Date: Tue, 04 Aug 2020 04:23:03 +0000
To: "lf-lists@mattcorallo.com" <lf-lists@mattcorallo.com>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Subject: Re: [bitcoin-dev] BIP 118 and SIGHASH_ANYPREVOUT
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Good morning Matt,

> While I admit I haven=E2=80=99t analyzed the feasibility, I want to throw=
 one additional design consideration into the ring.
>
> Namely, it would ideally be trivial, at the p2p protocol layer, to relay =
a transaction to a full node without knowing exactly which input transactio=
n that full node has in its mempool/active chain. This is at least potentia=
lly important for systems like lighting where you do not know which counter=
party commitment transaction(s) are in a random node=E2=80=99s mempool and =
you should be able to describe to that node that you are spending then none=
theless.
>
> This is (obviously) an incredibly nontrivial problem both in p2p protocol=
 complexity and mempool optimization, but it may leave SIGHASH_NOINPUT rath=
er useless for lighting without it.
>
> 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 =
make lighting transactions relay properly (presumably with miners running s=
uch software).

Ah, right.

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 you=
 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 see=
 B in the mempool.

On the other hand --- what the victim needs to react to is *onchain* confir=
med transactions.
So I think all the victim needs to do, in a Lightning universe utilizing pr=
imarily `SIGHASH_NOINPUT`-based mechanisms, is to monitor onchain events an=
d ignore mempool events.

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 =
without a reorg and a `SIGHASH_NOINPUT` signature can then be "locked" to t=
hat 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.

In addition, we want to implement scorch-the-earth, keep-bumping-the-fee st=
rategies anyway, so we would keep rebroadcasting new versions of the spendi=
ng transaction, and spending from a transaction that is confirmed.

Or are there other attack vectors you can see that I do not?
I think this is fixed by looking at the blockchain.

Regards,
ZmnSCPxj