SegWit inputs in PSBT must provide full UTXO

A security issue* in the design of BIP-143 allows an attacker to lie about segwit input amounts and get the user to pay an unexpectedly high transaction fee. The problem affects all HWW vendors.

For Trezor, we are fixing this by requiring the full UTXO for all types of inputs, so we can validate that the input amount is correct.

To facilitate that, we need BTCPay to provide full UTXOs for all input types. This goes against the recommendation in BIP-174, saying that NON_WITNESS_UTXO should not be provided for SegWit inputs. Nevertheless, the resulting PSBT is still valid AFAICT.

Without this modification, it will be impossible to sign SegWit transactions on Trezor firmwares starting with 1.9.1 and 2.3.1. Also, an issue in HWI must be fixed first: https://github.com/bitcoin-core/HWI/issues/338

*) Details in our blogpost: https://blog.trezor.io/details-of-firmware-updates-for-trezor-one-version-1-9-1-and-trezor-model-t-version-2-3-1-1eba8f60f2dd

[NicolasDorier]

@AvivMilner that said, I don’t know Ledger stance on it. I don’t think they will fix it as it breaks lot’s of thing, but I may be wrong.

This is a difficult decision for hardware wallets. I personally think that the pill is worse than the disease.

[Kukks]

But when Wasabi in btcpay? Keep everything in one place.

Thank you for your answers.

An issue for another day that I’m already looking into.