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

[ZmnSCPxj <ZmnSCPxj@protonmail.com>]

Good morning Ethan,


Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Friday, April 19, 2019 4:12 AM, Ethan Heilman <eth3rs@gmail.com> 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
> 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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