From: Rusty Russell <rusty@rustcorp.com.au>
To: Anthony Towns <aj@erisian.com.au>, bitcoin-dev@lists.linuxfoundation.org
In-Reply-To: <87k1gubdjm.fsf@rustcorp.com.au>
References: <20190313014143.ifffshwdux2jt7w5@erisian.com.au>
	<87k1gubdjm.fsf@rustcorp.com.au>
Date: Wed, 20 Mar 2019 14:03:55 +1030
Message-ID: <87woku9q3g.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Cc: lightning-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] [Lightning-dev] More thoughts on NOINPUT safety
Precedence: list

Sorry AJ, my prior email was not constructive :(

I consider the "my software reused my keys" the most reasonable attack
scenario, though still small compared to other lightning attack surfaces.

But I understand the general wariness of third-parties reusing
SIGHASH_NOINPUT signatures.

Since "must have a non-SIGHASH_NOINPUT" rule addresses the first reuse
scenario (as well as the second), I'd be content with that proposal.
Future segwit versions may choose to relax it.[1]

Cheers,
Rusty.
[1] Must be consensus, not standardness; my prev suggestion was bogus.

Rusty Russell <rusty@rustcorp.com.au> writes:
> 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.