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From: Johnson Lau <jl2012@xbt.hk>
In-Reply-To: <a7742e04-22b1-2015-440a-ef81b5fbeed4@gmail.com>
Date: Mon, 27 May 2019 01:24:13 +0800
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References: <e537e781-e10f-7299-fddb-67fab74124c0@gmail.com>
	<6DFB6C65-D123-40FD-9CE3-49FFCA81EE46@xbt.hk>
	<e342cb8f-a909-a806-bd76-91580234cd7f@gmail.com>
	<253DB697-BECF-4D68-8A1E-1549B52D5AFA@xbt.hk>
	<a7742e04-22b1-2015-440a-ef81b5fbeed4@gmail.com>
To: Aymeric Vitte <vitteaymeric@gmail.com>
Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Two questions about segwit implementation
Precedence: list

Empty scriptSig doesn=E2=80=99t imply segwit input: if the previous =
scriptPubKey is OP_1 (which does not allow witness), it could still be =
spent with an empty scriptSig

Similarly, a scriptSig looking like a spend of P2SH-segwit doesn=E2=80=99t=
 imply segwit input: if the previous scriptPubKey is empty, it could be =
spent with a push of any non-zero value.

> On 27 May 2019, at 1:09 AM, Aymeric Vitte <vitteaymeric@gmail.com> =
wrote:
>=20
> I did not phrase correctly in fact, what I meant is: if the validator
> sees empty or witness script in scriptSig, then this is a segwit =
input,
> and doing this one by one the validator can associate the correct =
segwit
> data to the correct segwit input, so 00 does not look to be needed
>=20
> Le 26/05/2019 =C3=A0 18:28, Johnson Lau a =C3=A9crit :
>> This is not how it works. While the transaction creator may know =
which inputs are segwit, the validators have no way to tell until they =
look up the UTXO set.
>>=20
>> In a transaction, all information about an input the validators have =
is the 36-byte outpoint (txid + index). Just by looking at the outpoint, =
there is no way to tell whether it is segwit-enabled or not. So there =
needs to be a way to tell the validator that =E2=80=9Cthe witness for =
this input is empty=E2=80=9D, and it is the =E2=80=9C00=E2=80=9D.
>>=20
>>> On 27 May 2019, at 12:18 AM, Aymeric Vitte <vitteaymeric@gmail.com> =
wrote:
>>>=20
>>> =E2=80=A6=E2=80=A6. for the 00 number of witness
>>> data for non segwit inputs the one that is doing the transaction =
knows
>>> which inputs are segwit or not, then parsing the transaction you can
>>> associate the correct input to the correct witness data, without the
>>> need of 00, so I must be missing the use case
>>=20