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From: Mark Friedenbach <mark@friedenbach.org>
In-Reply-To: <201710010247.42180.luke@dashjr.org>
Date: Sat, 30 Sep 2017 22:04:32 -0700
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Message-Id: <D811BF0D-8286-4A40-A443-09147E4EADDD@friedenbach.org>
References: <201710010113.30518.luke@dashjr.org>
	<5A220A8D-3A85-49D0-8DB2-6BDEC362EAEB@friedenbach.org>
	<201710010247.42180.luke@dashjr.org>
To: Luke Dashjr <luke@dashjr.org>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Version 1 witness programs (first draft)
Precedence: list

Clean stack should be eliminated for other possible future uses, the =
most obvious of which is recursive tail-call for general computation =
capability. I=E2=80=99m not arguing for that at this time, just arguing =
that we shouldn=E2=80=99t prematurely cut off an easy implementation of =
such should we want to. Clean stack must still exist as policy for =
future soft-fork safety, but being a consensus requirement was only to =
avoid witness malleability, which committing to the size of the witness =
also accomplishes.

Committing to the number of witness elements is fully sufficient, and =
using the number of elements avoids problems of not knowing the actual =
size in bytes at the time of signing, e.g. because the witness contains =
a merkle proof generated by another party from an unbalanced tree, and =
unbalanced trees are expected to be common (so that elements can be =
placed higher in the tree in accordance with their higher expected =
probability of usage). Other future extensions might also have =
variable-length proofs.

> On Sep 30, 2017, at 7:47 PM, Luke Dashjr <luke@dashjr.org> wrote:
>=20
> Should it perhaps commit to the length of the serialised witness data =
instead=20
> or additionally? Now that signatures are no longer variable-length, =
that'd be=20
> possible...
>=20
> As far as tail-call needs are concerned, CLEANSTACK wouldn't have been =
checked=20
> until AFTER the tail-call in the first draft. But I suppose =
eliminating it for=20
> other possible future purposes is still useful.
>=20
> Luke