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?
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
