Subject: Re: [bitcoin-dev] [Bitcoin-development] [RFC] Canonical input and output ordering in transactions

From: Kristov Atlas <[email-masked]>
Date: Wed, 24 Jun 2015 18:09:32 -0400

Following up on this topic...

gmaxwell has reserved BIP 69 for my proposal.

It has been implemented by Electrum in v2.3.2:
https://github.com/spesmilo/electrum/blob/master/RELEASE-NOTES

Rusty has kindly tweaked his original canonical ordering proposal
implementation for Bitcoin Core's wallet client to fit my proposal:
https://github.com/rustyrussell/bitcoin/tree/bip-69 (needs testing)

Outstanding objections appear to me to boil down to two points:

1) Some transactions cannot comply with this BIP because there are input
and/or put index dependencies.

My response: No big deal, it's informational. They simply won't be
compliant with the BIP, and that's fine with me.

2) If we set a standard now for transactions that is not apparently random
ordering from the perspective of passive blockchain observers, transactions
that can't comply with this BIP will stand out. Also, if we change our
collective minds in the future about how ordering should be handled, those
future transactions would stand out. Therefore, the "safe" course of action
is to come up with another scheme that appears to be random ordering from
the perspective of a passive blockchain observer.

My response: Apparently-random but owner-verifiable ordering is doable.
Discussion of this has revolved around what I have called a "sorting key"
-- sort lexicographically, and then reorder according to the bits in a
sorting key that is impossible to predict by an attacker. This means
passive observers cannot determine anything meaningful about the
transaction (e.g. which output is change, information leaked based on utxo
selection algorithm for inputs, etc.) but the owner of the funds and the
sorting key can verify that his transaction matches the canonical
specification. Ideally, I think the key should rotate for each transaction
to avoid the possibility that a static key can link multiple transactions
together. The key should be rotated in such a fashion that the next
iteration is not predictable to anyone except the key holder (e.g. put the
key through a secure pseudo-random function for each new iteration). This
could be done by generating a few bytes of entropy upon wallet creation and
keeping track of the current iteration of rotation. HD wallets could derive
the initial state of the sorting key by deriving it from the HD seed. There
are a variety of schemes that could work here.

My main objection to this family of approaches at present is complexity. I
suspect that many wallet clients will not want to implement the BIP if they
have to maintain a sorting key.

A second objection is that no one will be able to detect anomalies in BIP
compliance except for the sorting key holder. Most users probably will not
bother to verify this. For code reviewers, this means that the sorting key
is yet another aspect of the code base that must be scrutinized to ensure
it is not being used as a covert channel or has been underhandedly weakened
in some fashion.

Also, I will mention an ancillary benefit of a non-random canonical
ordering: it makes unit testing of transactions for Bitcoin wallets simpler.

Given all of the above, I will reiterate my preference to keep the proposal
as it is now. The pull request is here:
https://github.com/bitcoin/bips/pull/157

If there is market demand for it, a separate sorting key-based proposal
could be written which can compete with this BIP and over time successfully
deprecate it. I would currently envision that as an HD BIP with a new
purpose code.

-Kristov

On Mon, Jun 15, 2015 at 12:01 AM, Kristov Atlas <
[email-masked]> wrote:

> On Sun, Jun 14, 2015 at 7:02 PM, Gregory Maxwell <[email-masked]>
> wrote:
>
>> I'm not a great fan of this proposal for two reasons: The first is
>> that the strict ordering requirements is incompatible with future
>> soft-forks that may expose additional ordering constraints. Today we
>> have _SINGLE, which as noted this interacts with poorly, but there
>> have been other constraints proposed that this would also interact
>> with poorly.
>>
>
> I'm not clear on why this is a problem, so long as the canonical ordering
> BIP is *optional*. Unless there is a specific plan to soft fork a change
> that would break the BIP and it is fairly imminent, I see this only as a
> reason not to integrate it into isStandard().
>
>
>> The second is that even absent consensus rules there may be invisible
>> constraints in systems-- e.g. hardware wallets that sign top down, or
>> future transaction covenants that have constraints about ordering,  or
>> proof systems that use (yuck) midstate compression for efficiency. Right
>> now, with random ordering these applications are fairly
>> indistinguishable from other random uses (since their imposed order
>> could come about by chance) but if everyone else was ordered, even if
>> wasn't enforced.. these would be highly distinguishable. Which would
>> be unfortunate.
>
>
> Maybe they shouldn't be doing that. :-P
>
>
>> I think perhaps the motivations here are understated. We have not seen
>> any massive deployments of accidentally broken ordering that I'm aware
>> of-- and an implementation that got this wrong in a harmful way would
>> likely make far more fatal mistakes (e.g. non random private keys).
>>
>
> In my reading of various wallet client sources, it is common that wallet
> clients will use cryptographically weak sources of randomness to sort
> outputs -- that is, the ones that actually bother to randomly sort. I can
> hunt down some examples if this would substantially contribute to the
> discussion.
>
> As an alternative to this proposal the ordering can be privately
>> derandomized in the same way DSA is, to avoid the need for an actual
>> number source.  If getting the randomness right were really the only
>> motivation, I'd suggest we propose a simple derandomized randomization
>> algorithm--- e.g. take the order from (H(input ids||client secret)).
>>
>
> This sounds similar to an idea that Sergio pitched to me privately, which
> was for wallets to have a private sorting key that they can use to sort
> inputs and outputs. However, I suspect that adding yet another key which
> will necessarily require special key rotation rules and maybe special
> backup procedures will mean that this standard will not be widely adopted
> any time soon. Ideally, I'd like to see someone write a different BIP with
> the sorting key idea and let them compete in the wallet client market
> rather than trying to anticipate what is best for all clients and
> distilling two good ideas into one artificially.
>
> -Kristov
>
>