Re: [bitcoin-dev] New PSBT version proposal

[Andrew Chow <achow101-lists@achow101.com>]

Hi All,

I've made some reorganization changes to the way that new PSBT versions 
should be handled in BIP 174 (see 
https://github.com/bitcoin/bips/pull/1055) so PSBTv2 will be submitted 
as a separate BIP. The full document can be read at 
https://github.com/achow101/bips/blob/psbt2/bip-psbt2.mediawiki and I 
have also included it in this email.

I've included Rusty's suggestion for PSBT_GLOBAL_UNDER_CONSTRUCTION and 
made a few modifications. First, the field will be named 
PSBT_GLOBAL_TX_MODIFIABLE and only include the inputs modifiable and 
outputs modifiable flags. The SIGHASH_SINGLE bitmap will be included as 
a separate field PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS. This allows most 
PSBTs to not have to carry around a useless bitmap.

Andrew

***

<pre>
   BIP: PSBTv2
   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-PSBT2
   Status: Draft
   Type: Standards Track
   Created: 2021-01-14
   License: BSD-2-Clause
</pre>

==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.

===Copyright===

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 <tt><input-map></tt> or <tt><output-map></tt>. With 
Version 0, to retrieve
all of the information for an input or output, data would need to be 
found in two locations:
the <tt><input-map></tt>/<tt><output-map></tt> 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.

<tt>PSBT_GLOBAL_UNSIGNED_TX</tt> must be excluded in PSBTv2.
<tt>PSBT_GLOBAL_VERSION</tt> must be included in PSBTv2 and set to 
version number 2<ref>'''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</ref>.

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

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><keydata></tt> Description
! <tt><valuedata></tt>
! <tt><valuedata></tt> Description
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| Transaction Version
| <tt>PSBT_GLOBAL_TX_VERSION = 0x02</tt>
| None
| No key data
| <tt><32-bit uint></tt>
| 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.
| 2
| 0
| 2
|-
| Fallback Locktime
| <tt>PSBT_GLOBAL_FALLBACK_LOCKTIME = 0x03</tt>
| None
| No key data
| <tt><32-bit uint></tt>
| The 32-bit little endian unsigned integer representing the transaction 
locktime to use if no inputs specify a required locktime.
|
| 0
| 2
|-
| Input Count
| <tt>PSBT_GLOBAL_INPUT_COUNT = 0x04</tt>
| None
| No key data
| <tt><compact size uint></tt>
| Compact size unsigned integer representing the number of inputs in 
this PSBT.
| 2
| 0
| 2
|-
| Output Count
| <tt>PSBT_GLOBAL_OUTPUT_COUNT = 0x05</tt>
| None
| No key data
| <tt><compact size uint></tt>
| Compact size unsigned integer representing the number of outputs in 
this PSBT.
| 2
| 0
| 2
|-
| Transaction Modifiable Flags
| <tt>PSBT_GLOBAL_TX_MODIFIABLE = 0x06</tt>
| None
| No key data
| <tt><single byte boolean> <single byte boolean></tt>
| A single byte boolean (0 for False, 1 for True) representing whether 
inputs can be modified, referred to as the Inputs Modifiable Flag. This 
is followed by a single byte boolean representing whether outputs can be 
modified, referred to as the Outputs Modifiable Flag.
|
| 0
| 2
|-
| SIGHASH_SINGLE Inputs
| <tt>PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS = 0x07</tt>
| None
| No key data
| <tt><bit vector></tt>
| A bit vector representing which input indexes use SIGHASH_SINGLE. If 
the bit for an index is set to 1, then the input and output pair at that 
index are tied together with SIGHASH_SINGLE and must be moved together.
|
| 0
| 2
|}

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

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><keydata></tt> Description
! <tt><valuedata></tt>
! <tt><valuedata></tt> Description
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| Previous TXID
| <tt>PSBT_IN_PREVIOUS_TXID = 0x0e</tt>
| None
| No key data
| <tt><txid></tt>
| 32 byte txid of the previous transaction whose output at 
PSBT_IN_OUTPUT_INDEX is being spent.
| 2
| 0
| 2
|-
| Spent Output Index
| <tt>PSBT_IN_OUTPUT_INDEX = 0x0f</tt>
| None
| No key data
| <tt><32-bit uint></tt>
| 32 bit little endian integer representing the index of the output 
being spent in the transaction with the txid of PSBT_IN_PREVIOUS_TXID.
| 2
| 0
| 2
|-
| Sequence Number
| <tt>PSBT_IN_SEQUENCE = 0x10</tt>
| None
| No key data
| <tt><32-bit uint></tt>
| 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).
|
| 0
| 2
|-
| Required Time-based Locktime
| <tt>PSBT_IN_REQUIRED_TIME_LOCKTIME = 0x11</tt>
| None
| No key data
| <tt><32-bit uint></tt>
| 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.
|
| 0
| 2
|-
| Required Height-based Locktime
| <tt>PSBT_IN_REQUIRED_HEIGHT_LOCKTIME = 0x12</tt>
| None
| No key data
| <tt><32-bit uiht></tt>
| 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.
|
| 0
| 2
|}

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

{|
! Name
! <tt><keytype></tt>
! <tt><keydata></tt>
! <tt><keydata></tt> Description
! <tt><valuedata></tt>
! <tt><valuedata></tt> Description
! Versions Requiring Inclusion
! Versions Requiring Exclusion
! Versions Allowing Inclusion
|-
| Output Amount
| <tt>PSBT_OUT_AMOUNT = 0x03</tt>
| None
| No key data
| <tt><64-bit uint></tt>
| 64 bit signed little endian integer representing the output's amount 
in satoshis.
| 2
| 0
| 2
|-
| Output Script
| <tt>PSBT_OUT_SCRIPT = 0x03</tt>
| None
| No key data
| <tt><script></tt>
| 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. <ref>'''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.</ref>
The Creator should also 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 (first boolean) 
is True.
Outputs may only be added if the Outputs Modifiable flag (second 
boolean) is True.
If PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS is present and its bitvector 
indicates any inputs use SIGHASH_SINGLE, then the same number of inputs 
and outputs must be added before that input and its corresponding outputs.

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.
If the input has a required height based timelock, then 
PSBT_IN_REQUIRED_HEIGHT_TIMELOCK.
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.

When adding a new input, the new input must be compatible with the 
timelock types of all existing inputs.
Since Bitcoin requires that a transaction uses only one type of 
timelock, all of the inputs must be able to support the same type of 
timelock.
If the type of timelock is incompatible with the timelock type of the 
other inputs, then the input must not be added.

Since a Constructor may be adding inputs to a PSBT that has inputs with 
existing signatures, the Constructor must be careful to not invalidate 
any existing signatures.
The PSBT_GLOBAL_TX_MODIFIABLE and PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS 
fields caches this information so that constructors do not need to 
inspect every input.
If the PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS bit vector indicates an input 
index uses SIGHASH_SINGLE, the same number of inputs and outputs must be 
added before that input and its corresponding output.
When an input is added, the bit vector, if present, must be expanded to 
include a bit for this input in the correct position.
When there are signatures, in addition to respecting the lock time rules 
described previously, the newly added inputs must not change the lock 
time used by the transaction.
It cannot cause the lock time to change as that will invalidate all 
signatures since they all include the lock time regardless of the 
sighash type.

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 and 
PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS fields after signing inputs so
that these fields 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, a 
PSBT_GLOBAL_SIGHASH_SINGLE_INPUTS field must be added if not present, 
and the corresponding bit in its bit vector must be set to 1.

===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.

==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==

<references/>

==Reference implementation==

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


On 12/23/20 4:32 PM, Andrew Chow wrote:
> Hi All,
>
> The full modified BIP can be read at
> https://github.com/achow101/bips/blob/psbt2/bip-0174.mediawiki.
>
> I will open a PR to the BIPs repo soon after further discussion on this.
>
>
> Andrew
>
> On 12/22/20 3:12 PM, Andrew Chow wrote:
>> Hi All,
>>
>> I have some updates on this after speaking with some people off-list.
>>
>> Firstly, the version number will be set to 2. In most discussions, this
>> proposal was being referred to as PSBT version 2, so it'll be easier and
>> clearer to set the version number to 2.
>>
>> For lock times, instead of a single  PSBT_IN_REQUIRED_LOCKTIME field,
>> there will be 2 of them, one for a time based lock time, and the other
>> for height based. These will be:
>> * PSBT_IN_REQUIRED_TIME_LOCKTIME = 0x10
>>      * Key: empty
>>      * Value: 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. Must be omitted in
>> PSBTv0, and may be omitted in PSBTv2
>> * PSBT_IN_REQUIRED_HEIGHT_LOCKTIME = 0x11
>>      * Key: empty
>>      * Value: 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. Must be omitted in PSBTv0, and may be
>> omitted in PSBTv2.
>>
>> Having two lock time fields is necessary due to the behavior where all
>> inputs must use the same type of lock time (height or time). Thus if an
>> input requires a particular type of lock time, it must set the requisite
>> field. Any new inputs being added must be able to accommodate all
>> existing inputs' lock time type. This means they either must not have a
>> lock time specified (i.e. no OP_CLTV involved), or have branches that
>> allow the acceptance of either type. If an input has a lock time type
>> that is incompatible with the rest of the transaction, it must not be added.
>>
>> PSBT_GLOBAL_PREFERRED_LOCKTIME is changed to purely be the fallback
>> option if no input lock time fields are present. If there are input lock
>> times, all lock time calculations must ignore it.
>>
>> Any role which does lock time calculation will first check if there are
>> input lock time fields. If there are not, it must then check for a
>> PSBT_GLOBAL_PREFERRED_LOCKTIME. If this field exists, its value is the
>> transaction's lock time. If it does not, the lock time is 0. If there
>> are input lock time fields, it must choose the type which does not
>> invalidate any inputs. The lock time is then determined to be the
>> maximum value of all of the lock time fields for the chosen type.
>>
>>
>> Additionally, I would like to add a new global field:
>> * PSBT_GLOBAL_UNDER_CONSTRUCTION = 0x05
>>      * Key: empty
>>      * Value: A single byte as a boolean. 0 for False, 1 for True. All
>> other values ore prohibited. Must be omitted for PSBTv0, may be omitted
>> in PSBTv2.
>>
>> PSBT_GLOBAL_UNDER_CONSTRUCTION is used to signal whether inputs and
>> outputs can be added to the PSBT. This flag may be set to True when
>> inputs and outputs are being updated, signed, and finalized. However
>> care must be taken when there are existing signatures. If this field is
>> omitted or set to False, no further inputs and outputs may be added to
>> the PSBT.
>>
>> Several rules must be followed to ensure that adding additional inputs
>> and outputs will not invalidate existing signatures. First, an input or
>> output adder must check for any existing signatures in all of the other
>> inputs. If there are none, the input or output may be added in any
>> position. If there are one or more signatures, each signature's sighash
>> type must be examined. Inputs may only be added if all existing
>> signatures use SIGHASH_ANYONECANPAY. Outputs may only be added if all
>> existing signatures use SIGHASH_NONE. If an input has a signature using
>> SIGHASH_SINGLE, the same number of inputs and outputs must be added
>> before that input and it's corresponding output. For all other sighash
>> types (i.e. SIGHASH_ALL and any future sighash types), no inputs or
>> outputs may be added to the PSBT. Specific exceptions can be made in the
>> future for additional sighash types.
>>
>> Furthermore, these newly added inputs must follow additional lock time
>> rules. Because all signatures, regardless of sighash type, sign the
>> transaction lock time, newly added inputs when there are existing
>> signatures must have the same type of lock time used in the transaction,
>> and must be less than or equal to the transaction lock time. It must not
>> cause the transaction lock time to change, otherwise the signatures will
>> be invalidated.
>>
>>
>> Lastly, to uniquely identify transactions for combiners, a txid can be
>> computed from the information present in the PSBT. Internally, combiners
>> can create an unsigned transaction given the transaction version, the
>> input prevouts, the outputs, and the computed locktime. This can then be
>> used to calculate a txid and thus used as a way to identify PSBTs.
>> Combiners will need to do this for all version 2 PSBTs in order to avoid
>> combining distinct transactions.
>>
>>
>> Andrew Chow
>>
>> On 12/9/20 5:25 PM, Andrew Chow wrote:
>>> Hi All,
>>>
>>> I would like to propose a new PSBT version that addresses a few
>>> deficiencies in the current PSBT v0. As this will be backwards
>>> incompatible, a new PSBT version will be used, v1.
>>>
>>> The primary change is to truly have all input and output data for each
>>> in their respective maps. Instead of having to parse an unsigned
>>> transaction and lookup some data from there, and other data from the
>>> correct map, all of the data for an input will be contained in its map.
>>> Doing so also disallows PSBT_GLOBAL_UNSIGNED_TX in this new version.
>>> Thus I propose that the following fields be added:
>>>
>>> Global:
>>> * PSBT_GLOBAL_TX_VERSION = 0x02
>>>       * Key: empty
>>>       * Value: 32-bit little endian unsigned integer for the transaction
>>> version number. Must be provided in PSBT v1 and omitted in v0.
>>> * PSBT_GLOBAL_PREFERRED_LOCKTIME = 0x03
>>>       * Key: empty
>>>       * Value: 32 bit little endian unsigned integer for the preferred
>>> transaction lock time. Must be omitted in PSBT v0. May be provided in
>>> PSBT v1, assumed to be 0 if not provided.
>>> * PSBT_GLOBAL_INPUT_COUNT = 0x04
>>>       * Key: empty
>>>       * Value: Compact size unsigned integer. Number of inputs in this
>>> PSBT. Must be provided in PSBT v1 and omitted in v0.
>>> * PSBT_GLOBAL_OUTPUT_COUNT = 0x05
>>>       * Key: empty
>>>       * Value: Compact size unsigned integer. Number of outputs in this
>>> PSBT. Must be provided in PSBT v1 and omitted in v0.
>>>
>>> Input:
>>> * PSBT_IN_PREVIOUS_TXID = 0x0e
>>>       * Key: empty
>>>       * Value: 32 byte txid of the previous transaction whose output at
>>> PSBT_IN_OUTPUT_INDEX is being spent. Must be provided in PSBT v1 and
>>> omitted in v0.
>>> * PSBT_IN_OUTPUT_INDEX = 0x0f
>>>       * Key: empty
>>>       * Value: 32 bit little endian integer for the index of the output
>>> being spent. Must be provided in PSBT v1 and omitted in v0.
>>> * PSBT_IN_SEQUENCE = 0x0f
>>>       * Key: empty
>>>       * Value: 32 bit unsigned little endian integer for the sequence
>>> number. Must be omitted in PSBT v0. May be provided in PSBT v1 assumed
>>> to be max sequence (0xffffffff) if not provided.
>>> * PSBT_IN_REQUIRED_LOCKTIME = 0x10
>>>       * Key: empty
>>>       * Value: 32 bit unsigned little endian integer for the lock time that
>>> this input requires. Must be omitted in PSBT v0. May be provided in PSBT
>>> v1, assumed to be 0 if not provided.
>>>
>>> Output:
>>> * PSBT_OUT_VALUE = 0x03
>>>       * Key: empty
>>>       * Value: 64-bit unsigned little endian integer for the output's
>>> amount in satoshis. Must be provided in PSBT v1 and omitted in v0.
>>> * PSBT_OUT_OUTPUT_SCRIPT = 0x04
>>>       * Key: empty
>>>       * Value: The script for this output. Otherwise known as the
>>> scriptPubKey. Must be provided in PSBT v1 and omitted in v0.
>>>
>>> This change allows for PSBT to be used in the construction of
>>> transactions. With these new fields, inputs and outputs can be added as
>>> needed. One caveat is that there is no longer a unique transaction
>>> identifier so more care must be taken when combining PSBTs.
>>> Additionally, adding new inputs and outputs must be done such that
>>> signatures are not invalidated. This may be harder to specify.
>>>
>>> An important thing to note in this proposal are the fields
>>> PSBT_GLOBAL_PREFERRED_LOCKTIME and PSBT_IN_REQUIRED_LOCKTIME. A Bitcoin
>>> transaction only has a single locktime yet a PSBT may have multiple
>>> locktimes. To choose the locktime for the transaction, finalizers must
>>> choose the maximum of all of the *_LOCKTIME fields.
>>> PSBT_IN_REQUIRED_LOCKTIME is added because some inputs, such as those
>>> involving OP_CHECKLOCKTIMEVERIFY, require a specific minimum locktime to
>>> be set. This field allows finalizers to choose a locktime that is high
>>> enough for all inputs without needing to understand the scripts
>>> involved. The PSBT_GLOBAL_PREFERRED_LOCKTIME is the locktime to use if
>>> no inputs require a particular locktime.
>>>
>>> As these changes disallow the PSBT_GLOBAL_UNSIGNED_TX field, PSBT v1
>>> needs the version number bump to enforce backwards incompatibility.
>>> However once the inputs and outputs of a PSBT are decided, a PSBT could
>>> be "downgraded" back to v0 by creating the unsigned transaction from the
>>> above fields, and then dropping these new fields.
>>>
>>> If the list finds that these changes are reasonable, I will write a PR
>>> to modify BIP 174 to incorporate them.
>>>
>>> Thanks,
>>> Andrew Chow