Re: [bitcoin-dev] Bitcoin Core to disable Bloom-based Filtering by default

From: Matt Corallo <lf-lists@mattcorallo•com>
To: Andreas Schildbach <andreas@schildbach•de>,
	Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Bitcoin Core to disable Bloom-based Filtering by default
Date: Mon, 22 Jul 2019 05:01:58 +0000	[thread overview]
Message-ID: <72747be0-3760-a602-a540-37be6e4ad94c@mattcorallo.com> (raw)
In-Reply-To: <qh2qj1$7sg4$1@blaine.gmane.org>

Hey Andreas,

I think maybe some of the comments here were misunderstood - I don't
anticipate that most people will change their defaults, indeed, but
given the general upgrade cycles we've seen on the network over the
entire course of Bitcoin's history, there's little reason to believe
that many nodes with NODE_BLOOM publicly accessible will be around for
at least three or four years to come, though obviously any conscious
effort by folks who need those services to run nodes could extend that
significantly.

As for the DoS issues, a super old Proof-of-Concept of the I/O variant
is here: https://github.com/petertodd/bloom-io-attack though CPU DoS
attacks are also possible that use high hash counts to fill a node's CPU
usage (you can pretty trivially see when a bloom-based peer connects to
you just by looking at top...).

Finally, regarding alternatives, the filter-generation code for BIP
157/158 has been in Bitcoin Core for some time, though the P2P serving
side of things appears to have lost any champions working on it. I
presume one of the Lightning folks will eventually, given they appear to
be requiring their users connect to a handful of their own servers right
now, but if you really need it, its likely not a ton of work to pipe
them through.

Matt

On 7/21/19 10:56 PM, Andreas Schildbach via bitcoin-dev wrote:
> An estimated 10+ million wallets depend on that NODE_BLOOM to be
> updated. So far, I haven't heard of an alternative, except reading all
> transactions and full blocks.
> 
> It goes without saying pulling the rug under that many wallets is a
> disastrous idea for the adoption of Bitcoin.
> 
>> well-known DoS vectors
> 
> I asked many people, even some "core developers" at meetings, but nobody
> ever was able to explain the DoS vector. I think this is just a myth.
> 
> Yes, you can set an overly blurry filter and thus cause useless traffic,
> but it never exceeds just drinking from the full firehose (which this
> change doesn't prohibit). So where is the point? An attacker will just
> switch filtering off, or in fact has never used it.
> 
>> It is not anticipated that
>> this will result in a significant lack of availability of
>> NODE_BLOOM-enabled nodes in the coming years
> 
> Why don't you anticipate that? People almost never change defaults,
> especially if it's not for their own immediate benefit. At the same
> time, release notes in general recommend updating to the latest version.
> I *do* anticipate this will reduce the number of nodes usable by a large
> enough amount so that the feature will become unstable.
> 
>> clients
>> which rely on the availability of NODE_BLOOM-supporting nodes on the
>> P2P network should consider the process of migrating
>> to a more modern (and less trustful and privacy-violating) alternative
>> over the coming years.
> 
> There is no such alternative.
> 
> I strongly recommend postponing this change until an alternative exists
> and then give developers enough time to implement, test and roll out.
> 
> I also think as long as we don't have an alternative, we should improve
> the current filtering for segwit. E.g. testing the scripts themselves
> and each scriptPubKey spent by any input against the filter would do,
> and it also fixes the main privacy issue with server-side filtering
> (wallets have to add two items per address to the filter).
> 
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