Date: Sat, 23 Jun 2018 14:27:15 -0400
From: "Peter D. Gray" <peter@coinkite.com>
To: Achow101 <achow101-lists@achow101.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20180623182715.GC893@coinkite.com>
Reply-To: Peter Gray <peter@coinkite.com>
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Organization: Coinkite Cryptobank (www.coinkite.com)
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Subject: Re: [bitcoin-dev] BIP 174 thoughts
Precedence: list


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On Fri, Jun 22, 2018 at 06:28:33PM -0400, Achow101 wrote:
> After reading the comments here about BIP 174, I would like to propose th=
e following changes:
>=20
> - Moving redeemScripts, witnessScripts, and BIP 32 derivation paths to pe=
r-input and per-output data
=2E..

I like this. I agree it's making things easier and it's more flexible.

> - Finalized scriptSig and scriptWitness fields
>=20
> To determine whether two PSBTs are the same, we can compare the unsigned =
transaction. To ensure that the
> unsigned transactions are the same for two PSBTs with data for the same t=
x, we cannot put scriptSigs or
> scriptWitnesses into it. Thus for each input, two new fields have been ad=
ded to store the finalized scriptSig
> and finalized scriptWitness.
=2E..

To be honest, I don't understand the reasons/implications of this change, b=
ut it seems harmless.

> - Mandatory sighash
=2E..

Good improvement.

> - Encoding
>=20
> I have decided that PSBTs should either be in binary or encoded as a Base=
64 string. For the latter, several
> Bitcoin clients already support Base64 encoding of data (for signed messa=
ges) so this will not add any extra
> dependencies like Z85 would.
=2E..

Personally, I don't think you should spec an encoding. It should be binary =
only and hex for developers and JSON interfaces. My thinking is that PSBT's=
 are not user-visible things. They have a short lifetime and are nothing so=
mething that is "stored" or referenced much later. Hex is good enough and h=
as no downsides as an excoding except for density.

On the other hand, suggesting a filename extension (probably .PSBT?) and a =
mime-type to match, are helpful since wallets and such will want to registe=
r with their operating systems to become handlers of those mimetypes. Reall=
y that's a lot more important for interoperability at this point, than an e=
ncoding.

> A draft of the revised BIP can be found here: https://github.com/achow101=
/bips/blob/bip174-rev/bip-0174.mediawiki
> If these changes are satisfactory, I will open a PR to the BIPs repo to u=
pdate the BIP tomorrow. I will also
> create test vectors and update the implementation PR'ed to Core.
=2E..

Looking forward to test vectors, and I might have more to say once my code =
can handle them (again).

Feedback on the BIP as it stands now:=20

- Appendix A needs an update, and I suggest defining symbols (PK_PARTIAL_SI=
G =3D=3D 0x02) for the numeric key values. This helps implementers as we do=
n't all define our own symbols and/or use numeric constants in our code.

- Those tables are just not working. Might want to reformat as descriptive =
lists, point form, or generally anything else... sorry.

> Andrew
> _______________________________________________

---
Peter D. Gray  ||  Founder, Coinkite  ||  Twitter: @dochex  ||  GPG: A3A31B=
AD 5A2A5B10

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