[bitcoin-dev] Two questions about segwit implementation

Johnson Lau jl2012 at xbt.hk
Sun May 26 17:24:13 UTC 2019


Empty scriptSig doesn’t 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’t 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 at gmail.com> wrote:
> 
> 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
> 
> Le 26/05/2019 à 18:28, Johnson Lau a écrit :
>> 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.
>> 
>> 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 “the witness for this input is empty”, and it is the “00”.
>> 
>>> On 27 May 2019, at 12:18 AM, Aymeric Vitte <vitteaymeric at gmail.com> wrote:
>>> 
>>> ……. 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
>> 




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