Trying to understand SegWit

Question

Can someone please help to confirm my understanding of SegWit ?

My understanding:

• Originally, signatures are included in each block alongside each transaction, these signatures can be read by nodes to prove validity of each transactions.
• With SegWit, signatures are omitted from the blocks, but instead recorded in a side-chain called Witness blockchain.
• Whoever wants to verify a transaction can refer to the corresponding signature from the Witness side-chain
• The total amount of data remains the same, but since we now have two separate chains running in parallel, the size of each block is effectively doubled.

Questions:

• Is my understanding correct please?
• Does it mean that for old-fashioned Nodes who don’t look at Witness side-chain, they can only verify transactions up to the Segwit implementation, and will have to reject new blocks because they won't find a valid signature within the new blocks?
• In that case, how is it a soft-fork? Since all nodes will be forced to upgrade in order to operate. (by here I am sure I am wrong somewhere..)
• Finally, do miners and nodes now keep both chains and when they broadcast the new block, they also broadcast the new Witness block?

Thank you very much.

Answer 1

Is my understanding correct please?

The only nit above is the total amount of data is increased a little - so little that it can be ignored.

Does it mean that for old-fashioned Nodes who don’t look at Witness side-chain, they can only verify transactions up to the SegWit implementation, and will have to reject new blocks because they won't find a valid signature within the new blocks?

Wrong. SegWit outputs look like AnyoneCanSpend outputs to legacy nodes (AnyoneCanSpend = P2SH address that can be spent without a signature - you just have to know the script to spend it.) While new nodes understand the "Witness Program" and verify witness signatures, legacy nodes think it's just a usual AnyoneCanSpend script. Hence, they accept transactions that spend Witness addresses.