From: Luke Dashjr <luke@dashjr.org>
To: Johnson Lau <jl2012@xbt.hk>
Date: Thu, 21 Sep 2017 16:33:16 +0000
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Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] cleanstack alt stack & softfork improvements
	(Was: Merkle branch verification & tail-call semantics for
	generalized MAST)
Precedence: list

On Thursday 21 September 2017 8:02:42 AM Johnson Lau wrote:
> I think it=E2=80=99s possible only if you spend more witness space to sto=
re the
> (pubkey, message) pairs, so that old clients could understand the
> aggregation produced by new clients. But this completely defeats the
> purpose of doing aggregation.

SigAgg is a softfork, so old clients *won't* understand it... am I missing=
=20
something?

=46or example, perhaps the lookup opcode could have a data payload itself (=
eg,=20
like pushdata opcodes do), and the script can be parsed independently from=
=20
execution to collect the applicable ones.

> > This is another approach, and one that seems like a good idea in genera=
l.
> > I'm not sure it actually needs to take more witness space - in theory,
> > such stack items could be implied if the Script engine is designed for
> > it upfront. Then it would behave as if it were non-verify, while
> > retaining backward compatibility.
>=20
> Sounds interesting but I don=E2=80=99t get it. For example, how could you=
 make a
> OP_MUL out of OP_NOP?

The same as your OP_MULVERIFY at the consensus level, except new clients wo=
uld=20
execute it as an OP_MUL, and inject pops/pushes when sending such a=20
transaction to older clients. The hash committed to for the script would=20
include the inferred values, but not the actual on-chain data. This would=20
probably need to be part of some kind of MAST-like softfork to be viable, a=
nd=20
maybe not even then.

Luke