One thing I think should be added in V2 is the ability to specify sighash flags per-key as opposed to per-input. The per-key restriction is unfitting given that there are circumstances where multisig signers may validate heterogenous logic. -- @JeremyRubin On Wed, Dec 23, 2020 at 1:37 PM Andrew Chow via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> wrote: > Hi, > > On 12/22/20 10:30 PM, fiatjaf wrote: > > Hi Andrew. > > > > I'm just a lurker here and I have not much experience with PSBTs, but > still let me pose this very obvious question and concern: isn't this change > going to create a compatibility nightmare, with some software supporting > version 1, others supporting version 2, and the ones that care enough about > UX and are still maintained being forced to support both versions -- and > for no very important reason except some improvements in the way data is > structured? > No, it is not just "improvements in the way data is structured." > > The primary reason for these changes is to allow PSBT to properly > support adding inputs and outputs. This is a feature that many people > have requested, and the ways that people have been doing it are honestly > just hacks and not really the right way to be doing that. These changes > allow for that feature to be supported well. > > Furthermore, it is possible to downgrade and upgrade PSBTs between the > two versions, once all inputs and outputs have been decided. Since > PSBTv2 is essentially just taking all of the normal transaction fields > and grouping them all with the rest of the data for those inputs and > outputs, it is easy to reconstruct a global unsigned transaction and > turn a PSBTv2 into a PSBTv0. It is likewise just as easy to go the other > way and break apart the global unsigned tx to turn a PSBTv0 into a > PSBTv2. Originally, I had considered requiring that once a transaction > was fully constructed it must be downgraded to a PSBTv0, but the > structure changes that were made do make it easier to work with PSBT so > I decided not to add this requirement. > > Perhaps to maintain compatibility PSBT_GLOBAL_UNSIGNED_TX shouldn't be > disallowed in PSBTv2 once the transaction is constructed? It would make > things much more confusing though as it would no longer be a clean break. > > > Andrew Chow > > > Ultimately I don't think it should matter if some data is structured in > not-the-best-possible way, as long as it is clear enough for the computer > and for the libraries already written to deal with it. > Backwards-compatibility and general interoperability is worth much more > than anything else in these cases. > > > > Also let me leave this article here, which I find very important (even > if for some reason it ends up not being relevant to this specific case): > http://scripting.com/2017/05/09/rulesForStandardsmakers.html > > > > ---- On Tue, 22 Dec 2020 17:12:22 -0300 Andrew Chow via bitcoin-dev < > bitcoin-dev@lists.linuxfoundation.org> 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 > > > > > > > > > _______________________________________________ > > > bitcoin-dev mailing list > > > bitcoin-dev@lists.linuxfoundation.org > > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > > > > > > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >