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. 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
