[bitcoin-dev] BIP 174 thoughts

matejcik jan.matejek at satoshilabs.com
Tue Jun 19 14:22:30 UTC 2018

this is our second e-mail with replies to Pieter's suggestions.

On 16.6.2018 01:34, pieter.wuille at gmail.com (Pieter Wuille) wrote:
> * Key-value map model or set model.
> This was suggested in this thread:
> https://twitter.com/matejcik/status/1002618633472892929
> The motivation behind using a key-value model rather than a simple
> list of records was that PSBTs can be duplicated (given to multiple
> people for signing, for example), and merged back together after
> signing. With a generic key-value model, any implementation can remove
> the duplication even if they don't understand fields that may have
> been added in future extensions.
> However, almost the same can be accomplished by using the simpler set
> model (the file consists of a set of records, with no duplication
> allowed). This would mean that it would technically be legal to have
> two partial signatures with the same key for the same input, if a
> non-deterministic signer is used.

Strongly agree with this.

Just to note, we should probably use varint for the <type> field - this
allows us, e.g., to create “namespaces” for future extensions by using
one byte as namespace identifier and one as field identifier.

> On the other hand, this means that certain data currently encoded
> inside keys can be dropped, reducing the PSBT size. This is
> particularly true for redeemscripts and witnessscripts, as they can
> just be computed by the client when deserializing. The two types could
> even be merged into just "scripts" records - as they don't need to be
> separated based on the way they're looked up (Hash160 for P2SH, SHA256
> for P2WSH). The same could be done for the BIP32 derivation paths,
> though this may be expensive, as the client would need to derive all
> keys before being able to figure out which one(s) it needs.

It could be nice if the output scripts records would be ordered the same
as their corresponding outputs. But what if the Creator doesn’t want to
include a script for an output? Perhaps the Script record should have a
<vout> field to match it to the appropriate output.

As for input scripts, we suggest that they are per-input and not
included in the global record, see the other thread.

> One exception is the "transaction" record, which needs to be unique.
> That can either be done by adding an exception ("there can only be one
> transaction record"), or by encoding it separately outside the normal
> records (that may also be useful to make it clear that it is always
> required).

This seems to be the case for some fields already - i.e., an input field
must have exactly one of Non-witness UTXO or Witness Output. So “adding
an exception” is probably just a matter of language?

We’d also like to note that the “number of inputs” field should be
mandatory - and as such, possibly also a candidate for outside-record field.

> * Ability for Combiners to verify two PSBT are for the same transaction
> Clearly two PSBTs for incompatible transactions cannot be combined,
> and this should not be allowed.
> It may be easier to enforce this if the "transaction" record inside a
> PSBT was required to be in a canonical form, meaning with empty
> scriptSigs and witnesses. In order to do so, there could be per-input
> records for "finalized scriptSig" and "finalized witness". Actually
> placing those inside the transaction itself would only be allowed when
> all inputs are finalized.

Agreed! Also increases clarity, which is desired.

> * Derivation from xpub or fingerprint
> For BIP32 derivation paths, the spec currently only encodes the 32-bit
> fingerprint of the parent or master xpub. When the Signer only has a
> single xprv from which everything is derived, this is obviously
> sufficient. When there are many xprv, or when they're not available
> indexed by fingerprint, this may be less convenient for the signer.
> Furthermore, it violates the "PSBT contains all information necessary
> for signing, excluding private keys" idea - at least if we don't treat
> the chaincode as part of the private key.
> For that reason I would suggest that the derivation paths include the
> full public key and chaincode of the parent or master things are
> derived from. This does mean that the Creator needs to know the full
> xpub which things are derived from, rather than just its fingerprint.

We don’t understand the rationale for this idea. Do you see a scenario
where an index on master fingerprint is not available but index by xpubs
is? In our envisioned use cases at least, indexing private keys by xpubs
(as opposed to deriving from a BIP32 path) makes no sense.

Maybe this folds into the proposal for generic derivation below, or
something like implementation-specific derivation methods?

best regards

Jan Matejek
Tomas Susanka

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