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I've been reading a lot of docs on BTC transaction structure, and find it really confusing when it comes to SegWit story.

So, the very first thing is, if I've P2SH-P2WPKH address, does it mean that all outputs to this address (UTXOs) are by definition segwit UTXOs ?

In the docs (https://bitcoincore.org/en/segwit_wallet_dev/) it says:

Signature Generation and Verification for P2SH-P2WPKH

For spending of non-segwit UTXO, the signature generation algorithm is unchanged.

For spending of P2SH-P2WPKH:

  • The scriptSig MUST ONLY contain a push of the redeemScript

...

So, either all UTXOs for P2SH-P2WPKH address are segwit ones, or they are SegWit if SegWit transaction was made to this address (which does little sense actually for me, as then arises chicken-egg problem).

Why they wrote this "For spending of non-segwit UTXO, the signature generation algorithm is unchanged" part, what case it covers? If you've P2SH-P2WPKH address but you are taking some foreign UTXOs to spend(that were directed not to your address)?

Another not clear part are scripts for P2SH-P2WPKH address. So here is my example address: 3MxYPuvEcKDdaEG1xoGyddqir6C4b66MJi

And here is transaction that was made to it: https://blockchain.info/rawtx/6c2806587d5cdb3f0363e43f60b4a84baf37b3012fbde02f108f4eb1bdcede42

Based on it pubKeyScript of output is a914de523116b281c96ff1e204ea2ff435b75d48f0de87 which translates to:

OP_HASH160 de523116b281c96ff1e204ea2ff435b75d48f0de OP_EQUAL

So, de523116b281c96ff1e204ea2ff435b75d48f0de is hash160 of redeem script. The only redeem script that matches this hash I've found is following:

0 389f26d8d616cb96df1cfd6d9989248a30b933b9

Which is witness v0 program script(0 sha160sha256(pubkey), so it means I've to create SegWit transactions, right? I tried making P2SH ones and it says that I've non-nominal pushes in the redeem script, that's cause of "0" push there I believe.

I feel that doing something wrong in this part, and as a result signing either wrong thing or in a wrong way...

Very appreciate any explanations.

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if I've P2SH-P2WPKH address, does it mean that all outputs to this address (UTXOs) are by definition segwit UTXOs ?

Yes, Bitcoin sent to a P2SH-P2WPKH address will produce a segwit UTXO.

Why they wrote this "For spending of non-segwit UTXO, the signature generation algorithm is unchanged" part, what case it covers? If you've P2SH-P2WPKH address but you are taking some foreign UTXOs to spend(that were directed not to your address)?

I think you're right, it sounds like it's just saying if you want to send to a segwit address, you don't need to sign a non-segwit input any differently than before.

Spending P2SH(P2WPKH)

Here is a good reference example: BIP 141: P2SH(P2WPKH). In order to spend Bitcoin sent to your address 3MxYPuvEcKDdaEG1xoGyddqir6C4b66MJi, you have to provide the following:

witness:      <signature> <pubkey>
scriptSig:    <0 <20-byte-key-hash>>
              (0x160014{389f26d8d616cb96df1cfd6d9989248a30b933b9})

Perhaps you are forgetting the push ops 0x16 and/or 0x14 in scriptSig?

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    Yes, Bitcoin sent to a P2SH-P2WPKH address will produce a segwit UTXO. Technically, the utxo type is still p2sh. It's only revealed as segwit when it is spent. You could totally spend it as a p2sh if you can brute force a redeem script that satisfies the hash and is solvable by you, although that's rather impossible. Regardless, the output itself is p2sh.
    – Raghav Sood
    Commented Oct 11, 2018 at 15:15
  • @RaghavSood True, there would be no way to detect that it was a segwit UTXO, because it's the redeem script that is segwit.
    – JBaczuk
    Commented Oct 11, 2018 at 15:18
  • Thanks for the answer and comments, now it's more clear what is what :) I don't have 0x16, here is mine full script: "script": "0014389f26d8d616cb96df1cfd6d9989248a30b933b9", Is 0x16 required also?
    – Jaroslav Gorjatsev
    Commented Oct 11, 2018 at 16:25
  • Basically here is serialized tx I've 0100000000010142decebdb14e8f102fe0bd2f01b337af4ba8b4603fe463033fdb5c7d5806286c00000000160014389f26d8d616cb96df1cfd6d9989248a30b933b9ffffffff0210270000000000001976a914eeba84c4b62a6b6db8f1e0a1daffa0eccde33adf88acbe2100000000000017a914de523116b281c96ff1e204ea2ff435b75d48f0de8702483045022100fc44e10e8ab2e344c7e90f04f4f21a313850cd8eaebb0320edbc5e2d34c749b70220369da1b85125babd90a067d47e4034bc15ca1b9631ed5c8554148be3a4349d19012102ed991ef73f82488def8031fcccc4d51de840e7f17e3ad5d820781df0b4ee6e0600000000
    – Jaroslav Gorjatsev
    Commented Oct 11, 2018 at 16:25
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    You need the 0x16 because it tells the interpreter to push the following 22 bytes (redeem script) to the stack.
    – JBaczuk
    Commented Oct 11, 2018 at 16:30

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