BIP 371: Taproot Fields for PSBT

2021-06-21

  BIP: 371
  Layer: Applications
  Title: Taproot Fields for PSBT
  Author: Andrew Chow <andrew@achow101.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0371
  Status: Draft
  Type: Standards Track
  Created: 2021-06-21
  License: BSD-2-Clause

Introduction

Abstract

This document proposes additional fields for BIP 174 PSBTv0 and BIP 370 PSBTv2 that allow for BIP 340/341/342 Taproot data to be included in a PSBT of any version. These will be fields for signatures and scripts that are relevant to the creation of Taproot inputs.

This BIP is licensed under the 2-clause BSD license.

Motivation

BIPs 340, 341, and 342 specify Taproot which provides a wholly new way to create and spend Bitcoin outputs. The existing PSBT fields are unable to support Taproot due to the new signature algorithm and the method by which scripts are embedded inside of a Taproot output. Therefore new fields must be defined to allow PSBTs to carry the information necessary for signing Taproot inputs.

Specification

The new per-input types are defined as follows:

Name

Description

Description

Versions Requiring Inclusion

Versions Requiring Exclusion

Versions Allowing Inclusion

Taproot Key Spend Signature

PSBT_IN_TAP_KEY_SIG = 0x13

None

No key data [1]

The 64 or 65 byte Schnorr signature for key path spending a Taproot output. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed.

0, 2

Taproot Script Spend Signature

PSBT_IN_TAP_SCRIPT_SIG = 0x14

A 32 byte X-only public key involved in a leaf script concatenated with the 32 byte hash of the leaf it is part of.

The 64 or 65 byte Schnorr signature for this pubkey and leaf combination. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed.

0, 2

Taproot Leaf Script

PSBT_IN_TAP_LEAF_SCRIPT = 0x15

The control block for this leaf as specified in BIP 341. The control block contains the merkle tree path to this leaf.