Quote from: jonald_fyookball on April 01, 2015, 07:27:57 PM

I'm not particular familiar with NXT or various implementations, i'm speaking in terms
of general principles. Based on the whitepaper, there's a complex calculation involving
the UXTOs and the block headers of previous blocks. I still don't see how that prevents
"grinding" or using computational power to build a chain.

If it is difficult to compute, isn't that almost becoming proof of work and everything
that goes along with it? (If its difficult to compute for an "average" computer,
wouldnt an ASIC do it easily?)

The issue with discussion about grinding is that as long as you don't go into specifics it's difficult to really make progress!!!
I didn't say it was "difficult" to compute but that grinding was made extremely inefficient.
An order of magnitude, is that an attacker with 1 ASIC miner (1TH/S) would need ~33% of the mining coins to perform a 51% attack while an attacker with the entire hash rate of the bitcoin network (~300PH/S) would need ~30%. That's what ennificient means.
The advantage you can get through grinding is highly non-linear.

More generally, it's difficult to answer objections about grinding if the argument specify through which parameter you are trying to grind.

Quote

You seem to be saying that it is not difficult to build a chain of 1 block, but it
difficult to build a chain of many blocks under this implementation.
What exactly makes that possible? I haven't seen any explanation of that assertion,
if that's what is being claimed.

What is difficult (actually probabilistically impossible without large portion of the coins) is to build a chain that is longer than the main chain at any point.
Let me explain, using a relatively simple example:
Unlike in PoW, building a chain in PoS doesn't take time. You could create a fork and know practically immediately what the trust of your fork will be X days from now.
Let's imagine you've got 10% of the coins. What is the probability that you'll be longer than the main chain after it has built 10 blocks? The answer is ~10^-6

From that, you'd be tempted to conclude that you can try again ("grind") many times and that at some point you'll win, because after all 10^6 attempts is nothing even for a laptop.
However, and that's where specifying what you grind through is important, the only way to "try" more than once is to change the kernel of your 10% of coins mining. The best way to do that is to change the parameters of the kernel inherited form the UTXO by sending all the coins to the fork. That's when the minimum stake age kicks in. It will prevent these stakes from mining for 1.6 days (in NeuCoin's case) so the attacker's fork will basically be "losing" 1.6 days worth of blocks he could've mined had his stakes been allowed to.

This period during which he cannot mine is devastating for his performance. It's similar to starting 1.6 days behind in PoW. With 10% the probability to succeed is null.

Maybe another thing I should point out is that nodes do not accept blocks created with a proof that has a timestamp too far in the future (otherwise forking would obviously be trivial).

Maybe I'm missing something, but it sounds like a self-defeating argument:

"We'll prevent this from turning into proof of work by making it really
hard to compute." Tongue