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From: Gregory Maxwell <greg@xiph.org>
Date: Mon, 20 Nov 2017 18:59:34 +0000
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To: Praveen Baratam <praveen.baratam@gmail.com>, 
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Why SegWit Anyway?
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On Mon, Nov 20, 2017 at 5:24 PM, Praveen Baratam via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> Bitcoin Noob here. Please forgive my ignorance.
>
> From what I understand, in SegWit, the transaction needs to be serialized
> into a data structure that is different from the current one where
> signatures are separated from the rest of the transaction data.
>
> Why change the format at all? Why cant we just compute the Transaction ID
> the same way the hash for signing the transaction is computed?

That is effectively what segwit does, upto engineering minutia and
compatibility details.

Segwit does not serialize transactions in to a data structure where
signatures are separated from the rest of the transaction data; this
is a misunderstanding.  The "segregated" refers to them being excluded
from the TXID.   The serialization of segwit on the p2p network in
transactions and in blocks encodes the witness field inside the
transactions, immediately prior to the nlocktime field.