection and being rejected by honest nodes
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Would destroy the attacker’s trust and any coins they have
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🧠 Why It Matters
Immutability:
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Protects against fraud and censorship
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Guarantees data integrity
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Builds trust in decentralized systems