[bitcoin-dev] New PSBT version proposal

[Andrew Chow]

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