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From: Matt Corallo <lf-lists@mattcorallo.com>
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Date: Tue, 4 Aug 2020 09:10:02 -0400
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To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP 118 and SIGHASH_ANYPREVOUT
Precedence: list

Hmm, apologies that little context was provided - this was meant in the cont=
ext of the current crop of relay-based attacks that have been discovered. As=
 we learned in those contexts, =E2=80=9Cjust handle it when it confirms=E2=80=
=9D doesn=E2=80=99t provide the types of guarantees we were hoping for as pl=
acing commitment transactions in mempools can be used to prevent honest node=
s from broadcasting the latest state. This implies that HTLC security may be=
 at risk.

> On Aug 4, 2020, at 00:23, ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:
>=20
> =EF=BB=BFGood morning Matt,
>=20
>> While I admit I haven=E2=80=99t analyzed the feasibility, I want to throw=
 one additional design consideration into the ring.
>>=20
>> Namely, it would ideally be trivial, at the p2p protocol layer, to relay a=
 transaction to a full node without knowing exactly which input transaction t=
hat full node has in its mempool/active chain. This is at least potentially i=
mportant for systems like lighting where you do not know which counterparty c=
ommitment transaction(s) are in a random node=E2=80=99s mempool and you shou=
ld be able to describe to that node that you are spending then nonetheless.
>>=20
>> This is (obviously) an incredibly nontrivial problem both in p2p protocol=
 complexity and mempool optimization, but it may leave SIGHASH_NOINPUT rathe=
r useless for lighting without it.
>>=20
>> The least we could do is think about the consensus design in that context=
, even if we have to provide an external overlay relay network in order to m=
ake lighting transactions relay properly (presumably with miners running suc=
h software).
>=20
> Ah, right.
>=20
> A feasible attack, without the above, would be to connect to the fullnode o=
f the victim, and connect to miners separately.
> Then you broadcast to the victim one of the old txes, call it tx A, but yo=
u broadcast to the miners a *different* old tx, call it B.
> The victim reacts only to tA, but does not react to B since it does not se=
e B in the mempool.
>=20
> On the other hand --- what the victim needs to react to is *onchain* confi=
rmed transactions.
> So I think all the victim needs to do, in a Lightning universe utilizing p=
rimarily `SIGHASH_NOINPUT`-based mechanisms, is to monitor onchain events an=
d ignore mempool events.
>=20
> So if we give fairly long timeouts for our mechanisms, it should be enough=
, I think, since once a transaction is confirmed its txid does not malleate w=
ithout a reorg and a `SIGHASH_NOINPUT` signature can then be "locked" to tha=
t txid, unless a reorg unconfirms the transaction.
> We only need to be aware of deep reorgs and re-broadcast with a malleated p=
revout until the tx being spent is deeply confirmed.
>=20
> In addition, we want to implement scorch-the-earth, keep-bumping-the-fee s=
trategies anyway, so we would keep rebroadcasting new versions of the spendi=
ng transaction, and spending from a transaction that is confirmed.
>=20
> Or are there other attack vectors you can see that I do not?
> I think this is fixed by looking at the blockchain.
>=20
> Regards,
> ZmnSCPxj