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In BIP 174/PSBT, there is a PSBT Input key-value pair called "PSBT_IN_WITNESS_SCRIPT" (0x05) and another called "PSBT_IN_FINAL_SCRIPTWITNESS" (0x08). What is the difference between a WitnessScript and a scriptWitness?

From what I understood, a witness script is the witness data for a single input (the signatures and scripts that unlock it). Is the scriptWitness something different?

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PSBT_IN_WITNESS_SCRIPT is specifically the actually executed script for a P2WSH or P2SH-P2WSH scriptPubKey being spent. It is added by an Updater who knows it.

PSBT_IN_FINAL_SCRIPTWITNESS is the witness stack needed to spend an output, including scripts, signatures, public keys, ... whatever is needed. It is created by a Finalizer, using other fields necessary (including PSBT_IN_WITNESS_SCRIPT) to produce it.

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  • To clarify, SCRIPTWITNESS is the Segwit equivalent of ScriptSig, while WITNESS_SCRIPT is the Segwit equivalent of the redeemScript? Or is WITNESS_SCRIPT the script for ScriptHash outputs, and it is separate by the redeemScript? Dec 27 '20 at 0:08
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    The scriptWitness (PSBT_IN_FINAL_SCRIPTWITNESS) is the per-input witness stack, so indeed more or less equivalent to scriptSig (for all segwit spends, not just P2WSH). The witness script (PSBT_IN_WITNESS_SCRIPT) is the executed script in P2WSH - similar to the redeemScript is P2SH. Dec 27 '20 at 0:18

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