Date: Sat, 20 Apr 2019 01:59:25 +0000
To: Ruben Somsen <rsomsen@gmail.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
Reply-To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
	"tdryja@media.mit.edu" <tdryja@media.mit.edu>
Subject: Re: [bitcoin-dev] Improving SPV security with PoW fraud proofs
Precedence: list

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 yo=
u 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 Bitco=
in blockchain if so).

You would get the UTXO commitment from the previous block (if the UTXO comm=
itment is in the coinbase, then all you need is the Merkle proof of the coi=
nbase).


>
> > More difficult is: how can an SPV node acquire the UTXO set at a partic=
ular 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=3DedRun-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 va=
lid.
>
>     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 w=
e
>     are only interested in blocks which have forks, so it's more doable t=
o
>     download them.

This makes no sense.
In order to validate block N, you need to know that every UTXO spent by a t=
ransaction in block N is valid.
The UTXO you want to validate is located in some other block, not on the si=
ngle block you are verifying.

Thus the non-existent fraud proof can only be validated by loading the bloc=
k of the UTXO purported to be spent, and every block between that and the c=
urrent block you are verifying, i.e. fullnode.
Either that or you trust that every peer you have is not omitting the proof=
.

Regards,
ZmnSCPxj