Sign SegWit transaction without downloading blocks

[liorwavebl]

I managed to sign a SegWit transaction. I have two servers- one with access to the blockchain and the second has no access at all. The second server can get requests from the first server.
Can I sign a transaction from the second server without downloading blocks?

[guggero]

You can use PSBTs (https://github.com/bitcoin/bips/blob/master/bip-0174.mediawiki) for this. On the node with blockchain access you would add all required information (mot notably the witness UTXO field PSBT_IN_WITNESS_UTXO) for each input. The signing node then just uses that meta information to sign.
Although btcd itself doesn't have PSBT support yet, so you'll need to write code yourself or use lnd as the on-chain wallet.

[liorwavebl]

Can I send the data from the one who has access to the blockchain to the second one without use PSBT?
If I used non-segwit I can access the UTXO scriptPubKey and sign the transaction with it by sending it from the first server to the second one.
How can I do it with SegWit transactions?

[guggero]

Sure, you can use whatever method you want to send the UTXO information from one server to another. Using a PSBT is just the standard way to do it.

[liorwavebl]

How can I extract the equivalent data to scriptPubKey for SegWit transactions?
ASFAIU it's called witnessScript. I cannot get the it by calling decoderawtransaction.

[guggero]

Should be gettxout.

[liorwavebl]

According to bitcoin API gettxout returns the scriptPubKey from the chain. If i passed the scriptPubKey to txscript.WitnessSignature I'm getting an exception that says: non-mandatory-script-verify-flag (Witness requires empty scriptSig)