Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs

[Ethan Heilman <eth3rs@gmail.com>]

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
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