The key to understanding how it works is to stop thinking in terms of a block size limit, but rather a block weight limit. 1 byte of witness data counts as 1 weight, the rest counts for 4 weight. A block must be less than 4 million weight. There's no separate limits at all, so any saving in the witness space (e.g. through signature aggregation) is useful for both witness/non-witness data.

-Ryan

> -------- Original Message --------
> Subject: [bitcoin-dev] Single signature for all transactions in a block?
> Local Time: December 31, 2017 5:39 PM
> UTC Time: December 31, 2017 11:39 PM
> From: bitcoin-dev@lists.linuxfoundation.org
> To: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA512
>
> I had a question relating to scaling and privacy enhancements.
> I believe that segwit combined with aggregated signatures
> and coinjoin can potentially achieve such. The idea is to
> use aggregated signatures in conjunction with coinjoin. So
> that all inputs of a coinjoin transaction would have a single
> signature vastly decreasing size while having privacy at the
> same time. If majority of transactions in a block did this I
> assume that significant more transactions could be fit into a
> block? However the question I have, with the extra blockspace
> made possible by segwit, is this extra blockspace limited to only
> witness data or can it be used for transaction data such as the
> scenario I have described here?
>
> ---------------------------------------------------------------
>
> Cannon
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> Email: cannon@cannon-ciota.info
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