Hi all,
While implementing PSBT support in the btclib library (https://github.com/btclib-org/btclib), I have failed to understand the rationale for the proprietary and proof-of-reserves types.
First off, at face value they have nothing to do with the operations intrinsically required to finalize a valid transaction from PSBT manipulation.
Moreover, whatever information content they can provide for non-standard PSBT manipulation, that content could stay in the unknown field without any loss of generality. How to structure and deal with unknown data would be the responsibility of proprietary software or users wanting to provide proof-of-reserve. As long as BIP174 clearly prescribes that unknown data must be kept during PSBT manipulation, that should be enough.
Let me stress the above point: I have a project where we include proprietary information in the PSBT. Any PSBT software supporting unknown data gently keeps our proprietary information and our proprietary software retrieves that data from serialized PSBT with no problem. There is no need for a PSBT implementation to provide explicit support for proprietary and proof-of-reserves types.
My last conclusion is reinforced by the evidence of all PSBT implementations I know of, including bitcoin core and HWI, not implementing proprietary and proof-of-reserve types. There is a high probability that part of BIP174 would be just ignored.
Am I missing something?
Thanks