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From: Ruben Somsen <rsomsen@gmail.com>
Date: Mon, 9 Sep 2019 08:53:28 +0200
Message-ID: <CAPv7TjYE1rp2EEo247fKmmN=q9QBqCHPBy56xvBymOv418LFDw@mail.gmail.com>
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] PoW fraud proofs without a soft fork
Precedence: list

Hi ZmnSCPxj,

Thank you for your comments. You raise an important point that I should cla=
rify.

>1.  In event of a sybil attack, a fullnode will stall and think the blockc=
hain has no more miners.

You can still attack the full node by feeding it a minority PoW chain,
then it won't stall.

>2.  In event of a sybil attack, an SPV, even using this style, will follow=
 the false blockchain.

Correct, but this false blockchain does need to have valid PoW.

So in both cases valid PoW is required to fool nodes. The one
difference is that for a full node, the blocks themselves also need to
be valid (except for the fact that they are in a minority chain), but
the end result is still that a victim can be successfully double spent
and lose money.

I hope this clarifies why I consider the security for these two
situations to be roughly equivalent. In either situation, victims can
be fooled into accepting invalid payments.

Cheers,
Ruben

On Mon, Sep 9, 2019 at 6:14 AM ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:
>
> Good morning Ruben,
>
>
> >     One might intuitively feel that the lack of a commitment is unsafe,
> >     but there seems to be no impact on security (only bandwidth). The o=
nly
> >     way you can be fooled is if all peers lie to you (Sybil), causing y=
ou
> >     to follow a malicious minority chain. But even full nodes (or the
> >     committed version of PoW fraud proofs) can be fooled in this way if
> >     they are denied access to the valid most PoW chain. If there are
> >     additional security concerns I overlooked, I=E2=80=99d love to hear=
 them.
>
>
> I think it would be better to more precisely say that:
>
> 1.  In event of a sybil attack, a fullnode will stall and think the block=
chain has no more miners.
> 2.  In event of a sybil attack, an SPV, even using this style, will follo=
w the false blockchain.
>
> This has some differences when considering automated systems.
>
> Onchain automated payment processing systems, which use a fullnode, will =
refuse to acknowledge any incoming payments.
> This will lead to noisy complaints from clients of the automated payment =
processor, but this is a good thing since it warns the automated payment pr=
ocessor of the possibility of this attack occurring on them.
> The use of a timeout wherein if the fullnode is unable to see a new block=
 for, say, 6 hours, could be done, to warn higher-layer management systems =
to pay attention.
> While it is sometimes the case that the real network will be unable to fi=
nd a new block for hours at a time, this warning can be used to confirm if =
such an event is occurring, rather than a sybil attack targeting that fulln=
ode.
>
> On the other hand, such a payment processing system, which uses an SPV wi=
th PoW fraud proofs, will be able to at least see incoming payments, and co=
ntinue to release product in exchange for payment.
> Yet this is precisely a point of attack, where the automated payment proc=
essing system is sybilled and then false payments are given to the payment =
processor on the attack chain, which are double-spent on the global consens=
us chain.
> And the automated system may very well not be able to notice this.
>
> Regards,
> ZmnSCPxj