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Author Topic: How are blocks secure?  (Read 40 times)
Creatiive
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August 12, 2018, 06:41:41 PM
 #1

Hi and sorry if this question doesn’t even make sense!

I have read that bitcoin is secure because if someone wanted to tamper with a transaction in a past block, they would have to rehash that block and all the blocks that came after it - which would be prohibitive / impossible to catch up.

Assuming the above is correct, what if someone operating a node actually managed to do it. Wouldn’t they now have a chain that was valid, but different to everyone elses chain? I think this is the bit I have got wrong.

Im trying to understand how it prevents someone from changing a block.

Thanks for any pointers!
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August 12, 2018, 11:04:39 PM
Merited by OgNasty (1)
 #2

Hi and sorry if this question doesn’t even make sense!

I have read that bitcoin is secure because if someone wanted to tamper with a transaction in a past block, they would have to rehash that block and all the blocks that came after it - which would be prohibitive / impossible to catch up.

Assuming the above is correct, what if someone operating a node actually managed to do it. Wouldn’t they now have a chain that was valid, but different to everyone elses chain? I think this is the bit I have got wrong.

Im trying to understand how it prevents someone from changing a block.

Thanks for any pointers!

You are asking about how "proof of work" (POW) works. POW is the algorithm that secures the network.

I made this summary about it few days ago
https://bitcointalk.org/index.php?topic=3317586.0;topicseen


Assuming the above is correct, what if someone operating a node actually managed to do it. Wouldn’t they now have a chain that was valid, but different to everyone elses chain? I think this is the bit I have got wrong.

Im trying to understand how it prevents someone from changing a block.

Thanks for any pointers!

More straight forward answer:

- finding the target hash (solving proof of work algorithm) for a block requires a lot of processing power, which costs energy and hardware.
- If someone creates a side chain, it will be a smaller chain. Only the longest chain is considered valid, which is the chain with the most processing power. All the other nodes will consider this side chain invalid, unless this node could have more processing power than the rest of the network. This is called a 51% attack.

Bitcoin is safe if the honest nodes have more than 51% of the network processing power.


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August 12, 2018, 11:42:04 PM
 #3

I have read that bitcoin is secure because if someone wanted to tamper with a transaction in a past block, they would have to rehash that block and all the blocks that came after it - which would be prohibitive / impossible to catch up.

Its not correct.
You won't be able to tamper with blocks because noone would care you changed it.
There would be needed many many more factors to be able to disrupt the bitcoin network.

Don't be afraid Smiley, best computer scientist in the world taking care of Bitcoin

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