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Author Topic: There can be only one! ... or well, maybe a few  (Read 271 times)
TCraver
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December 08, 2017, 08:28:25 AM
 #1

Suppose there are (for example) ten independent blockchains that all achieve consensus using the same physical compute tech (CPU/GPU, a specific ASIC, whichever).
Suppose one of those becomes dominant - say getting 91% of compute resources, while the others get about 1% each.

Then anyone having about 1.2% of compute power on the dominant chain could shift their resources to a minor chain and have 51% of all compute.
Which would allow them to cheat, unless that chain is somehow immune to 51% attacks.

But whether or not anyone DOES cheat, users of the lesser chains would know that the potential exists to cheat on that chain.
Which would make users less willing to use the lesser chains.
Similar reasoning would apply if the most heavily processed chain had less than 91%, just to a lesser degree.

The only way I can see around this is for all of the lesser chains to immediately post hashes of accepted blocks into the dominant chain to enable historical block checking.
But that essentially makes them just sub-chains of the dominant blockchain, not independent chains, no matter how different their protocols may otherwise be.
They can't be used safely without relying on the existance of the dominant chain.

Is this correct?  Does it affect any of the ICOs out there?  I know a lot are based off Ethereum - but maybe some are vulnerable?
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aleksej996
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December 08, 2017, 04:29:16 PM
 #2

You are exactly right. When Bitcoin was designed it was never meant there to be multiple blockchains and there really is no need for that.
There are obvious security problems with smaller blockchains. Even the biggest one can be in danger if all the smaller ones leave their own chain and attack the big one.

Optimally there would be only one blockchain for each PoW algorithm and there pretty much isn't any reason to use your own blockchain if you are just going to use an algorithm that is already in use, since you can always host your own chain on top of it and increase the security on both of them by doing so.

Most ICOs are horrible and useless and by most I mean practically all if not all. My advice is always to stick with Bitcoin as there is nothing else that really contributes anything useful.

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hatshepsut93
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December 08, 2017, 06:13:08 PM
 #3

In general you are correct, small blockchains with tiny PoW are not suitable for transferring big amounts of value. However, many people are oversimplifying the 51% attack- on practice it is way more harder than simply getting 51% of hashpower. The easiest way to defend against double spends is to increase the number of required confirmations by receivers - it is possible to calculate it dynamically, basing on current price of the coin,  hashrate and received amount, and the resulting amount of confirmations will make it unprofitable to charge back corresponding transactions. So the hard part is to find someone who will accept big transactions with small amount of confirmations, but even if such attack will succeed, the news about it will spread quickly, and attacker will be left with very small amount of value from his chargeback and mining rewards - in some cases it will not be enough to cover electricity costs. However, there can be other incentives to perform 51% attacks like for example damaging the reputation of a smaller blockchain - but this can also be analyzed in a similar way. The fact that we have thousands of altcoins on the market, while we almost never hear about any successful attacks on them via mining, proves that such attacks are not very attractive for atackers.

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December 17, 2017, 07:56:32 PM
 #4

Small blockchains are indeed multiple. It may affect the bigger chains as well And yes, it may affect any of the ICOs.
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December 18, 2017, 05:05:52 AM
 #5

Optimally there would be only one blockchain for each PoW algorithm and there pretty much isn't any reason to use your own blockchain if you are just going to use an algorithm that is already in use, since you can always host your own chain on top of it and increase the security on both of them by doing so.

Can you direct me to learn more about how hosting one blockchain on top of another works?

aleksej996
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December 18, 2017, 05:22:48 PM
 #6

Optimally there would be only one blockchain for each PoW algorithm and there pretty much isn't any reason to use your own blockchain if you are just going to use an algorithm that is already in use, since you can always host your own chain on top of it and increase the security on both of them by doing so.

Can you direct me to learn more about how hosting one blockchain on top of another works?

Not really, I just thought about it a bit and it makes sense to me that you can do something as easy as that.
For example, you can simply define as a consensus protocol in your blockchain that a first valid block that has a hash referenced in the transaction of a host blockchain that is n blocks away (on the host) blockchain from the transaction that references last guest block, is the longest chain.

So "miners" in your chain don't need to do any PoW, they just need to publish a transaction on the host blockchain that references the hash of their block (for example as an address to which coins are sent to or in OP_RETURN or whatever is easiest for that chain).
Then the first such transaction that gets into a block (multiple can be listed, but only the first one is counted) is the one that is valid and seen as the tip of the longest chain in your blockchain.

I am not sure if someone came up with this before, but it is quite simple, so I assume they did.

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nullius
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December 18, 2017, 07:23:03 PM
 #7

Optimally there would be only one blockchain for each PoW algorithm and there pretty much isn't any reason to use your own blockchain if you are just going to use an algorithm that is already in use, since you can always host your own chain on top of it and increase the security on both of them by doing so.

I think the pertinent existing concept is merged mining.  A special form thereof, blind merged mining, is part of the Drivechain proposal for sidechains.

Most ICOs are horrible and useless and by most I mean practically all if not all. My advice is always to stick with Bitcoin as there is nothing else that really contributes anything useful.

Quoted, because you can say that again!



OP, the problem you sketch is part of why some of the better altcoins chose to start with their own POW algorithm, not (then yet) used by any other coin.  For example, Zcash uses Equihash; so Bitcoin miners with SHA-256 ASICs can’t suddenly redirect their hashrate to attack and overwhelm Zcash.  Given the breathtaking size of SHA-256 hashpower currently in existence, this is quite important.  Any altcoin which uses a SHA-256-based POW is incredibly foolish, and not only for reasons of a potential “51% attack”; see what happened with miners gaming BCH’s DAA before their November hardfork.  (Though in that case, the vulnerability was by design; in essence, it was a de facto premine for Jihan & Co.  I focus here on the technical issues and possibilities.)



To subject line, on multiple levels:  Yes, there can be only one Bitcoin!

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