From: Johnson Lau <jl2012@xbt.hk>
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Date: Sun, 26 May 2019 22:33:06 +0800
References: <e537e781-e10f-7299-fddb-67fab74124c0@gmail.com>
To: Aymeric Vitte <vitteaymeric@gmail.com>,
	bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
In-Reply-To: <e537e781-e10f-7299-fddb-67fab74124c0@gmail.com>
Message-Id: <6DFB6C65-D123-40FD-9CE3-49FFCA81EE46@xbt.hk>
Subject: Re: [bitcoin-dev] Two questions about segwit implementation
Precedence: list


> On 26 May 2019, at 7:56 AM, Aymeric Vitte via bitcoin-dev =
<bitcoin-dev@lists.linuxfoundation.org> wrote:
>=20
> I realized recently that my segwit implementation was not correct,
> basically some time ago, wrongly reading the specs (and misleaded by
> what follows), I thought that scriptsig would go into witness data as =
it
> was, but that's not the case, op_pushdata is replaced by varlen
>=20

Witness is not script. There is no op_pushdata or any other opcodes.

Witness is a stack. For each input, the witness starts with a =
CCompactSize for the number of stack elements for this input. Each stack =
element in turns starts with a CCompactSize for the size of this =
element, followed by the actual data


> Now reading correctly the specs, they seem to be not totally correct,
> then the first question is: why OP_0 is 00 in witness data and not =
0100?
> Does this apply to other op_codes? This does not look logical at all
>=20

A =E2=80=9C00=E2=80=9D element means the size of this element is zero. =
Since it=E2=80=99s zero size, no data is followed. This will create an =
empty element on the stack. It=E2=80=99s effectively same as OP_0 =
(Again, witness is not script)

A =E2=80=9C0100=E2=80=9D element means the element size is one, and the =
data for this element is =E2=80=9C00=E2=80=9D. So it will leave an =
1-byte element on the stack.


> The second question is: why for non segwit inputs there is a 00 length
> in segwit data, what is the rational for that? It should just be =
nothing
> since you don't need this to reconciliate things

The =E2=80=9C00=E2=80=9D here means "this input has no witness stack =
element=E2=80=9D. You need this even for non segwit inputs, because =
there is no way to tell whether an input is segwit-enabled or not, until =
you look up the UTXO, which might not be always available. Transaction =
serialization couldn=E2=80=99t rely on contextual information.

However, if all inputs have no stack element, the spec requires you to =
always use the non-segwit serialization.

>=20
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