Good morning Ethan, Thank you for clarifying, I understand better now.
It seems that minority miners can disrupt SPV clients such that SPV clients will download 2 blocks for every block the minority miner can find, not 1. This can be done by simply making multiple 1-block chainsplits, rather than a single persistent chainsplit, and alternating split-off and non-split-off. For instance, such a minority miner might split at S+1, forcing SPV clients to download S+1 and S+2. Then the minority miner splits at S+3, forcing SPV clients to download S+3 and S+4. With a mere 33% hashrate, this can force SPV clients to download every block, i.e. become a fullnode anyway. Since there exist pools with >33% hashrate, the above attack is possible so the only solution is to become a fullnode anyway. Regards, ZmnSCPxj Sent with ProtonMail Secure Email. ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Friday, April 19, 2019 9:13 AM, Ethan Heilman <eth...@gmail.com> wrote: > Hi ZmnSCPxj, > > Let's see if I understand what you are saying. In your scenario chain > A consists of honest miners (10% of the hash rate) and chain B (90% > of the hash rate) consists of dishonest miners who are inflating the > coin supply. > > Chain A: S, S+1 > Chain B: S, S+1 (invalid), S+2, S+3, S+4, S+5, S+6, S+7, S+8, S+9 > > Chain B S+1 has a invalid coinbase > > > At around height S+9, the minority miners generate an alternate block at > > height S+1. So SPV nodes download S+9 and S+8 on the longer chain, and see > > nothing wrong with those blocks. > > What I am suggesting is that when the minority miners generate an > alternate block at S+1 (chain A) the SPV node would download blocks > S+1 and S+2 from chain B (the dishonest chain). Since S+1 has the > invalid coinbase the SPV node would learn that chain B is invalid and > abandon it. > > Bitcoin is in big trouble if a malicious party controls 90% of the > mining power. The malicious miners can spend +11% of their mining > power ensuring that the honest chain never reaches consensus by > continuously forking it. The malicious miners can then extend their > favored chain using the other 79% of the mining power. This would > produce a scenario in which users are forced to choose between a > stable chain that violates a consensus rule and an unstable honest > chain that is completely unusable and which never pays out mining > rewards. I agree that SPV nodes and many wallets would make this even > worse especially in their current condition where they just trust the > hash rate/wallet provider and there are no fraud proofs. > > On Thu, Apr 18, 2019 at 8:25 PM ZmnSCPxj zmnsc...@protonmail.com wrote: > > > Good morning Ethan, > > Sent with ProtonMail Secure Email. > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > On Friday, April 19, 2019 4:12 AM, Ethan Heilman eth...@gmail.com wrote: > > > > > I'm probably repeating a point which has been said before. > > > > > > > I suppose a minority miner that wants to disrupt the network could > > > > simply create a valid block at block N+1 and deliberately ignore every > > > > other valid block at N+1, N+2, N+3 etc. that it did not create itself. > > > > > > If this minority miner has > 10% of network hashrate, then the rule of > > > thumb above would, on average, give it the ability to disrupt the > > > SPV-using network. > > > Proposed rule: > > > Whenever a chainsplit occurs SPV clients should download and validate > > > the "longest chain" up to more than one block greater than the height > > > of the losing chain. > > > Lets say a block split causes chain A and chain B: Chain A is N blocks > > > long, chain B is M blocks long, and N < M. Then the SPV client should > > > download all the block data of N+1 blocks from Chain B to verify > > > availability of chain B. Once the SPV client has verified that chain B > > > is available they can use fraud proofs determine if chain B is valid. > > > > Let us then revert to the original scenario. > > Suppose a supermajority (90%) of miners decide to increase inflation of the > > currency. > > They do this by imposing the rule: > > > > 1. For 1 block, the coinbase is 21,000,000 times the pre-fork coinbase > > value. > > 2. For 9 blocks, the coinbase is the pre-fork value. > > 3. Repeat this pattern every 10 blocks. > > > > The above is a hardfork. > > However, as they believe that SPV nodes dominate the economy, this mining > > supermajority believes it can take over the network hashpower and impose > > its will on the network. > > At height S+1, they begin the above rule. > > This implies that at heights S+1, S+11, S+21, s+31... the coinbase violates > > the pre-hardfork rules. > > At around height S+9, the minority miners generate an alternate block at > > height S+1. > > So SPV nodes download S+9 and S+8 on the longer chain, and see nothing > > wrong with those blocks. > > At around height S+18, the minority miners generate an alternate block at > > height S+2. > > So SPV nodes download S+18, S+17, S+16 and again see nothing wrong with > > those blocsk. > > This can go on for a good amount of time. > > With a "rare enough" inflation event, miners may even be able to spend some > > coinbases on SPV nodes that SPV nodes become unwilling to revert to the > > minority pre-hardfork chain, economically locking in the post-hardfork > > inflation. > > Again: every rule is an opportunity to loophole. > > Regards, > > ZmnSCPxj > > > > > An attacker could use this to force SPV clients to download 1 block > > > per block the attacker mines. This is strictly weaker security than > > > provided by a full-node because chain B will only be validated if the > > > client knows chain A exists. If the SPV client's view of the > > > blockchain is eclipsed then the client will never learn that chain A > > > exists and thus never validate chain B's availability nor will the > > > client be able to learn fraud proofs about chain B. A full node in > > > this circumstance would notice that the chain B is invalid and reject > > > it because a full node would not depend on fraud proofs. That being > > > said this rule would provide strictly more security than current SPV > > > clients. > > > On Thu, Apr 18, 2019 at 3:08 PM ZmnSCPxj via bitcoin-dev > > > bitcoin-dev@lists.linuxfoundation.org wrote: > > > > > > > Good morning Ruben, > > > > Sent with ProtonMail Secure Email. > > > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > > > On Thursday, April 18, 2019 9:44 PM, Ruben Somsen via bitcoin-dev > > > > bitcoin-dev@lists.linuxfoundation.org wrote: > > > > > > > > > Simplified-Payment-Verification (SPV) is secure under the assumption > > > > > that the chain with the most Proof-of-Work (PoW) is valid. As many > > > > > have pointed out before, and attacks like Segwit2x have shown, this is > > > > > not a safe assumption. What I propose below improves this assumption > > > > > -- invalid blocks will be rejected as long as there are enough honest > > > > > miners to create a block within a reasonable time frame. This still > > > > > doesn’t fully inoculate SPV clients against dishonest miners, but is a > > > > > clear improvement over regular SPV (and compatible with the privacy > > > > > improvements of BIP157[0]). > > > > > The idea is that a fork is an indication of potential misbehavior -- > > > > > its block header can serve as a PoW fraud proof. Conversely, the lack > > > > > of a fork is an indication that a block is valid. If a fork is created > > > > > from a block at height N, this means a subset of miners may disagree > > > > > on the validity of block N+1. If SPV clients download and verify this > > > > > block, they can judge for themselves whether or not the chain should > > > > > be rejected. Of course it could simply be a natural fork, in which > > > > > case we continue following the chain with the most PoW. > > > > > > > > I presume you mean a chain split? > > > > > > > > > The way Bitcoin currently works, it is impossible to verify the > > > > > validity of block N+1 without knowing the UTXO set at block N, even if > > > > > you are willing to assume that block N (and everything before it) is > > > > > valid. This would change with the introduction of UTXO set > > > > > commitments, allowing block N+1 to be validated by verifying whether > > > > > its inputs are present in the UTXO set that was committed to in block > > > > > N. An open question is whether a similar result can be achieved > > > > > without a soft fork that commits to the UTXO set[0][1]. > > > > > If an invalid block is created and only 10% of the miners are honest, > > > > > on average it would take 100 minutes for a valid block to appear. > > > > > During this time, the SPV client will be following the invalid chain > > > > > and see roughly 9 confirmations before the chain gets rejected. It may > > > > > therefore be prudent to wait for a number of confirmations that > > > > > corresponds to the time it may take for the conservative percentage of > > > > > miners that you think may behave honestly to create a block (including > > > > > variance). > > > > > > > > I suppose a minority miner that wants to disrupt the network could > > > > simply create a valid block at block N+1 and deliberately ignore every > > > > other valid block at N+1, N+2, N+3 etc. that it did not create itself. > > > > If this minority miner has > 10% of network hashrate, then the rule of > > > > thumb above would, on average, give it the ability to disrupt the > > > > SPV-using network. > > > > > > > > > 10% of network hashrate to disrupt the SPV-using nodes would be a > > > > > rather low bar to disruption. > > > > > Consider that SPV-using nodes would be disrupted, without this rule, > > > > > only by >50% network hashrate. > > > > > > > > It is helpful to consider that every rule you impose is potentially a > > > > loophole by which a new attack is possible. > > > > Regards, > > > > ZmnSCPxj > > > > bitcoin-dev mailing list > > > > bitcoin-dev@lists.linuxfoundation.org > > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev _______________________________________________ bitcoin-dev mailing list bitcoin-dev@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
