Subject: Re: [Bitcoin-development] Lexicographical Indexing of Transaction Inputs and Outputs

From: Peter Todd <[email-masked]>
Date: Tue, 9 Jun 2015 16:14:36 -0400

On Mon, Jun 08, 2015 at 06:53:54PM -0400, Kristov Atlas wrote:

Two other things:

> On Sat, Jun 6, 2015 at 10:35 PM, Peter Todd <[email-masked]> wrote:
> 
> > Why mention SIGHASH_SINGLE at all? Its use-case is highly specialized
> > protocols; you haven't taken into account the needs of those protocols.
> > For BIP's it's better to stick to the use-cases where the need is clear
> > and there exists running code that to speculate too much on future uses.
> > With signature hashing in particular it's not yet clear at all what
> > future OP_CHECKSIG's will look like, let alone the various ways people
> > will use sighash for smart contract type stuff.
> >
> > You'd be better off presenting the BIP in terms of a generic statement
> > that "except when otherwise prevented by advanced signature hashing
> > requirements, wallet software must emit transactions sorted according to
> > the following" You can then specify the two common cases in detail:
> >
> > 1) SIGHASH_ALL: input and output order signed, so sort appropriately
> >
> > 2) SIGHASH_ANYONECANPAY: input order not signed, so software should emit
> >    transactions sorted, recognising that the actual mined order may be
> >    changed.
> >
> 
> That makes sense. I updated the language as follows -- your thoughts? Keep
> in mind this BIP is informational, and so people are free to ignore it.
> 
> "Applicability: This BIP applies to all transactions of signature hash type
> SIGHASH_ALL. Additionally,  software compliant with this BIP that allows
> later parties to update the transaction (e.g. using signature hash types
> SIGHASH_NONE or a variant of SIGHASH_ANYONECANPAY) should emit
> lexicographically sorted inputs and outputs, although they may later be
> modified. Transactions that have index dependencies between transactions or
> within the same transaction are covered under the section of this BIP
> entitled “Handling Input/Output Dependencies.”"

I'd keep it even simpler than that, and just say for now that such
use-cases are out of the scope of this BIP, however those standards
should come up with some kind of deterministic standard that meets the
needs of the protocol. Again, there's a bunch of possible use-cases here
and we just can't predict them; focus on the fact that the *spirit* of
what this BIP is about is applicable and future standards should be
developed.

So I'd change the "Applicability" section to:

This BIP applies to all transactions where the order of inputs and
outputs does not matter. This is true for the vast majority of
transactions as they simply move funds from one place to another.

Currently this generally refers to transactions where SIGHASH_ALL is
used, in which case the signatures commit to the exact order of input
and outputs. In the case where SIGHASH_ANYONECANPAY and/or SIGHASH_NONE
has been used (e.g. crowdfunds) the order of inputs and/or outputs may
not be signed, however compliant software should still emit transactions
with sorted inputs and outputs, even though they may later be modified
by others.

In the event that future protocol upgrades introduce new signature hash
types, compliant software should apply the lexographic ordering
principle analogously.

While out of scope of this BIP, protocols that do require a specified
order of inputs/outputs (e.g. due to use of SIGHASH_SINGLE) should
consider the goals of this BIP and how best to adapt them to the
specifics needs of those protocols.

Then remove the "handling input/output deps" section.

> > Do you have a patch implementing deterministic tx ordering for Bitcoin
> > Core yet?
> >
> 
> I'm not a frequent C programmer, so I'd prefer to let someone else take
> care of it, as a frequent committer of code would do a faster and more
> stylistically consistent job of it. If no one else will, however, I will.

re: the actual ordering algorithm, having txids be sorted by with the
hex-based algorithm is odd. I'd simply say they're sorted as
little-endian byte arrays, or in other words, with the bytearr_cmp()
function, but with the order of bytes reversed. You also should say that
we're doing that to make the user see them in visually sorted order to
match expectations because txids are displayed as little-endian.

For outputs, don't say "locking script", say "scriptPubKey". Secondly,
scriptPubKeys are not in little-endian representation - they have no
endianness to them. With output amount, there's no need to say that
they're unsigned or little-endian satoshies, just say they're sorted
largest/smallest amount first.

"For the sake of efficiency, amounts will be considered first for
sorting, since they contain fewer bytes of information (7 bytes)
compared to a standard P2PKH locking script (800 bytes)." <- where the
heck did you get these numbers from? Amounts are 8 bytes, and P2PKH
scriptPubKeys are 25 bytes.

"Backwards Compatibility" <- I'd just remove this whole section; we're
unlikely to make this an IsStandard() rule anytime soon.

-- 
'peter'[:-1]@petertodd.org
0000000000000000127ab1d576dc851f374424f1269c4700ccaba2c42d97e778