Re: [bitcoin-dev] Continuing the discussion about noinput / anyprevout

[Anthony Towns <aj@erisian.com.au>]

On Mon, Sep 30, 2019 at 03:23:56PM +0200, Christian Decker via bitcoin-dev wrote:
> With the recently renewed interest in eltoo, a proof-of-concept implementation
> [1], and the discussions regarding clean abstractions for off-chain protocols
> [2,3], I thought it might be time to revisit the `sighash_noinput` proposal
> (BIP-118 [4]), and AJ's `bip-anyprevout` proposal [5].

Hey Christian, thanks for the write up!

> ## Open questions
> The questions that remain to be addressed are the following:
> 1.  General agreement on the usefulness of noinput / anyprevoutanyscript /
>     anyprevout[?]
> 2.  Is there strong support or opposition to the chaperone signatures[?]
> 3.  The same for output tagging / explicit opt-in[?]
> 4.  Shall we merge BIP-118 and bip-anyprevout. This would likely reduce the
>     confusion and make for simpler discussions in the end.

I think there's an important open question you missed from this list:
(1.5) do we really understand what the dangers of noinput/anyprevout-style
constructions actually are?

My impression on the first 3.5 q's is: (1) yes, (1.5) not really,
(2) weak opposition for requiring chaperone sigs, (3) mixed (weak)
support/opposition for output tagging.

My thinking at the moment (subject to change!) is:

 * anyprevout signatures make the address you're signing for less safe,
   which may cause you to lose funds when additional coins are sent to
   the same address; this can be avoided if handled with care (or if you
   don't care about losing funds in the event of address reuse)

 * being able to guarantee that an address can never be signed for with
   an anyprevout signature is therefore valuable; so having it be opt-in
   at the tapscript level, rather than a sighash flag available for
   key-path spends is valuable (I call this "opt-in", but it's hidden
   until use via taproot rather than "explicit" as output tagging
   would be)

 * receiving funds spent via an anyprevout signature does not involve any
   qualitatively new double-spending/malleability risks.
   
   (eltoo is unavoidably malleable if there are multiple update
   transactions (and chaperone signatures aren't used or are used with
   well known keys), but while it is better to avoid this where possible,
   it's something that's already easily dealt with simply by waiting
   for confirmations, and whether a transaction is malleable is always
   under the control of the sender not the receiver)

 * as such, output tagging is also unnecessary, and there is also no
   need for users to mark anyprevout spends as "tainted" in order to
   wait for more confirmations than normal before considering those funds
   "safe"

I think it might be good to have a public testnet (based on Richard Myers
et al's signet2 work?) where we have some fake exchanges/merchants/etc
and scheduled reorgs, and demo every weird noinput/anyprevout case anyone
can think of, and just work out if we need any extra code/tagging/whatever
to keep those fake exchanges/merchants from losing money (and write up
the weird cases we've found in a wiki or a paper so people can easily
tell if we missed something obvious).

Cheers,
aj