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
