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I am fairly new to the bitcoin and blockchain technology and have recently started reading about it. So my understanding and the question bellow may not be very accurate.

As I have understood so far, proof of work is the basic building block for a bitcoin block chain and because of it, an attacker will have to produce more than 50% of the total compute power (i.e control more than 50% of the nodes) in order to manipulate the block chain by being able to produce longest block chain consistently.

Now bitcoin guys were bit lucky as they were the first and nobody paid attention in there early days. Once bitcoin gathered momentum, number of honest nodes become predominant and system became inherently secure.

But now, how someone can start a new public blockchain (for completely different application) safely ? Because, if a new blockchain is floated with few mining nodes, any attacker can come with more compute nodes and hijack the blockchain as there are small number of honest nodes.

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how someone can start a new public blockchain

This is impossible to secure PoW blockchain without "work". No work by honest nodes - no security. Point.

  • Thanks amaclin ! Just to understand more, even if honest nodes are doing 'work', they can be outnumbered by a sufficiently large group of dishonest nodes in nascent stage of public PoW blockchain application. How to overcome this ? If it is impossible, then how to bootstrap entirely new public system based on PoW blockchain avoiding above scenario ? – Saurabh Deshpande Jul 19 '17 at 7:00
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if I may add two lines :-) Generally, @amacline described it very well. I think it is also a question of value. If you have only 1.000 Euros in your blockchain, who will invest in creating a 51% attack? But if the value gets bigger, do the math! Starting a new PoW blockchain can be done in a way, that you created enough blocks at the beginning (so you invested computing power), to raise the difficulty high enough, so that PoW requires a corresponding (high) investment. Not sure if it is even thinkable, to manually set difficulty to a high level at the very beginning.

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