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From: Ruben Somsen <rsomsen@gmail.com>
Date: Sat, 20 Apr 2019 05:26:03 +0200
Message-ID: <CAPv7TjbbYkXf62ApadMgOLef-Z9xTU1HQy_f+L3vNVUfRhChrw@mail.gmail.com>
To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Content-Type: text/plain; charset="UTF-8"
Subject: Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs
Precedence: list

Hi ZmnSCPxj,

>There is no safe way to use UTXO sets without identifying who is telling you those sets are valid, or making it expensive to lie
>The first option requires trust and is weaker than SPV, the second requires committing to a proof-of-work

Olaoluwa Osuntokun's BIP157 manages to function without a commitment:
"If the client receives conflicting filter headers from different
peers for any block and filter type, it SHOULD interrogate them to
determine which is faulty."

I am wondering if the same logic can be applied to UTXO sets or the
fraud proofs I just described.

>This makes no sense
>or you trust that every peer you have is not omitting the proof.

It's the latter, you trust every peer you have is not omitting the
proof. It requires one honest peer. The reason this is acceptable is
because you're already making that assumption. If none of your peers
are honest, you have no guarantee of hearing about the chain with the
most PoW.

Again, this is not a new observation. I am just recalling the fraud
proof debate from when it was being considered for segwit (though of
course it's possible I got some details wrong).

-- Ruben Somsen

On Sat, Apr 20, 2019 at 3:59 AM ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:
>
> Good morning,
>
>
> > > As I understand it, this requires that UTXO commitments be mandatory.
> >
> > Perhaps UTXO sets can be made useful without committing them. I have
> > some very loose thoughts on the subject, I consider it an open
> > question.
>
> There is no safe way to use UTXO sets without identifying who is telling you those sets are valid, or making it expensive to lie.
> The first option requires trust and is weaker than SPV, the second requires committing to a proof-of-work (and probably best to fold it into the Bitcoin blockchain if so).
>
> You would get the UTXO commitment from the previous block (if the UTXO commitment is in the coinbase, then all you need is the Merkle proof of the coinbase).
>
>
> >
> > > More difficult is: how can an SPV node acquire the UTXO set at a particular block?
> >
> > I think you are asking fair questions about how the UTXO set
> > commitments would work in practice, and how viable that makes it. I'm
> > not sure. The most comprehensive work I have seen on this topic has
> > been the utreexo proposal by Tadge Dryja:
> > https://www.youtube.com/watch?v=edRun-6ubCc
> >
> > Actually, now that I think about it... As an alternative to UTXO set
> > commitments, the old fraud proofs idea for segwit can be applied here.
> >
> > We get miners to commit to the location of the UTXOs that are being
> > spent (e.g. transaction 5 in block 12). This allows full nodes to
> > succinctly prove invalidity to SPV clients in the following ways:
> >
> > -   a committed location does not contain the stated UTXO
> > -   the UTXO has already been spent in a prior block
> >
> >     If no fraud proofs are given, then the inputs can be assumed to be valid.
> >
> >     As you may recall, these kinds of fraud proofs were abandoned mainly
> >     because the data unavailability claim could only be verified by
> >     downloading the data, resulting in a DoS vector where all blocks had
> >     to be downloaded. This problem does not seem to apply here, because we
> >     are only interested in blocks which have forks, so it's more doable to
> >     download them.
>
> This makes no sense.
> In order to validate block N, you need to know that every UTXO spent by a transaction in block N is valid.
> The UTXO you want to validate is located in some other block, not on the single block you are verifying.
>
> Thus the non-existent fraud proof can only be validated by loading the block of the UTXO purported to be spent, and every block between that and the current block you are verifying, i.e. fullnode.
> Either that or you trust that every peer you have is not omitting the proof.
>
> Regards,
> ZmnSCPxj