walletprocesspsbt produces invalid signatures, finalizepsbt doesn't finalize tx with valid signatures

[stepansnigirev]

Trying to create and sign psbt with elements produces an invalid transaction.

The flow is:

• create psbt with walletcreatefundedpsbt (bip32deriv is true)
• blind it with blindpsbt
• sign it with the wallet using walletprocesspsbt
• finalize the transaction with finalizepsbt
• check that transaction will be accepted by the mempool with testmempoolaccept - signatures are invalid.

As I figured out the walletprocesspsbt is using a raw unblinded transaction from the global scope of the psbt, so in the digest unblinded values and assets of the outputs are used. If I manually sign the transaction with asset and value commitments available in the output scope of the PSBT I can create a valid signature, but then finalizepsbt rpc call refuses to finalize the transaction because it is also checking the signatures without taking into account the commitments.

I am using elements-0.18.1.9 from the releases.

Currently I am using my own script to sign and finalize the transaction, but it's not very convenient... Would be nice to fix this.

[apoelstra]

Definitely. Reimplementing PSET is our first priority after #935 gets merged (and we are working on that in parallel based on the spec in #951)