[BIP Proposal] Add PSBT_IN_SP_TWEAK field

[nymius]

Hi all,

I'm working on the implementation of silent payments in BDK.

BDK's transaction creation process is structured around PSBTs. Because of this, a stable implementation of silent payments in the project depends of the specifications of BIP 352 for this format. BIP 375 and BIP 374 are core component for this.

However, there is a need for the inclusion of silent payments tweaks in PSBTs to spend silent payment outputs, which was considered before [1][2] but never specified.

I would like to propose the following as a base for a new BIP proposal addressing this gap:

### Abstract

This document proposes additional fields for BIP 370 PSBTv2 that allow for BIP 352 silent payment tweaks to be included in a PSBT of version 2. These will be fields for scripts that are relevant to the spending of silent payment outputs, but may be also useful to other protocols using taproot tweaks not following BIP 340 spec.

### Motivation

BIPs 352 specify silent payments protocol, which provides a new way to create P2TR outputs and spend them. The existing PSBT fields are unable to support silent payments without changes, due to the new method by which outputs are created. BIP 375 and complementary BIP 374 specify how to create outputs locked with silent payment keys using PSBTs. But they don't specify how to unlock these outputs in a transaction. Therefore new fields must be defined to allow PSBTs to carry the information necessary for tweaking taproot keys without following the BIP 340 tagging scheme.

### Specification

The new per-input types are defined as follows:

| Name              | \<keytype\>               | \<keydata\> | <keydata\><br>Description | \<valuedata\>    | \<valuedata\><br>Description                                                                                 | \<Versions Requiring Inlusion\> | \<Versions Requiring Exclusion\> | \<Versions Allowing Inclusion\> |

| ----------------- | ------------------------- | ----------- | ------------------------- | ---------------- | ------------------------------------------------------------------------------------------------------------ | ------------------------------- | -------------------------------- | ------------------------------- |

| Taproot Raw Tweak | `PSBT_IN_SP_TWEAK = 0x1f` | None        | No key data               | `<32-byte hash>` | A 32 byte raw tweak. Finalizers should remove this field after `PSBT_IN_FINAL_SCRIPTWITNESS` is constructed. |                                 | 0                                | 2                               |

### Rationale

On PSBTs, when spending non silent payment outputs, one can rely on the `PSBT_IN_BIP32_DERIVATION` or any of the allowed `PSBT_IN_TAP_*` combinations available to get the right private keys to sign for each input.

To spend silent payment outputs you have to combine the private key with the tweak obtained from the transaction corpus.

On PSBT there is no field prescribed for this raw tweaks. The `PSBT_IN_BIP32_DERIVATION` field cannot be used because its different nature, neither the `PSBT_IN_TAP_MERKLE_ROOT` field because of the tagged hash used for tweaking.

A change of the hash tag used for silent payments to `TapTweak` or something compatible with taproot tweaking wouldn't make sense. Although the raw tweak can be disguised as the tap tree merkle root for spending, at the moment of verifying change outputs, you need the full tap tree, and there would be none backing this fake merkle root.

The use of proprietary fields is possible but brittle, as one may end up having to perform extra lookups for keys that are not unified across implementations.

Assuming different tweaking schemes available, `PSBT_IN_TAP_RAW_TWEAK` would be a more general solution. However is unclear how a hardware wallet will determine what the content of the field were in the first more general case. In addition, PSBT fields are usually specified as to the nature of the contents.

### Backward Compatibility

These are new fields added to the existing PSBT format. Because PSBT is designed to be extensible, old software will ignore the new fields.

### Test Vectors

To be added

### Related Work

BIP 352, BIP 375, BIP 371

A version of this document is also available at https://gist.github.com/nymius/b3dd0b8a08c6735d617e6216b73c4260.

Any feedback is appreciated.

Thanks, nymius.

[1]: https://delvingbitcoin.org/t/bip352-psbt-support/877

[2]: https://groups.google.com/g/bitcoindev/c/5G5wzqUXyk4/m/1qEST5b7AQAJ