Subject: Re: [bitcoin-dev] BIP 174 thoughts

From: Pieter Wuille <[email-masked]>
Date: Thu, 21 Jun 2018 10:39:57 -0700

On Thu, Jun 21, 2018 at 4:29 AM, matejcik <[email-masked]> wrote:
> In the case of everything per-input, the naive Signer can do this:
> 1. (in the global section) pre-serialize the transaction
> 2. (in each input) find and fill out scriptPubKey from the provided UTXO
> 3. (for a given BIP32 path) check if the master fingerprint matches
> mine, if yes, derive secret key, output pubkey, signature
> 4. goto 3 (more keys per input), goto 2 (next input)
>
> Note that this flow works perfectly for multisig; it’s going to be the
> job of a Finalizer to build the final scriptSig, but each input can have
> multiple partial signatures -- and, interestingly, the naive Signer
> doesn’t even need to know about multisig.

Ah, you're thinking of an even simpler signer than I was imagining. I
don't think this works in general, because the hash being signed
depends on the structure of the script. For example, if it is P2SH, it
is the redeemscript that goes into the scriptCode serialization rather
than the scriptPubKey. If it is segwit, BIP143 serialization needs to
be used, etc. It may work if your signing is restricted to just one of
those structures, though.

> A less naive Signer will want to check things, maybe derive a scriptSig
> itself and check if it matches the given hash, etc., but it can do this
> all in place. You go linearly through the signing flow and place a
> couple strategic assertions along the way.

Right - but I think anything but the simplest signer must do this,
just to be able to distinguish between different kinds of signature
hashing.

But you're right, having per-input redeemscript/witnessscript
simplifies things still - instead of needing to look a script hash in
a map, you can just compare it with *the* redeemscript/witnessscript.

> However, if the data is global, as is now, it gets more complicated:
> 1. (in the global section) pre-serialize the transaction, prefill lookup
> tables
> 2. (for a given BIP32 path) check if mine, then derive public key and
> store in a dictionary
> 3. (for each input) find _and parse_ scriptPubKey, extract (PK or)
> script hash
> 4. lookup redeem script based on script-hash; if not found, goto 2; if
> found, parse out public key
> 5. lookup public key in the BIP32 dictionary; if not found, goto 2
> 6. output pubkey, signature

I understand your point now. I hadn't considered the possibility of
just signing with all BIP32 derivation paths given for which the
master matches, instead of extracting pubkeys/pkhs from the script.
That's a major simplification for signers indeed. I do think you need
some conditions before to determine the script structure (see above),
but this is a good point in favour of making the derivation paths
per-input.

> In general, you seem to focus a lot on the role of Combiners, esp.
> simple Combiners. To me, that doesn’t look like a significant role. As I
> envision it, a Combiner really doesn’t need to do anything more
> complicated than merge and deduplicate records, simply based on the
> uniqueness of the whole record.

It's more a side-effect of focusing on forward compatibility. I expect
that we will have transactions with inputs spending different kinds of
outputs, and some signers may not be able to understand all of them.
However, as long as the design goal of having Combiners function
correctly for things they don't understand, everything should be able
to work together fine.

> It’s the Finalizer’s job to reconstruct and validate the result. Also
> ISTM if something messes up the PSBT (such as including multiple
> conflicting fields anywhere), it’s OK to leave it to Finalizer to fail.

Agree.

> An aside to this in particular, I’ve been thinking about the requirement
> to share derivation paths and public keys with the Creator. The spec
> assumes that this will happen; you’re talking about providing full
> xpub+chaincode too. At least, the Creator must prefill BIP32 paths and
> master key fingerprints. Possibly also prefill public keys in the redeem
> scripts?
>
> This might not be an improvement proposal, but a point worth being
> raised and maybe explained in the spec. Perhaps the original Creator
> doesn’t have access to this data, and delegates this to some
> “sub-Creators”  - I imagine a coordinator sending a PSBT to signing
> parties, each of which acts as a sub-Creator (fills out derivation paths
> and public keys) and a Signer (forwarding to a HWW). Some of the
> discussion even suggests some sort of generic “key derivation field”
> with arbitrary contents - fingerprint + bip32 path? xpub + chain code?
> derivation points? encrypted xprv?

That makes sense - I think we've already touched this when discussing
the requirement for UTXOs to be added. Perhaps those aren't added by
the Creator, but by some index server. The same could be true for the
scripts or derivations paths.

And indeed, most of the information in the derivation paths is
effectively opaque to the Creator - it's just some data given out by
the Signer about its keys that gets passed back to it so it can
identify the key. There is benefit in keeping it in a fixed structure
(like xpub/chaincode, or fingerprint + derivation indexes), to
guarantee compatibility between multiple signer implementations with
access to the same key.

On Tue, Jun 19, 2018 at 5:39 PM, Jason Les via bitcoin-dev
<[email-masked]> wrote:
>>Hex encoding?
>
> I was hoping for some standard here was well and I agree using something
> more compact than hex is important. My understanding is Z85 is better for
> use with JSON than Base64, which is probably a good benefit to have here.

Both Base64 and Z85 can be stored in JSON strings without quoting
(neither uses quotation characters or backslashes), but Z85 is
slightly more compact (Z85 is 5 characters for 4 bytes, Base64 is 4
characters for 3 bytes). Both use non-alphanumeric characters, so I
don't think there is much difference w.r.t. copy-pastability either.
Z85 is far less common though.

On Thu, Jun 21, 2018 at 4:44 AM, Tomas Susanka via bitcoin-dev
<[email-masked]> wrote:
>> I think here it makes sense because there can actually only be (up to)
>> one redeemscript and (up to) one witnessscript. So if we made those
>> per-input and per-output, it may simplify signers as they don't need a
>> table lookup to find the correct one. That would also mean we can drop
>> their hashes, even if we keep a key-value model.
> Yes, indeed. Just to clarify: in the first sentence you mean "per
> output", right? There can actually only be (up to) one redeemscript and
> (up to) one witnessscript *per output*.

Up to one per output, and up to one per input - indeed.

On Thu, Jun 21, 2018 at 7:32 AM, Tomas Susanka via bitcoin-dev
<[email-masked]> wrote:
>>> A question to consider is,
>>> will there be more per-output data? If yes, it might make sense to have
>>> an output section.
>> I think it is unlikely that there would be anymore per-output data.
>
> Hmm, upon further reflection, maybe it's not even worth including *any*
> per-output data, aside from what the original transaction contains.
>
> The output redeem script is either:
> - unknown, because we have received only an address from the receiver
> - or it is known, because it is ours and in that case it doesn’t make
> sense to include it in PSBT
>
> We got stuck on the idea of the Creator providing future (output)
> redeem/witness scripts. But that seems to be a minority use case and can
> be solved efficiently via the same channels that coordinate the PSBT
> creation. Sorry to change opinions so quickly on this one.

Perhaps you're missing the reason for having output scripts? It is so
that signers that wish to known the amounts transferred can be told
which outputs of the to-be transaction are change, and thus shouldn't
be counted towards the balance. By providing the scripts and
derivation paths in a PSBT, the Creator can prove to the Signer that
certain outputs do not actually move funds to some other entity.

Based on the points before, my preference is having everything
per-input and per-output except the transaction (with empty
scriptSig/witness) itself, and having exactly one set/map per input
and output (which may include a "finalized scriptSig/witness field"
for finalized inputs). The overhead of having at least one separator
byte for every input and output in the transaction is at most a few
percent compared to the data in the transaction itself. If size is
really an issue (but I think we've already established that small size
gains aren't worth much extra complexity), we could also serialize the
transaction without scriptSigs/witnesses (which are at least one byte
each, and guaranteed to be empty).

I'm unsure about typed record vs. key-value model. If we'd go with a
per-input script approach, the key would just be a single byte ("the
redeemscript" and "the witnessscript"), so the advantage of being able
to drop the script hashes applies equally to both models. After that,
it seems the only difference seems to be that a well-defined prefix of
the records is enforced to be unique as opposed to the entire record
being enforced to be unique. I don't think there is much difference in
complexity, as Combiners and Signers still need to enforce some kind
of uniqueness even in a typed records model.

Cheers,

-- 
Pieter