Good morning Ethan,
Sent with ProtonMail Secure Email. ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Friday, April 19, 2019 4:12 AM, Ethan Heilman <[email protected]> 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 > [email protected] wrote: > > > Good morning Ruben, > > Sent with ProtonMail Secure Email. > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > On Thursday, April 18, 2019 9:44 PM, Ruben Somsen via bitcoin-dev > > [email protected] 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 > > [email protected] > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev _______________________________________________ bitcoin-dev mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
