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I understand that non-segwit nodes interpret segwit txes as Anyone-Can-Spend txes, and they cannot validate txes that spend segwit outputs.

But when an old style miner receives a segwit tx to mine he consideres it invalid, right? (That's because it appears to them as a strange transaction with no input and 1 output.) So, does it follow that segwit txes can only be mined in a block by upgraded miners?

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An old style miner would only receive a stripped transaction, i.e. a transaction from which the witness has been removed. They will consider a segwit transaction to be valid but non-standard because one element remains on the stack when resolving the redeem script.

If a miner included this transaction in their block, the block would appear valid to segwit-incapable nodes, but invalid to any segwit-capable nodes as any segwit input would be missing the required witness and thus would not be properly signed. This would lead to the block being rejected by 98% of the nodes on the network, and thus the segwit-incapable miner to lose their block reward as it would not become part of the heaviest blockchain. Segwit-incapable miners can however contribute blocks that only include non-segwit transactions that build on top of the chain that includes segwit-blocks.

This means that only a miner collecting transactions with a segwit-capable node is able to create a valid block that includes segwit transactions.

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  • So, old miners can't create a block with segwit txs but can reiceve this block from new miners (stripped), right? Thank you for your explaination
    – Zaitsev
    Oct 12, 2017 at 2:01
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    Exactly. And not only can't they produce SegWit blocks, they also won't, as SegWit transactions are non-standard.
    – Pieter Wuille
    Oct 12, 2017 at 4:48
  • they won't receive the block from other (upgrade) miner? so old miner refuse them?
    – Zaitsev
    Oct 12, 2017 at 15:09
  • The old nodes can redistribute the stripped block amongst each other, or any new nodes will create the stripped block when it is requested from any old node.
    – Murch ♦
    Oct 12, 2017 at 22:24
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    I meant that non-SegWit miners won't try to include SegWit txn, as they are non-standard to them, so they won't accept or relay them. They will however accept (stripped) SegWit txn if they occur in a block miner by a SegWit miner.
    – Pieter Wuille
    Dec 9, 2017 at 17:58

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