Bitcoin Book of BIPs

BIP 371

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 <keytype> <keydata> <keydata> Description <valuedata> <valuedata> Description Versions Requiring Inclusion Versions Requiring Exclusion Versions Allowing Inclusion
Taproot Key Spend Signature PSBT_IN_TAP_KEY_SIG = 0x13 None No key data Why is there no key data for PSBT_IN_TAP_KEY_SIGThe signature in a key path spend corresponds directly with the pubkey provided in the output script. Thus it is not necessary to provide any metadata that attaches the key path spend signature to a particular pubkey. <64 or 65 byte signature> 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 <xonlypubkey> <leafhash> 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. <64 or 65 byte signature> 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 <bytes control block> The control block for this leaf as specified in BIP 341. The control block contains the merkle tree path to this leaf. <bytes script> <8-bit uint leaf version> The script for this leaf as would be provided in the witness stack followed by the single byte leaf version. Note that the leaves included in this field should be those that the signers of this input are expected to be able to sign for. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed. 0, 2
Taproot Key BIP 32 Derivation Path PSBT_IN_TAP_BIP32_DERIVATION = 0x16 <32 byte xonlypubkey> A 32 byte X-only public key involved in this input. It may be the output key, the internal key, or a key present in a leaf script. <compact size uint number of hashes> <32 byte leaf hash>* <4 byte fingerprint> <32-bit little endian uint path element>* A compact size unsigned integer representing the number of leaf hashes, followed by a list of leaf hashes, followed by the 4 byte master key fingerprint concatenated with the derivation path of the public key. The derivation path is represented as 32-bit little endian unsigned integer indexes concatenated with each other. Public keys are those needed to spend this output. The leaf hashes are of the leaves which involve this public key. The internal key does not have leaf hashes, so can be indicated with a hashes len of 0. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed. 0, 2
Taproot Internal Key PSBT_IN_TAP_INTERNAL_KEY = 0x17 None No key data <32 byte xonlypubkey> The X-only pubkey used as the internal key in this output.Why is the internal key provided?The internal key is not necessarily the same key as in the Taproot output script. BIP 341 recommends tweaking the key with the hash of itself. It may be necessary for signers to know what the internal key actually is so that they are able to determine whether an input can be signed by it. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed. 0, 2
Taproot Merkle Root PSBT_IN_TAP_MERKLE_ROOT = 0x18 None No key data <32-byte hash> The 32 byte Merkle root hash. Finalizers should remove this field after PSBT_IN_FINAL_SCRIPTWITNESS is constructed. 0, 2

The new per-output types are defined as follows:

Name <keytype> <keydata> <keydata> Description <valuedata> <valuedata> Description Versions Requiring Inclusion Versions Requiring Exclusion Versions Allowing Inclusion
Taproot Internal Key PSBT_OUT_TAP_INTERNAL_KEY = 0x05 None No key data <32 byte xonlypubkey> The X-only pubkey used as the internal key in this output. 0, 2
Taproot Tree PSBT_OUT_TAP_TREE = 0x06 None No key data {<8-bit uint depth> <8-bit uint leaf version> <compact size uint scriptlen> <bytes script>}* One or more tuples representing the depth, leaf version, and script for a leaf in the Taproot tree, allowing the entire tree to be reconstructed. The tuples must be in depth first search order so that the tree is correctly reconstructed. Each tuple is an 8-bit unsigned integer representing the depth in the Taproot tree for this script, an 8-bit unsigned integer representing the leaf version, the length of the script as a compact size unsigned integer, and the script itself. 0, 2
Taproot Key BIP 32 Derivation Path PSBT_OUT_TAP_BIP32_DERIVATION = 0x07 <32 byte xonlypubkey> A 32 byte X-only public key involved in this output. It may be the output key, the internal key, or a key present in a leaf script. <compact size uint number of hashes> <32 byte leaf hash>* <4 byte fingerprint> <32-bit little endian uint path element>* A compact size unsigned integer representing the number of leaf hashes, followed by a list of leaf hashes, followed by the 4 byte master key fingerprint concatenated with the derivation path of the public key. The derivation path is represented as 32-bit little endian unsigned integer indexes concatenated with each other. Public keys are those needed to spend this output. The leaf hashes are of the leaves which involve this public key. The internal key does not have leaf hashes, so can be indicated with a hashes len of 0. 0, 2

UTXO Types

BIP 174 recommends using PSBT_IN_NON_WITNESS_UTXO for all inputs because of potential attacks involving an updater lying about the amounts in an output. Because a Taproot signature will commit to all of the amounts and output scripts spent by the inputs of the transaction, such attacks are prevented as any such lying would result in an invalid signature. Thus Taproot inputs can use just PSBT_IN_WITNESS_UTXO.

Compatibility

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

Test Vectors

The following are invalid PSBTs:

The following are valid PSBTs:

Rationale

Reference implementation

The reference implementation of the PSBT format is available at https://github.com/achow101/bitcoin/tree/taproot-psbt.

Acknowledgements

TBD

  BIP: 373
  Layer: Applications
  Title: MuSig2 PSBT Fields
  Authors: Ava Chow <me@achow101.com>
  Status: Complete
  Type: Specification
  Assigned: 2024-06-04
  License: CC0-1.0
  Requires: 32, 174, 327, 328, 370