Hi darosior,

It's interesting to note that APOAS|SINGLE (with the ANYONECANPAY behaviour
and without covering the spent input index) has some interesting uses for
cases where the covenant only needs to restrict a single output (so useful
for e.g. vaults or spacechains, but not for batch channels or congestion
control).

For example in the vault use-case, it makes it possible to bump fees on the
unvault tx by adding more inputs and a change output, as well as unvault
multiple vaulted outputs in a single transaction.

For spacechains, it makes it possible to add the spaceblock hash OP_RETURN
and pay fees directly in the tx chain, instead of having to use an
additional tx to prepare an output that gets spent in the tx chain  (see
the diagram in [0]).

> via `sha_sequences` and maybe also `sha_amounts`

CTV does not commit to the input amounts. This has some practical
implications:

1. If it is committed, sending an even slightly incorrect amount will make
the covenant-encumbered spend path unusable.

With CTV, sending a slightly lower amount results in slightly lower fees,
while any extra gets spent/burned on fees. The covenant spend path only
becomes unusable if the amount is too low to cover for the outputs (+relay
fee for it to also be standard).

2. The ability to allow for additional inputs with unknown amounts makes it
possible to fee-bump the covenant spending transaction (with whole utxos
and no change). You can have one tapleaf for spending the covenant output
alone, and another one for attaching an extra fee input to it.

This also makes it possible to resolve the under-payment issue described in
(1), by adding an input that covers the original intended amount.

So my suggestion would be to either not cover `sha_amounts` in the msg
hash, or to make it optional behind a flag.

shesek

[0] https://github.com/fiatjaf/simple-ctv-spacechain

On Fri, Apr 22, 2022 at 2:23 PM darosior via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> I would like to know people's sentiment about doing (a very slightly
> tweaked version of) BIP118 in place of
> (or before doing) BIP119.
>
> SIGHASH_ANYPREVOUT and its precedent iterations have been discussed for
> over 6 years. It presents proven and
> implemented usecases, that are demanded and (please someone correct me if
> i'm wrong) more widely accepted than
> CTV's.
>
> SIGHASH_ANYPREVOUTANYSCRIPT, if its "ANYONECANPAY" behaviour is made
> optional [0], can emulate CTV just fine.
> Sure then you can't have bare or Segwit v0 CTV, and it's a bit more
> expensive to use. But we can consider CTV
> an optimization of APO-AS covenants.
>
> CTV advocates have been presenting vaults as the flagship usecase.
> Although as someone who've been trying to
> implement practical vaults for the past 2 years i doubt CTV is necessary
> nor sufficient for this (but still
> useful!), using APO-AS covers it. And it's not a couple dozen more virtual
> bytes that are going to matter for
> a potential vault user.
>
> If after some time all of us who are currently dubious about CTV's stated
> usecases are proven wrong by onchain
> usage of a less efficient construction to achieve the same goal, we could
> roll-out CTV as an optimization.  In
> the meantime others will have been able to deploy new applications
> leveraging ANYPREVOUT (Eltoo, blind
> statechains, etc..[1]).
>
>
> Given the interest in, and demand for, both simple covenants and better
> offchain protocols it seems to me that
> BIP118 is a soft fork candidate that could benefit more (if not most of)
> Bitcoin users.
> Actually i'd also be interested in knowing if people would oppose the
> APO-AS part of BIP118, since it enables
> CTV's features, for the same reason they'd oppose BIP119.
>
>
> [0] That is, to not commit to the other inputs of the transaction (via
> `sha_sequences` and maybe also
> `sha_amounts`). Cf
> https://github.com/bitcoin/bips/blob/master/bip-0118.mediawiki#signature-message
> .
>
> [1] https://anyprevout.xyz/ "Use Cases" section
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