Subject: Re: [bitcoin-dev] Altruistic Rebroadcasting - A Partial Replacement Cycling Mitigation

From: Antoine Riard <[email-masked]>
Date: Thu, 21 Dec 2023 21:59:04 +0000

Hi Peter,

> Obviously a local replacement cache is a possibility. The whole point of
my
> email is to show that a local replacement cache isn't necessarily needed,
as
> any alturistic party can implement that cache for all nodes.

Sure, note as soon as you start to introduce an altruistic party
rebroadcasting for everyone in the network, we're talking about a different
security model for time-sensitive second-layers. And unfortunately, one can
expect the same dynamics we're seeing with BIP157 filter servers, a handful
of altruistic nodes for a large swarm of clients.

> 1) That is trivially avoided by only broadcasting txs that meet the local
> mempool min fee, plus some buffer. There's no point to broadcasting txs
that
> aren't going to get mined any time soon.
>
> 2) BIP-133 feefilter avoids this as well, as Bitcoin Core uses the
feefilter
> P2P message to tell peers not to send transactions below a threshold fee
rate.
>
> https://github.com/bitcoin/bips/blob/master/bip-0133.mediawiki

I know we can introduce BIP-133 style of filtering here only the top % of
the block template is altruistically rebroadcast. I still think this is an
open question how a second-layer node would discover the "global" mempool
min fee, and note an adversary might inflate the top % of block template to
prevent rebroadcast of malicious HTLC-preimage.

> Did you actually read my email? I worked out the budget required in a
> reasonable worst case scenario:

Yes. I think my uncertainty lies in the fact that an adversary can
rebroadcast _multiple_ times the same UTXO amount under different txids,
occupying all your altruistic bandwidth. Your economic estimation makes an
assumption per-block (i.e 3.125 BTC / 10 minutes). Where it might be
pernicious is that an adversary should be able to reuse those 3.125 BTC of
value for each inter-block time.

Of course, you can introduce an additional rate-limitation per-UTXO, though
I think this is very unsure how this rate-limit would not introduce a new
pinning vector for "honest" counterparty transaction.

> Here, we're talking about an attacker that has financial resources high
enough
> to possibly do 51% attacks. And even in that scenario, storing the entire
> replacement database in RAM costs under $1000

> The reality is such an attack would probably be limited by P2P bandwidth,
not
> financial resources, as 2MB/s of tx traffic would likely leave mempools
in an
> somewhat inconsistent state across the network due to bandwidth
limitations.
> And that is *regardless* of whether or not anyone implements alturistic tx
> broadcasting.

Sure, I think we considered bandwidth limitations in the design of the
package relay. Though see my point above, the real difficulty in your
proposed design sounds to me in the ability to reuse the same UTXO
liquidity for an unbounded number of concurrent replacement transactions
exhausting all the altruistic bandwidth.

One can just use a 0.1 BTC UTXO and just fan-out 3.125 BTC of concurrent
replacement transactions from then. So I don't think the assumed adversary
financial resources are accurate.

I think the best long-term way to fix replacement cycling is still more in
the direction of things like:
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2023-December/022191.html

With the additional constraint of removing package malleability, where all
the feerate is coming from package self-contained fee-bumping reserves
UTXOs.

Best,
Antoine

Le dim. 17 déc. 2023 à 10:57, Peter Todd <[email-masked]> a écrit :

> On Fri, Dec 15, 2023 at 10:29:22PM +0000, Antoine Riard wrote:
> > Hi Peter,
> >
> > > Altruistic third parties can partially mitigate replacement cycling(1)
> > attacks
> > > by simply rebroadcasting the replaced transactions once the replacement
> > cycle
> > > completes. Since the replaced transaction is in fact fully valid, and
> the
> > > "cost" of broadcasting it has been paid by the replacement
> transactions,
> > it can
> > > be rebroadcast by anyone at all, and will propagate in a similar way to
> > when it
> > > was initially propagated. Actually implementing this simply requires
> code
> > to be
> > > written to keep track of all replaced transactions, and detect
> > opportunities to
> > > rebroadcast transactions that have since become valid again. Since any
> > > interested third party can do this, the memory/disk space requirements
> of
> > > keeping track of these replacements aren't important; normal nodes can
> > continue
> > > to operate exactly as they have before.
> >
> > I think there is an alternative to altruistic and periodic rebroadcasting
> > still solving replacement cycling at the transaction-relay level, namely
> > introducing a local replacement cache.
> >
> > https://github.com/bitcoin/bitcoin/issues/28699
> >
> > One would keep in bounded memory a list of all seen transactions, which
> > have entered our mempool at least once at current mempool min fee.
>
> Obviously a local replacement cache is a possibility. The whole point of my
> email is to show that a local replacement cache isn't necessarily needed,
> as
> any alturistic party can implement that cache for all nodes.
>
> > For the full-nodes which cannot afford extensive storage in face of
> > medium-liquidity capable attackers, could imagine replacement cache nodes
> > entering into periodic altruistic rebroadcast. This would introduce a
> > tiered hierarchy of full-nodes participating in transaction-relay. I
> think
> > such topology would be more frail in face of any sybil attack or scarce
> > inbound slots connections malicious squatting.
> >
> > The altruistic rebroadcasting default rate could be also a source of
> > amplification attacks, where there is a discrepancy between the feerate
> of
> > the rebroadcast traffic and the current dynamic mempool min fee of the
> > majority of network mempools. As such wasting bandwidth for everyone.
>
> 1) That is trivially avoided by only broadcasting txs that meet the local
> mempool min fee, plus some buffer. There's no point to broadcasting txs
> that
> aren't going to get mined any time soon.
>
> 2) BIP-133 feefilter avoids this as well, as Bitcoin Core uses the
> feefilter
> P2P message to tell peers not to send transactions below a threshold fee
> rate.
>
> https://github.com/bitcoin/bips/blob/master/bip-0133.mediawiki
>
> > Assuming some medium-liquidity or high-liquidity attackers might reveal
> any
> > mitigation as insufficient, as an unbounded number of replacement
> > transactions might be issued from a very limited number of UTXOs, all
> > concurrent spends. In the context of multi-party time-sensitive protocol,
> > the highest feerate spend of an "honest" counterparty might fall under
> the
> > lowest concurrent replacement spend of a malicious counterparty,
> occupying
> > all the additional replacement cache storage.
>
> Did you actually read my email? I worked out the budget required in a
> reasonable worst case scenario:
>
> > > Suppose the DoS attacker has a budget equal to 50% of the total block
> > > reward.
> > > That means they can spend 3.125 BTC / 10 minutes, or 520,833sats/s.
> > >
> > >     520,833 sats/s
> > >     -------------- = 2,083,332 bytes / s
> > >     0.25 sats/byte
> > >
> > > Even in this absurd case, storing a one day worth of replacements would
> > > require
> > > just 172GB of storage. 256GB of RAM costs well under $1000 these days,
> > > making
> > > altruistic rebroadcasting a service that could be provided to the
> network
> > > for
> > > just a few thousand dollars worth of hardware even in this absurd case.
>
> Here, we're talking about an attacker that has financial resources high
> enough
> to possibly do 51% attacks. And even in that scenario, storing the entire
> replacement database in RAM costs under $1000
>
> The reality is such an attack would probably be limited by P2P bandwidth,
> not
> financial resources, as 2MB/s of tx traffic would likely leave mempools in
> an
> somewhat inconsistent state across the network due to bandwidth
> limitations.
> And that is *regardless* of whether or not anyone implements alturistic tx
> broadcasting.
>
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>