[bitcoin-dev] Witness serialization in PSBT non-witness UTXOs

[Pieter Wuille <pieter.wuille@gmail.com>]

Hello all,

BIP174 currently specifies that non-witness UTXOs (the transactions
being spent by non-witness inputs) should be serialized in network
format.

I believe there are two issues with this.

1. Even in case the transaction whose output being spent itself has a
witness, this witness is immaterial to PSBT. It's only there to be
able to verify the txid commits to the output/amount being spent,
which can be done without witness.

2. "Network format" is a bit ambiguous. We can imagine a future
softfork that introduces a new type of witness. Network format could
be interpreted as including that new witness type, which is clearly
unnecessary (by the above argument), and would gratuitously break
compatibility with existing signers if implemented pedantically.

So my suggestion is to update the specification to state that
non-witness UTXOs must be serialized without witness. If it's too late
for that, it should instead be updated to explicitly specify with or
witnout witness, but it's safe to drop the witness.

Opinions?

Cheers,

-- 
Pieter