Subject: Re: [bitcoin-dev] bitcoin scripting and lisp

From: Bram Cohen <[email-masked]>
Date: Tue, 15 Mar 2022 23:40:51 -0700

On Wed, Mar 9, 2022 at 6:30 AM ZmnSCPxj <[email-masked]> wrote:

> I am pointing out that:
>
> * We want to save bytes by having multiple inputs of a transaction use the
> same single signature (i.e. sigagg).
>
> is not much different from:
>
> * We want to save bytes by having multiple inputs of a transaction use the
> same `scriptPubKey` template.
>

Fair point. In the past Bitcoin has been resistant to such things because
for example reusing pubkeys can save you from having to separately pay for
the reveals of all of them but letting people get credit for that
incentivizes key reuse which isn't such a great thing.

>
> > > For example you might have multiple HTLCs, with mostly the same code
> except for details like who the acceptor and offerrer are, exact hash, and
> timelock, and you could claim multiple HTLCs in a single tx and feed the
> details separately but the code for the HTLC is common to all of the HTLCs.
> > > You do not even need to come from the same protocol if multiple
> protocols use the same code for implementing HTLC.
> >
> > HTLCs, at least in Chia, have embarrassingly little code in them. Like,
> so little that there's almost nothing to compress.
>
> In Bitcoin at least an HTLC has, if you remove the `OP_PUSH`es, by my
> count, 13 bytes.
> If you have a bunch of HTLCs you want to claim, you can reduce your
> witness data by 13 bytes minus whatever number of bytes you need to
> indicate this.
> That amounts to about 3 vbytes per HTLC, which can be significant enough
> to be worth it (consider that Taproot moving away from encoded signatures
> saves only 9 weight units per signature, i.e. about 2 vbytes).
>

Oh I see. That's already extremely small overhead. When you start
optimizing at that level you wind up doing things like pulling all the
HTLCs into the same block to take the overhead of pulling in the template
only once.

>
> Do note that PTLCs remain more space-efficient though, so forget about
> HTLCs and just use PTLCs.
>

It makes a lot of sense to make a payment channel system using PTLCs and
eltoo right off the bat but then you wind up rewriting everything from
scratch.

> > > This does not apply to current Bitcoin since we no longer accept a
> SCRIPT from the spender, we now have a witness stack.
> >
> > My mental model of Bitcoin is to pretend that segwit was always there
> and the separation of different sections of data is a semantic quibble.
>
> This is not a semantic quibble --- `witness` contains only the equivalent
> of `OP_PUSH`es, while `scriptSig` can in theory contain non-`OP_PUSH`
> opcodes.
> xref. `1 RETURN`.
>

It's very normal when you're using lisp for snippets of code to be passed
in as data and then verified and executed. That's enabled by the extreme
adherence to no side effects.

> This makes me kinda wary of using such covenant features at all, and if
> stuff like `SIGHASH_ANYPREVOUT` or `OP_CHECKTEMPLATEVERIFY` are not added
> but must be reimplemented via a covenant feature, I would be saddened, as I
> now have to contend with the complexity of covenant features and carefully
> check that `SIGHASH_ANYPREVOUT`/`OP_CHECKTEMPLATEVERIFY` were implemented
> correctly.
>

Even the 'standard format' transaction which supports taproot and graftroot
is implemented in CLVM. The benefit of this approach is that new
functionality can be implemented and deployed immediately rather than
having to painstakingly go through a soft fork deployment for each thing.