Quote from: amincd on Today at 06:27:12 am
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The whole double-spending scenario is proportional to bitcoin value. If value were to go up orders of magnitude, the number of blocks needed to get your investment back goes down the same order of magnitude.
But difficulty, and therefore cost of a double spend attack, is also proportional to bitcoin value, so the rise in the potential reward of a double spend attack, is canceled out by the rise in cost in pulling it off, as bitcoin value increases.
I don't agree, as the cost of pulling it of is proportional to transaction fee cost, which is much lower than transaction value.
But the ratio of cost to transaction value stays the same regardless of what the value of bitcoin is, since the cost of pulling it off increases at the same rate as the transaction value does, as the value of bitcoin increases.
If nothing were to change in the value of bitcoins or transaction fees, I'd eventually have to produce 23.6 Ghash / second for a succesful attack. Which would cost me roughly 15 kW electricity (ca. 3$ per hour) and (very) roughly 1$ depreciation per hour of my hardware. Let's say it costs 5$ per hour all together.
It would definitely be much easier to attack bitcoin if transaction volume doesn't increase by the time coin generation becomes negligible, but your calculation of cost doesn't take into account the large initial investment required to acquire a large amount of hashing power, which is not just purchasing the hardware, but getting the facility, putting in the man-hours to set it up, etc all of which have a huge fixed cost, and therefore the need to double spend for many blocks, to make back the cost of the investment.
I can then sustain an attack where I forge (double spend) say 100$ (which will not be conspicuous) for 20 hours (which should be more than enough to collect).
So after all that effort in getting 23 GH/s, you only make $2,000?