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Bitcoin: Virtual money created by CPU cycles

Bitcoin: Virtual money created by CPU cycles

Posted Nov 23, 2010 0:33 UTC (Tue) by creighto (guest, #71377)
In reply to: Bitcoin: Virtual money created by CPU cycles by nybble41
Parent article: Bitcoin: Virtual money created by CPU cycles

"It is the size of the network which protects against "brute force" attacks, not the difficultly of each individual block."

Those two variables are on opposite sides of an equation. When the network grows, the difficulty automaticly increases to compensate and maintain a relatively consistant block interval.


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Bitcoin: Virtual money created by CPU cycles

Posted Nov 23, 2010 8:21 UTC (Tue) by nybble41 (subscriber, #55106) [Link]

I understand, and I think we actually do agree on this if we could just put it into the right terms. My use of "difficulty" was wrong; that is a term with a very specific meaning in Bitcoin, but I was using it in a generic sense to refer to the time it takes to generate a block (the target rate), not the number of hashes or upper limit for acceptable solutions (which are derived from that target rate and the actual rate).

Obviously you don't want the rate at which blocks are generated to increase sharply, since that would make it relatively easy to invalidate some or all of the work that went into creating the existing block chain. If it takes a year to get to block 1000, but only another week to get to block 2000, then you have a problem--a concerted effort could supplant the original block chain, or at least a significant suffix of it, with a new one of the attacker's choice, causing the system to break down (double-spending, etc.). For that to happen to the last day's transactions is one thing, but with a large rate of change a few day's effort could invalidate a much longer period of the chain history.

To avoid that the network has to regulate the rate at which new blocks are generated such that acceleration of that rate, if any, is extremely gradual; currently that means no average change at all, although there is some variation above and below the goal rate between difficulty adjustments.

What I'm saying, however, is that this goal rate doesn't have to be six blocks per hour to prevent so-called "brute force" attacks; provided the clients remain synchronized, it could just as easily be six blocks per *second*. I refer, of course, to a constant rate of six blocks per second since the network was formed, not a sudden 3600x drop in the difficulty from an existing six-blocks-per-hour chain. If there was to be a transition from one rate to the other it would have to be extremely gradual.

Bitcoin: Virtual money created by CPU cycles

Posted Nov 23, 2010 23:01 UTC (Tue) by creighto (guest, #71377) [Link]

"What I'm saying, however, is that this goal rate doesn't have to be six blocks per hour to prevent so-called "brute force" attacks; provided the clients remain synchronized, it could just as easily be six blocks per *second*. I refer, of course, to a constant rate of six blocks per second since the network was formed, not a sudden 3600x drop in the difficulty from an existing six-blocks-per-hour chain. If there was to be a transition from one rate to the other it would have to be extremely gradual."

The clients could not stay synchronized anywhere near such a rate, and the rate of blocks does directly affect the growth of the total size of the blockchain. The target interval is an arbitrary decision, and one that could be changed with consensus in the future; but if it does change I would guess that it would be reduced from 6 blocks an hour to 4 or 3 per hour. The p2p network is currently very small, but in a future with Bitcoin as much of the online economy as PayPal; the network would likely be bogged down with latency at anything faster than 10 per minute.

Bitcoin: Virtual money created by CPU cycles

Posted Nov 23, 2010 23:22 UTC (Tue) by creighto (guest, #71377) [Link]

Sorry, that last line should read "10 per hour"

Bitcoin: Virtual money created by CPU cycles

Posted Nov 24, 2010 3:21 UTC (Wed) by nybble41 (subscriber, #55106) [Link]

So we do agree, then. It is only the practice (synchronization and block-chain size), and not any issue with the theory of operation, which leads to low rate of block generation. If synchronization and the size of the block-chain were not significant factors then the rate at which blocks are generated could be arbitrarily high without impacting the security of the system.

Bitcoin: Virtual money created by CPU cycles

Posted Nov 24, 2010 20:00 UTC (Wed) by creighto (guest, #71377) [Link]

Generally speaking, yes. There might be other technical issues that I'm not aware of, but we do agree.


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