<p>The way the BIP141 segwit rules consider transactions is simply that <em>every</em> transaction input (even non-segwit ones) have a witness stack. Non-segwit inputs are required to have an empty witness stack however.</p>
<p>If every transaction input has an empty witness stack, the old pre-segwit transaction serialization is used for the transaction. If at least one inout has a non-empty witness stack, the BIP144 transaction serialization format (including marker &amp; flag) is used. The non-segwit inputs then simply have an empty witness.</p>
<p>Now, there is a consensus rule that says that unless a transaction input is a segwit spend, its witness stack <em>must</em> be empty. An input is a segwit spend if the UTXO it spends is a witness program (OP_n + push of 2-40 bytes), or if it is a P2SH UTXO, whose redeemScript is such a witness program. <em>If</em> an input is a segwit spend, the scriptSig <em>must</em> be exactly empty or exactly the P2SH redeemScript and nothing else. If an input is <em>not</em> a segwit spend, the witness stack must be empty. But all of this is rules that govern whether a transaction is valid w.r.t. its witnesses and scriptSigs; it is unrelated to whether the transaction <em>has</em> witnesses.</p>

CC BY-SA 4.0
2022-11-25T19:15:51.080Z
Thanks for getting me going... you're an invaluable asset for the Bitcoin industry!
RooSoft
CC BY-SA 4.0
2022-11-25T19:06:20.590Z
2022-11-25T19:12:21.630Z
2022-11-25T19:12:21.630Z
Pieter Wuille
<p>This is a discussion that has already been had <a href="https://bitcoin.stackexchange.com/questions/68413/how-do-i-determine-whether-an-input-or-output-is-segwit">here</a>, but bear with me.</p>
<p>We're gonna be talking about tx <code>faae7e55db14a32e083cbf6a91db8a5ab6a3b05e050d9cefdec80b47f966848e</code></p>
<ul>
<li>all I got is the encoded transaction</li>
<li>it has been marked as segwit (marker + flag)</li>
<li>tx has 3 inputs, only the last one is segwit</li>
<li>the segwit input has a script sig: <code>1600140c6259927541c4f8e88fc1398691e2661d15591a</code></li>
<li>got to decode the witnesses section correctly, implied there is <code>only one</code></li>
</ul>
<p>In the previous discussion, they mentioned that a segwit input is easily spotted as it has an empty script sig.</p>
<p>The segwit input we have here <code>has a script sig</code>... I know it's segwit cause I looked in a blockchain explorer. How can my software programatically tell it's segwit just by looking at the transaction's data?</p>

CC BY-SA 4.0
2022-11-25T18:40:45.017Z
Native segwit inputs have an empty scriptSig; P2SH-wrapped segwit inputs have a P2SH redeemScript in the scriptSig.
Pieter Wuille
CC BY-SA 4.0
2022-11-25T18:47:29.350Z
The way to determine whether an input has a witness is simply by seeing if it had witness data. I realize that's tautological, but there is nothing else to it: the BIP144 transaction serialization simply encodes the witness data for every input.
Pieter Wuille
CC BY-SA 4.0
2022-11-25T19:02:45.490Z
Ohhh I see, the witness section starts by 0, 0, 2... Thanks again @PieterWuille
RooSoft
<transactions><segregated-witness><transaction-input><witness-data>
How do I determine whether an input or output is segwit? - revisited
RooSoft
# How do I determine whether an input or output is segwit? - revisited

(Asked by: username_0 on 2022-11-25)

This is a discussion that has already been had [here](https://bitcoin.stackexchange.com/questions/68413/how-do-i-determine-whether-an-input-or-output-is-segwit), but bear with me.

We're gonna be talking about tx `faae7e55db14a32e083cbf6a91db8a5ab6a3b05e050d9cefdec80b47f966848e`

* all I got is the encoded transaction
* it has been marked as segwit (marker + flag)
* tx has 3 inputs, only the last one is segwit
* the segwit input has a script sig: `1600140c6259927541c4f8e88fc1398691e2661d15591a`
* got to decode the witnesses section correctly, implied there is `only one`

In the previous discussion, they mentioned that a segwit input is easily spotted as it has an empty script sig.

The segwit input we have here `has a script sig`... I know it's segwit cause I looked in a blockchain explorer. How can my software programatically tell it's segwit just by looking at the transaction's data?

**Tags:** transactions, segregated-witness, transaction-input, witness-data

> **Comments:**
> **username_1**: Native segwit inputs have an empty scriptSig; P2SH-wrapped segwit inputs have a P2SH redeemScript in the scriptSig.  
> **username_1**: The way to determine whether an input has a witness is simply by seeing if it had witness data. I realize that's tautological, but there is nothing else to it: the BIP144 transaction serialization simply encodes the witness data for every input.  
> **username_0**: Ohhh I see, the witness section starts by 0, 0, 2... Thanks again @username_1  

---

## 1 answers

### Answer by username_1 on 2022-11-25. Score: 5 | Accepted answer

The way the BIP141 segwit rules consider transactions is simply that *every* transaction input (even non-segwit ones) have a witness stack. Non-segwit inputs are required to have an empty witness stack however.

If every transaction input has an empty witness stack, the old pre-segwit transaction serialization is used for the transaction. If at least one inout has a non-empty witness stack, the BIP144 transaction serialization format (including marker & flag) is used. The non-segwit inputs then simply have an empty witness.

Now, there is a consensus rule that says that unless a transaction input is a segwit spend, its witness stack *must* be empty. An input is a segwit spend if the UTXO it spends is a witness program (OP\_n + push of 2-40 bytes), or if it is a P2SH UTXO, whose redeemScript is such a witness program. *If* an input is a segwit spend, the scriptSig *must* be exactly empty or exactly the P2SH redeemScript and nothing else. If an input is *not* a segwit spend, the witness stack must be empty. But all of this is rules that govern whether a transaction is valid w.r.t. its witnesses and scriptSigs; it is unrelated to whether the transaction *has* witnesses.

> **Comments:**
> **username_0**: Thanks for getting me going... you're an invaluable asset for the Bitcoin industry!  

---

