But generally before one signs a transaction one does not know the
signature size (which may be variable). One can only estimate the maximum
size.

On Fri, Sep 22, 2017 at 6:11 PM, Mark Friedenbach <mark@friedenbach.org>
wrote:

>
> On Sep 22, 2017, at 1:32 PM, Sergio Demian Lerner <
> sergio.d.lerner@gmail.com> wrote:
>
>
>>
>> There are other solutions to this problem that could have been taken
>> instead, such as committing to the number of items or maximum size of
>> the stack as part of the sighash data, but cleanstack was the approach
>> taken.
>
>
> The lack of signed maximum segwit stack size was one of the objections to
> segwit I presented last year. This together with the unlimited segwit stack
> size.
>
> However, committing to the maximum stack size (in bytes) for an input is
> tricky. The only place where this could be packed is in sequence_no, with a
> soft-fork. E.g. when transaction version is 2 and and only when lock_time
> is zero.
>
> For transactions with locktime >0, we could soft-fork so transactions add
> a last zero-satoshi output whose scriptPub contains OP_RETURN and followed
> by N VarInts, containing the maximum stack size of each input.
> Normally, for a 400 byte, 2-input transaction, this will add 11 bytes, or
> a 2.5% overhead.
>
>
> There’s no need to put it in the transaction itself. You put it in the
> witness and it is either committed to as part of the witness (in which case
> it has to hold for all possible spend paths), or at spend time by including
> it in the data signed by CHECKSIG.
>
>