Date: Sun, 24 Apr 2022 20:41:54 +0000
To: darosior <darosior@protonmail.com>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: Richard Myers <remyers@yakshaver.org>
Reply-To: Richard Myers <remyers@yakshaver.org>
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Subject: Re: [bitcoin-dev] ANYPREVOUT in place of CTV
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Hi darosior,

Thanks for sharing your thoughts on this.

> I would like to know people's sentiment about doing (a very slightly twea=
ked version of) BIP118 in place of
> (or before doing) BIP119.

Sounds good to me. Although from an activation perspective it may not be ei=
ther/or, both proposals do compete for scarce reviewer time so their orderi=
ng will necessarily be driven by reviewer's priorities. My priority is elto=
o which is why I focus on BIP-118.

> SIGHASH_ANYPREVOUTANYSCRIPT, if its "ANYONECANPAY" behaviour is made opti=
onal [0], can emulate CTV just fine.

For someone not as versed in CTV, why is it necessary that ANYONECANPAY be =
optional to emulate CTV? Is there a write-up that explains how APO-AS w/out=
 ANYONECANPAY approximates CTV?

In the case of eltoo commit txs, we use bring-your-own-fee (BYOF) to late-b=
ind fees; that means ANYONECANPAY will always be paired with APO-AS for elt=
oo. Settlement txs in eltoo use just APO and do not necessarily need to be =
paired with ANYONECANPAY.

I would guess making ANYONECANPAY the default for APO-AS was a way to squee=
ze in one more sighash flag. Perhaps there's another way to do it?

Including SIGHASH_GROUP with APO for eltoo is also tempting. Specifically s=
o the counter-party who commits a settlement tx can use for fees their sett=
led to_self balance. How to rejigger the sighash flags to accommodate both =
APO and GROUP may be worth some discussion.

The BIP-118 proposal will certainly benefit from having input from reviewer=
s looking at other protocols than eltoo.

  -- Richard