Re: [bitcoin-dev] BIP 174 thoughts

[Achow101 <achow101-lists@achow101.com>]

Hi all,

After reading the comments here about BIP 174, I would like to propose the following changes:

- Moving redeemScripts, witnessScripts, and BIP 32 derivation paths to per-input and per-output data

I think that by moving these three fields into input and output specific maps, the format will be
easier to read and simpler for signers to parse. Instead of having to be able to parse entire
scripts and extract pubkeys, the signer can simply look at which pubkeys are provided in the inputs
and sign the input based upon the presence of a pubkey for which the signer has a privkey.

A neat trick that fits well with this model is that a plain pubkey (one that is not part of a BIP 32
derivation) can still be put in a BIP 32 derivation path field where the value is just the fingerprint
of the pubkey itself. This would indicate that no derivation needs to be done from the master key, and
the master key is just the specified key itself.

Additionally, by having the redeemScript and witnessScript readily available in the input, signers
do not need to construct a map to find a redeemScript or witnessScript and can instead just look
directly in the input data. There is also no need to include the hashes of these scripts, so the key
is just the type. This also allows us to enforce the requirement for only one redeemScript and one
witnessScript per input easily by continuing to follow the generic rule of unique keys.

By using input specific and output specific fields, there is no need for the input index and the input
count types as all inputs will be accounted for.

- Finalized scriptSig and scriptWitness fields

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 tx, we cannot put scriptSigs or
scriptWitnesses into it. Thus for each input, two new fields have been added to store the finalized scriptSig
and finalized scriptWitness.

- Mandatory sighash

The sighash type field will be changed from a recommendation to a requirement. Signatures will need to 
use the specified sighash type for that input. If a Signer cannot sign for a particular sighash type, it
must not add a partial signature.

- Encoding

I have decided that PSBTs should either be in binary or encoded as a Base64 string. For the latter, several
Bitcoin clients already support Base64 encoding of data (for signed messages) so this will not add any extra
dependencies like Z85 would.


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 update the BIP tomorrow. I will also
create test vectors and update the implementation PR'ed to Core.

Andrew