Jeremy [ARCHIVE] on Nostr: 📅 Original date posted:2021-01-02 📝 Original message:One thing I think should ...

📅 Original date posted:2021-01-02
📝 Original message: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 <https://twitter.com/JeremyRubin>;
<https://twitter.com/JeremyRubin>;


On Wed, Dec 23, 2020 at 1:37 PM Andrew Chow via bitcoin-dev <
bitcoin-dev at 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 at 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 at lists.linuxfoundation.org
> > > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> > >
>
>
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