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From my understanding, segwit blocks send transaction information without the witness, which includes the signature. I thought nodes needed the signature to verify the transactions?

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This is not correct.

All segwit blocks do differently is having the witnesses (which include the signature) in a separately-hashed area. They're still an integral part of the block, and full validation cannot proceed without the witness data. Removing or altering the witness data will invalidate a block.

What this does permit is stripping the witness data without affecting validity according to pre-segwit validation rules. This is the basis for using segwit as a blocksize increase (the old 1 MB block size limit thus only applies to the non-witness part, as old nodes don't see the witness data). However, nodes that implement the segwit rules need the full witness.

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  • Can you please explain what a non-segwit node sees when a segwit node notifies it of a new block, vs what a non-segwit node would see from a non-segwit node? I think I'm having trouble understanding how the witness data can be stripped and still be seen as valid – Jefumaru Jan 14 at 21:31
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    There is no difference. Non-witness nodes are never sent the witness data. Witness nodes strip it before sending it to them. – Pieter Wuille Jan 14 at 21:40
  • So they strip the portion of their block that contains witness data, correct? If this is true, then the non-witness node doesn't receive the witness data (which includes the signature). If so, how do non-witness nodes verify the block? – Jefumaru Jan 15 at 17:36
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    Yes. To them, the block is valid without witnesses. Every script that requires a witness looks like one that is always true to non-witness nodes. – Pieter Wuille Jan 15 at 17:49
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    They validate everything they care about. That does not include the validation of witness spends, as those look like anyonecanspend to them. But non-segwit scripts are still validated, PoW is validated, inflation is checked, UTXOs must exist, ... – Pieter Wuille Jan 15 at 21:38

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