[Bitcoin-development] Enforcing inflation rules for SPV clients
I've been having a discussion with d'aniel from the forums about how to handle the possibility of a majority-miner conspiracy to raise inflation, if most economic actors use SPV clients. Because of how blocks are formatted you cannot check the coinbase of a transaction without knowing the fees in the block, and the fees can only be calculated if you have all the input transactions for every transaction in that block. Because the attack scenario is an attempted takeover of the economy by miners, attempting to put hints into the blocks won't work - we have to assume the hardest chain is in fact wrong according to the rules signed up to by the Bitcoin user. The most obvious goal for a cartel of miners is to change the inflation formula, either for purely selfish reasons (they want more money than can be obtained by fees) or due to coercion by governments/central banks who still subscribe to the inflation is good idea. Whilst good nodes (still on the old ruleset) won't relay blocks that violate the rules no matter how hard they are, in a situation where an SPV client DOES hear about the bad best chain, it would switch to it automatically. And who knows how the network might look in future - perhaps most nodes would end up run by miners, or other entities that upgrade to the new ruleset for other reasons. d'aniel made a good proposal - having good nodes broadcast announcements when they detect a rule that breaks the rules, along with a proof that it did so. Checking the proof might be very expensive, but it would only have to be done for split points, limiting the potential for DoS. If a node announces that it has a weaker chain and that the split point is a rule-breaker, the SPV client would download the headers for the side chain to verify the split, then download all the transactions in the split block along with all their inputs, and the merkle branches linking the inputs to the associated block headers. In this way the fee can be calculated, the inflation formula applied and the coinbase value checked. If the block is indeed found to be a rule-breaker, it'd be blacklisted and chains from that point forward ignored. Miners may decide to allow themselves to create money with non-index-zero transactions to work around this. In that case the good node can announce that a given tx in the rule-breaker block is invalid. The SPV node would then challenge nodes announcing the longer chain to provide the inputs for the bad tx all the way back to a pre-split coinbase. Doing these checks would be rather time consuming with huge blocks, but it's a last resort only. In the absence of bugs, the mere presence of the mechanism should ensure it never has to be used. -- Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
Re: [Bitcoin-development] Enforcing inflation rules for SPV clients
Very interesting for you to bring this up. I had a similar idea for a totally different use case. Greg recently pointed out an interesting dilemma saying that (significantly) larger blocks would lead to centralization. So I've been working on a design for a decentralized pool that can handle gigabyte sized blocks by splitting up the work among its members. At the moment P2Pool nodes all verify all transactions in all blocks. But it seems feasible to create a system where miners who have over the last 1 blocks contributed to the pool's proof-of-work are allocated a proportional piece of verification work with redundancy and deterministic randomness that makes manipulation of the allocation extremely difficult. Such a pool would be very unlikely to accept an invalid block or transaction in practice. However, with these block sizes obviously non miners are going to have to be SPV, so even just a 0.0001% chance of an invalid block being accepted has profound implications for the network. If a decentralized pool like that had more than 50% of the hashing power and it accepted a single invalid block, that tainted chain would be forever regarded as valid by SPV clients. There needs to be some way to recover once an invalid block has been accidentally accepted by an imperfect miner. Based on that I also started to think about proofs of invalidity that would circulate. Basically you would add a new network message that would contain the proof that a specific signature and therefore the whole block is invalid. As long as the block's proof-of-work is valid and the block's parent is one of the last n = 5 blocks, the message is relayed (subject to a cooldown, warnings would be less frequently relayed the older the offending block is.) The mechanism works in exactly the way Mike mentions: It allows even SPV clients to punish any miner who is dishonest or negligent with their verification work. That gives miners a good reason not to be dishonest or negligent in the first place. (Motivation: Processing more transactions means that hashing is a smaller part of the overall cost for miners. For example, paying for 50 BTC worth of hashing per block costs 0.05 BTC per tx at 1000 tx/block, but only 0.0005 BTC at 10 tx/block. Number of transactions is a lever that lets us have lower fees and more network security at the same time. Like Greg correctly pointed out, this is not worth having if we have to sacrifice decentralization. But if we don't, it becomes a no-brainer. My IMTUO proposal [1] showed a way where miners don't need a copy of the set of unspent outputs at all. This means the minimum storage requirements per node no longer grow with the number of transactions. However, the price for this was about five times greater bandwidth usage per verified transaction. Since every miner still had to verify every transaction it looked like bandwidth would become an even bigger problem with IMTUO than storage would have been without. However, if a small miner can do less than 100% verifications and still contribute, suddenly IMTUO may become viable. That would accomplish the holy grail of Bitcoin scalability where the network successfully runs on trust-atomic entities all of which can choose to store only a small fraction of the block chain, verify a small fraction of transactions and perform a small fraction of the hashing.) [1] https://en.bitcoin.it/wiki/User:Justmoon/IMTUO On 6/24/2012 2:45 PM, Mike Hearn wrote: I've been having a discussion with d'aniel from the forums about how to handle the possibility of a majority-miner conspiracy to raise inflation, if most economic actors use SPV clients. Because of how blocks are formatted you cannot check the coinbase of a transaction without knowing the fees in the block, and the fees can only be calculated if you have all the input transactions for every transaction in that block. Because the attack scenario is an attempted takeover of the economy by miners, attempting to put hints into the blocks won't work - we have to assume the hardest chain is in fact wrong according to the rules signed up to by the Bitcoin user. The most obvious goal for a cartel of miners is to change the inflation formula, either for purely selfish reasons (they want more money than can be obtained by fees) or due to coercion by governments/central banks who still subscribe to the inflation is good idea. Whilst good nodes (still on the old ruleset) won't relay blocks that violate the rules no matter how hard they are, in a situation where an SPV client DOES hear about the bad best chain, it would switch to it automatically. And who knows how the network might look in future - perhaps most nodes would end up run by miners, or other entities that upgrade to the new ruleset for other reasons. d'aniel made a good proposal - having good nodes broadcast announcements when they detect a rule that breaks the rules, along with a proof that it did so. Checking the
Re: [Bitcoin-development] Enforcing inflation rules for SPV clients
On Sun, Jun 24, 2012 at 8:45 AM, Mike Hearn m...@plan99.net wrote: d'aniel made a good proposal - having good nodes broadcast announcements when they detect a rule that breaks the rules, along with a proof that it did so. Checking the proof might be very Link? I also proposed this on this list (see the response in the tree datastructures thread) along with more elaboration on IRC. If multiple people are coming up with it thats a good sign that it it might actually be viable. :) I was going for a slightly different angle and pointing out that the proofs would mean that a node doing validation with TxOUT tree which hasn't personally wittnessed the complete history of Bitcoin actually has basically the same security— including resistance to miners creating fake coin in the past— as a full node today because in order to get away with a lie every single node must conspire: It's adequate that only one honest node wittness the lie because once it has the proof information is hard to suppress. To save people from having to dig through the public IRC logs for what I wrote there: --- Day changed Thu Jun 21 2012 15:10 gmaxwell etotheipi_: amiller: an interesting point with all this txout tree stuff is that if you join the network late and just trust that the history is correct based on the headers, any other node who has witnessed a rule violation in the past can prepare a small message which you would take to be conclusive proof of a rule violation and then ignore that chain. 15:11 gmaxwell e.g. if someone doublespends I just take the conflicting transactions out and the segments connecting them to the chain... and show them to you. And without trusting me you can now ignore the entire child chain past that point. 15:13 gmaxwell This fits nicely with the Satoshi comment It takes advantage of the nature of information being easy to spread but hard to stifle ... it would be safe to late-join a txout tree chain, because if there is only a single other honest node in the world who was around long enough to wittness the cheating, he could still tell you and it would be as good as if you saw it yourself. 15:17 gmaxwell (this is akin to the provable doublespend alert stuff we talked about before, but applied to blocks) -- Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ ___ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
