Re: [bitcoin-dev] [Lightning-dev] More thoughts on NOINPUT safety

[Rusty Russell <rusty@rustcorp.com.au>]

Anthony Towns <aj@erisian.com.au> writes:
> If you publish to the blockchain:
...
> 4 can be dropped, state 5 and finish can be altered). Since the CSV delay
> is chosen by the participants, the above is still a possible scenario
> in eltoo, though, and it means there's some risk for someone accepting
> bitcoins that result from a non-cooperative close of an eltoo channel.

AJ, this was a meandering random walk which shed very little light.

I don't find the differentiation between malicious and non-malicious
double-spends convincing.  Even if you trust A, you already have to
worry about person-who-sent-the-coins-to-A.  This expands that set to be
"miner who mined coins sent-to-A", but it's very hard to see what
difference that makes to how you'd handle coins from A.

> Beyond that, I think NOINPUT has two fundamental ways to cause problems
> for the people doing NOINPUT sigs:
>
>  1) your signature gets applied to a unexpectedly different
>     script, perhaps making it look like you've being dealing
>     with some blacklisted entity. OP_MASK and similar solves
>     this.

... followed by two paragraphs describing how it's not a "fundamental
way to cause problems" that you (or I) can see.

> For the second case, that seems a little more concerning. The nightmare
> scenario is maybe something like:
>
>  * naive users do silly things with NOINPUT signatures, and end up
>    losing funds due to replays like the above

As we've never seen with SIGHASH_NONE?

>  * initial source of funds was some major exchange, who decide it's
>    cheaper to refund the lost funds than deal with the customer complaints
>
>  * the lost funds end up costing enough that major exchanges just outright
>    ban sending funds to any address capable of NOINPUT, which also bans
>    all taproot/schnorr addresses

I don't find this remotely credible.

> FWIW, I don't have a strong opinion here yet, but:
>
>  - I'm still inclined to err on the side of putting more safety
>    measures in for NOINPUT, rather than fewer

In theory, sure.  But not feel-good and complex "safety measures" which
don't actually help in practical failure scenarios.

>  - the "must have a sig that commits to the input tx" seems like it
>    should be pretty safe, not too expensive, and keeps taproot's privacy
>    benefits in the cases where you end up needing to use NOINPUT

If this is considered necessary, can it be a standardness rule rather
than consensus?

Thanks,
Rusty.