Subject: Re: [bitcoin-dev] New PSBT version proposal

From: Andrew Chow <[email-masked]>
Date: Tue, 22 Dec 2020 20:12:22 +0000

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