BIP: 370 source
  Layer: Applications
  Title: PSBT Version 2
  Author: Andrew Chow <achow101@gmail.com>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0370
  Status: Draft
  Type: Standards Track
  Created: 2021-01-14
  License: BSD-2-Clause

Introduction

Abstract

This document proposes a second version of the Partially Signed Bitcoin Transaction format described in BIP 174 which allows for inputs and outputs to be added to the PSBT after creation.

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

Motivation

Partially Signed Bitcoin Transaction Version 0 as described in BIP 174 is unable to have new inputs and outputs be added to the transaction. The fixed global unsigned transaction cannot be changed which prevents any additional inputs or outputs to be added. PSBT Version 2 is intended to rectify this problem.

An additional benficial side effect is that all information for a given input or output will be provided by its or . With Version 0, to retrieve all of the information for an input or output, data would need to be found in two locations: the / and the global unsigned transaction. PSBT Version 2 now moves all related information to one place.

Specification

PSBT Version 2 (PSBTv2) only specifies new fields and field inclusion/exclusion requirements.

PSBT_GLOBAL_UNSIGNED_TX must be excluded in PSBTv2. PSBT_GLOBAL_VERSION must be included in PSBTv2 and set to version number 2[1].

The new global types for PSBT Version 2 are as follows:

Name Description DescriptionVersions Requiring InclusionVersions Requiring ExclusionVersions Allowing Inclusion
Transaction VersionPSBT_GLOBAL_TX_VERSION = 0x02NoneNo key data<32-bit uint>The 32-bit little endian signed integer representing the version number of the transaction being created. Note that this is not the same as the PSBT version number specified by the PSBT_GLOBAL_VERSION field.202
Fallback LocktimePSBT_GLOBAL_FALLBACK_LOCKTIME = 0x03NoneNo key data<32-bit uint>The 32-bit little endian unsigned integer representing the transaction locktime to use if no inputs specify a required locktime.02
Input CountPSBT_GLOBAL_INPUT_COUNT = 0x04NoneNo key dataCompact size unsigned integer representing the number of inputs in this PSBT.202
Output CountPSBT_GLOBAL_OUTPUT_COUNT = 0x05NoneNo key dataCompact size unsigned integer representing the number of outputs in this PSBT.202
Transaction Modifiable FlagsPSBT_GLOBAL_TX_MODIFIABLE = 0x06NoneNo key data<8-bit uint>An 8 bit little endian unsigned integer as a bitfield for various transaction modification flags. Bit 0 is the Inputs Modifiable Flag and indicates whether inputs can be modified. Bit 1 is the Outputs Modifiable Flag and indicates whether outputs can be modified. Bit 2 is the Has SIGHASH_SINGLE flag and indicates whether the transaction has a SIGHASH_SINGLE signature who's input and output pairing must be preserved. Bit 2 essentially indicates that the Constructor must iterate the inputs to determine whether and how to add an input.02

The new per-input types for PSBT Version 2 are defined as follows:

Name Description DescriptionVersions Requiring InclusionVersions Requiring ExclusionVersions Allowing Inclusion
Previous TXIDPSBT_IN_PREVIOUS_TXID = 0x0eNoneNo key data32 byte txid of the previous transaction whose output at PSBT_IN_OUTPUT_INDEX is being spent.202
Spent Output IndexPSBT_IN_OUTPUT_INDEX = 0x0fNoneNo key data<32-bit uint>32 bit little endian integer representing the index of the output being spent in the transaction with the txid of PSBT_IN_PREVIOUS_TXID.202
Sequence NumberPSBT_IN_SEQUENCE = 0x10NoneNo key data<32-bit uint>The 32 bit unsigned little endian integer for the sequence number of this input. If omitted, the sequence number is assumed to be the final sequence number (0xffffffff).02
Required Time-based LocktimePSBT_IN_REQUIRED_TIME_LOCKTIME = 0x11NoneNo key data<32-bit uint>32 bit unsigned little endian integer greater than or equal to 500000000 representing the minimum Unix timestamp that this input requires to be set as the transaction's lock time.02
Required Height-based LocktimePSBT_IN_REQUIRED_HEIGHT_LOCKTIME = 0x12NoneNo key data<32-bit uiht>32 bit unsigned little endian integer less than 500000000 representing the minimum block height that this input requires to be set as the transaction's lock time.02

The new per-output types for PSBT Version 2 are defined as follows:

Name

Description

Description

Versions Requiring Inclusion

Versions Requiring Exclusion

Versions Allowing Inclusion

Output Amount

PSBT_OUT_AMOUNT = 0x03

None

No key data

<64-bit int>

64 bit signed little endian integer representing the output's amount in satoshis.

2

0

2

Output Script

PSBT_OUT_SCRIPT = 0x03

None

No key data

<script>

The script for this output, also known as the scriptPubKey. Must be omitted in PSBTv0. Must be provided in PSBTv2.

2

0

2

Determining Lock Time

The nLockTime field of a transaction is determined by inspecting the PSBT_GLOBAL_PREFERRED_LOCKTIME and each input's PSBT_IN_REQUIRED_TIME_LOCKTIME and PSBT_IN_REQUIRED_HEIGHT_LOCKTIME fields. If none of the inputs have a PSBT_IN_REQUIRED_TIME_LOCKTIME and PSBT_IN_REQUIRED_HEIGHT_LOCKTIME, then PSBT_GLOBAL_PREFERRED_LOCKTIME must be used. If PSBT_GLOBAL_PREFERRED_LOCKTIME is not provided, then it is assumed to be 0.

If one or more inuts have a PSBT_IN_REQUIRED_TIME_LOCKTIME or PSBT_IN_REQUIRED_HEIGHT_LOCKTIME, then the field chosen is the one which is supported by all of the inputs. This can be determined by looking at all of the inputs which specify a locktime in either of those fields, and choosing the field which is present in all of those inputs. Inputs not specifying a lock time field can take both types of lock times, as can those that specify both. The lock time chosen is then the maximum value of the chosen type of lock time.

Unique Identification

PSBTv2s can be uniquely identified by constructing an unsigned transaction given the information provided in the PSBT and computing the transaction ID of that transaction. Since PSBT_IN_SEQUENCE can be changed by Updaters and Combiners, the sequence number in this unsigned transaction must be set to 0 (not final, nor the sequence in PSBT_IN_SEQUENCE). The lock time in this unsigned transaction must be computed as described previously.

Roles

PSBTv2 introduces new roles and modifies some existing roles.

Creator

In PSBTv2, the Creator initializes the PSBT with 0 inputs and 0 outputs. The PSBT version number is set to 2. The transaction version number must be set to at least 2. [2] The Creator should also set PSBT_GLOBAL_PREFERRED_LOCKTIME. If the Creator is not also a Constructor and will be giving the PSBT to others to add inputs and outputs, the PSBT_GLOBAL_TX_MODIFIABLE field must be present and and the Inputs Modifiable and Outputs Modifiable flags set appropriately. If the Creator is a Constructor and no inputs and outputs will be added by other entities, PSBT_GLOBAL_TX_MODIFIABLE may be omitted.

Constructor

This Constructor is only present for PSBTv2. Once a Creator initializes the PSBT, a constructor will add inputs and outputs. Before any input or output may be added, the constructor must check the PSBT_GLOBAL_TX_MODIFIABLE field. Inputs may only be added if the Inputs Modifiable flag is True. Outputs may only be added if the Outputs Modifiable flag is True.

When an input or output is added, the corresponding PSBT_GLOBAL_INPUT_COUNT or PSBT_GLOBAL_OUTPUT_COUNT must be incremeted to reflect the number of inputs and outputs in the PSBT. When an input is added, it must have PSBT_IN_PREVIOUS_TXID and PSBT_IN_OUTPUT_INDEX set. When an output is added, it must have PSBT_OUT_VALUE and PSBT_OUT_OUTPUT_SCRIPT set. If the input has a required timelock, Constructors must set the requisite timelock field. If the input has a required time based timelock, then PSBT_IN_REQUIRED_TIME_TIMELOCK must be set If the input has a required height based timelock, then PSBT_IN_REQUIRED_HEIGHT_TIMELOCK must be set. If an input has both types of timelocks, then both may be set. In some cases, an input that can allow both types, but a particular branch supporting only one type of timelock will be taken, then the type of timelock that will be used can be the only one set.

If an input being added specifies a required time lock, then the Constructor must iterate through all of the existing inputs and ensure that the time lock types are compatible. Additionally, if during this iteration, it finds that any inputs have signatures, it must ensure that the newly added input does not change the transaction's locktime. If the newly added input has an incompatible time lock, then it must not be added. If it changes the transaction's locktime when there are existing signatures, it must not be added.

If the Has SIGHASH_SINGLE flag is True, then the Constructor must iterate through the inputs and find the inputs which have signatures that use SIGHASH_SINGLE. The same number of inputs and outputs must be added before those inputs and their corresponding outputs.

A Constructor may choose to declare that no further inputs and outputs can be added to the transaction by setting the booleans in PSBT_GLOBAL_TX_MODIFIABLE to False or by removing this field entirely.

A single entity is likely to be both a Creator and Constructor.

Updater

For PSBTv2, an Updater can set the sequence number.

Signer

For PSBTv2s, a signer must update the PSBT_GLOBAL_TX_MODIFIABLE field after signing inputs so that it accurately reflects the state of the PSBT. If the Signer added a signature that does not use SIGHASH_ANYONECANPAY, the Input Modifiable flag must be set to False. If the Signer added a signature that does not use SIGHASH_NONE, the Outputs Modifiable flag must be set to False. If the Signer added a signature that uses SIGHASH_SINGLE, the Has SIGHASH_SINGLE flag must be set to True.

Transaction Extractor

For PSBTv2s, the transaction is constructed using the PSBTv2 fields. The lock time for this transaction is determined as described in the Determining Lock Time section. The Extractor should produce a fully valid, network serialized transaction if all inputs are complete.

Backwards Compatibility

PSBTv2 shares the same gemeric format as PSBTv0 as defined in BIP 174. Parsers for PSBTv0 should be able to deserialize PSBTv2 with only changes to support the new fields.

However PSBTv2 is incompatible with PSBTv0, and vice versa due to the use of the PSBT_GLOBAL_VERSION. This incompatibility is intentional so that PSBT_GLOBAL_UNSIGNED_TX could be removed in PSBTv2. However it is possible to convert a PSBTv2 to a PSBTv0 by creating an unsigned transaction from the PSBTv2 fields.

Test Vectors

TBD

Rationale

Reference implementation

The reference implementation of the PSBT format is available at https://github.com/achow101/bitcoin/tree/psbt2.

[1] What happened to version number 1? Version number 1 is skipped because PSBT Version 0 has been colloquially referred to as version 1. Originally this BIP was to be version 1, but because it has been colloquially referred to as version 2 during its design phrase, it was decided to change the version number to 2 so that there would not be any confusion

[2] Why does the transaction version number need to be at least 2? The transaction version number is part of the validation rules for some features such as OP_CHECKSEQUENCEVERIFY. Since it is backwards compatible, and there are other ways to disable those features (e.g. through sequence numbers), it is easier to require transactions be able to support these features than to try to negotiate the transaction version number.