Re: [bitcoin-dev] BIP 158 Flexibility and Filter Size

[Gregory Maxwell <greg@xiph.org>]

On Fri, Jun 8, 2018 at 5:03 AM, Olaoluwa Osuntokun via bitcoin-dev
<bitcoin-dev@lists.linuxfoundation.org> wrote:
> As someone who's written and reviews code integrating the proposal all the
> way up the stack (from node to wallet, to application), IMO, there's no
> immediate cost to deferring the inclusion/creation of a filter that includes
> prev scripts (b) instead of the outpoint as the "regular" filter does now.
> Switching to prev script in the _short term_ would be costly for the set of
> applications already deployed (or deployed in a minimal or flag flip gated
> fashion) as the move from prev script to outpoint is a cascading one that
> impacts wallet operation, rescans, HD seed imports, etc.

It seems to me that you're making the argument against your own case
here: I'm reading this as a "it's hard to switch so it should be done
the inferior way".  That in argument against adopting the inferior
version, as that will contribute more momentum to doing it in a way
that doesn't make sense long term.

> Such a proposal would need to be generalized enough to allow several components to be committed,

I don't agree at all, and I can't see why you say so.

> likely have versioning,

This is inherent in how e.g. the segwit commitment is encoded, the
initial bytes are an identifying cookies. Different commitments would
have different cookies.

> and also provide the necessary extensibility to allow additional items to be committed in the future

What was previously proposed is that the commitment be required to be
consistent if present but not be required to be present.  This would
allow changing whats used by simply abandoning the old one.  Sparsity
in an optional commitment can be addressed when there is less than
100% participation by having each block that includes a commitment
commit to the missing filters ones from their immediate ancestors.

Additional optionality can be provided by the other well known
mechanisms,  e.g. have the soft fork expire at a block 5 years out
past deployment, and continue to soft-fork it in for a longer term so
long as its in use (or eventually without expiration if its clear that
it's not going away).

> wallets which wish to primarily use the filters for rescan purposes can't
> just construct them locally for this particular use case independent of
> what's currently deployed on the p2p network.

Absolutely, but given the failure of BIP37 on the network-- and the
apparent strong preference of end users for alternatives that don't
scan (e.g. electrum and web wallets)-- supporting making this
available via P2P was already only interesting to many as a nearly
free side effect of having filters for local scanning.  If it's a
different filter, it's no longer attractive.

It seems to me that some people have forgotten that this whole idea
was originally proposed to be a committed data-- but with an added
advantage of permitting expirementation ahead of the commitment.

> Maintaining the outpoint also allows us to rely on a "single honest peer"security model in the short term.

You can still scan blocks directly when peers disagree on the filter
content, regardless of how the filter is constructed-- yes, it uses
more bandwidth if you're attacked, but it makes the attack ineffective
and using outpoints considerably increases bandwidth for everyone
without an attack.  These ineffective (except for increasing
bandwidth) attacks would have to be common to offset the savings. It
seems to me this point is being overplayed, especially considering the
current state of non-existing validation in SPV software (if SPV
software doesn't validate anything else they could be validating, why
would they implement a considerable amount of logic for this?).