Merge mining lets Bitcoin miners support or attack an alt-coin without any  
additional cost for their proof-of-work.

Since bitcoin miners have to install software to build and claim blocks in  
the alt-coin, the percentage of bitcoin hashing power reflected toward the  
alt-coin will follow some adoption curve based on convincing bitcoin  
miners to opt-in.

Depending on where you are on that adoption curve or 'participation rate',  
you need [a lot] less than 51% of of total Bitcoin hashing power in order  
to 51% attack the alt-coin.

But there's so much 'dry powder' out there (GPUs), I wonder if *not*  
supporting merge-mining is any better? At least the attacker has to do  
some unique PoW, so you hope it's costing them something. Relatively large  
amounts of hashing can definitely be deployed on target with zero startup  
cost, and perhaps very little runtime cost (botnets).

I think the absolute cost of the PoW is very likely *not* the determining  
factor in preventing a 51% attack on all but one or two blockchains  
currently in existence.

Do I understand correctly, the question here is mostly a matter a game  

On Mon, 30 Dec 2013 17:14:05 -0800, Luke-Jr <> wrote:

> On Monday, December 30, 2013 11:22:25 PM Peter Todd wrote:
>> that you are using merge-mining is a red-flag because without majority,  
>> or
>> at least near-majority, hashing power an attacker can 51% attack your
>> altcoin at negligible cost by re-using existing hashing power.
> I strongly disagree on this isolated point. Using the same logic,  
> Bitcoin is
> vulnerable to an attacker at negligible cost by re-using existing hashing
> power from mining Namecoin. Any non-scam altcoin is pretty safe using  
> merged
> mining, since any would-be attacker is going to have it in their  
> interests to
> invest in the altcoin instead of attacking it. It's only the scam ones  
> that want to pump & dump with no improvements, that are really at risk  
> here.
> The rational decision for a non-scam altcoin, is to take advantage of  
> merged mining to get as much security as possible. There are also some  
> possible
> tricks to get the full security of the bitcoin miners even when not all
> participate in your altcoin (but this area probably needs some studying  
> to get right).
> Luke

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