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I understand that P2SH addresses are used to allow "segwit unaware" wallets to create outputs spendable by segwit wallets (please correct me if I got something wrong). For new clients I think I got it: they will check that the script cointaned in the witness matches the script hash and execute it.

What I do not understand is how this output is spendable by a segwit aware client to the eyes of an old client. Because it clearly doesn't look like an anyone can spend. How does it work?

Thanks.

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  • Why doesn't it look like an anyone can spend?
    – Osias Jota
    Mar 15, 2018 at 22:21
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    My statement came from the belief that the p2sh address was generated by encoding hash160 <witness program hash> op_equal. I found this: github.com/bitcoinbook/bitcoinbook/issues/440 . This means that decoding it makes it an anyone can spend for old clients right?
    – domegabri
    Mar 15, 2018 at 23:26
  • In addition to the previous comment: github.com/bitcoin/bips/blob/master/… . Segwit clients use a non-empty scriptSig when spending outputs received from old clients.
    – domegabri
    Mar 16, 2018 at 17:39

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There was a longer discussion thread in bitcointalk, I proide two references:

https://bitcointalk.org/index.php?topic=2333913.msg23758008#msg23758008

https://bitcointalk.org/index.php?topic=1682183.msg21389041#msg21389041

(Both threads are worthwhile to read in it's entirety). In short: before nodes talk to each other, they verify the capabilities. And a segwit node would reduce the information for a non-segwit node, so that he can understand the transaction details.

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