Quote from: DeathAndTaxes on May 15, 2014, 11:58:15 PM

Quote from: jonald_fyookball on May 15, 2014, 11:53:38 PM

Quote from: DeathAndTaxes on May 15, 2014, 11:41:36 PM

Quote from: jonald_fyookball link=topic=27787.msg6753438#msg6ing 753438 date=1400196716

I think they are saying you could 'roll back' on your own node to a point where you did have coin age, and try to attack from that point.

But it's not clear how you would get very far because there would be a longer chain soon and your coin age would be used up fast.

That is like saying you can't 51% attack a PoW network because the main chain is growing. Yeah if you have less computing power than the "good guys" you can't but what if you have more? With more than half of the network computing power you will eventually build the longest chain. With a PoS you will eventually build the longest chain if you have more than half of the network stake.

True. In both cases a majority ownership either in stake or hashing power would make an attack possible.
But how does that make PoS inferior ?

The attack has no cost or risk.

Very simplified example:
The network stake is 2M xCoins.
I acquired 1.1M xCoins as of block 1,000.
I sell you 1.1 M xCoins for $$$$$$$ and the transfer is recorded in block 1,001.
I now no longer have any xCoins (effective block 1,001+), I have no cost as I received $$$$$$ in return for the 1.1M xCoins.
I start building an attack chain as of block 1,000 double spending my transfer.

Eventually even if the main chain has a head start, my attack chain will be longer. This is no different than a 51% attack on a PoW based network however my attack has no cost and no risk. I already sold the coins. I am merely using my history of prior ownership to attack the network.

Compare that to PoW. I build a hashing farm with 51% of network capacity. If I attack with it then the attack has cost and risk. The farm wasn't free, I may not succeed in which case I would lose all the legit blocks I could build. If I sell the hashing farm I can't engage in an attack based on the history that at one point in the past I had more hashing power than the rest of the network.

Both are vulnerable to a 51% attack however PoS allows the attacker to exploit the history (your security mechanism is recorded in the very thing you are attempting to secure) to attack without cost or risk.

What if a PoS/PoW hybrid system? Let say PoS and PoW blocks have to occur alternatively?