[bitcoin-dev] Two questions about segwit implementation

Thomas Kerin me at thomaskerin.io
Sun May 26 19:34:55 UTC 2019

The scriptSig when evaluated populates the stack so opcodes can operate
on them. A witness is essentially a list of data elements, quite similar
to the script stack (the witness is passed in as the script stack in fact)

OP_0 when evaluated pushes a _zero length_ value onto the stack, hence
the 00 (the varlen) in the witness serialization. OP_1 (51 in decimal)
pushes 0x01 to the stack, so when serialized would be 0101.

It may help to consider the entire witness structure as
vector<vector<data element>> and it's length must equal the number of
inputs - so a non-segwit input must have a zero sized witness.

On 5/26/19 12:56 AM, Aymeric Vitte via bitcoin-dev wrote:
> 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
> 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
> 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
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