Date: Thu, 18 Apr 2019 16:55:10 +0000
To: Ruben Somsen <rsomsen@gmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs
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Good morning Ruben,


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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=E2=80=99t 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 c=
reate a *valid* block at block N+1 and deliberately ignore every other vali=
d 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 thum=
b above would, on average, give it the ability to disrupt the SPV-using net=
work.

>10% of network hashrate to disrupt the SPV-using nodes would be a rather l=
ow bar to disruption.
Consider that SPV-using nodes would be disrupted, without this rule, only b=
y >50% network hashrate.

It is helpful to consider that every rule you impose is potentially a looph=
ole by which a new attack is possible.

Regards,
ZmnSCPxj