[bitcoin-dev] New PSBT version proposal

[Andrew Chow]

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