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Date: Fri, 19 Jul 2024 05:41:06 -0700 (PDT)
From: /dev /fd0 <alicexbtong@gmail.com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
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Subject: [bitcoindev] Re: A "Free" Relay Attack Taking Advantage of The Lack
 of Full-RBF In Core
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Hi Peter,

> I didn't get a substantive
> response from Bitcoin Core, other than Core closing the my pull-req=20
enabling
> full-RBF by default that would fix this specific vulnerability.

The last comment in the pull request suggests opening a new pull request to=
=20
enable full RBF by default, referencing the one closed due to off-topic=20
comments.=20

> But read on, this is quite an odd case of Core politics, and the story is=
=20
not
> as simple as Core refusing to fix a vulnerability.

It seems that you are the one trying to politicize this issue.=20

/dev/fd0
floppy disk guy

On Thursday, July 18, 2024 at 4:04:26=E2=80=AFPM UTC Peter Todd wrote:

> # Summary
>
> This is a public disclosure of a vulnerability that I previously disclose=
d=20
> to
> the bitcoin-security mailing list. It's an easy vulnerability to fix.=20
> Although
> as with other "free" relay attacks I've disclosed, I didn't get a=20
> substantive
> response from Bitcoin Core, other than Core closing the my pull-req=20
> enabling
> full-RBF by default that would fix this specific vulnerability.
>
> But read on, this is quite an odd case of Core politics, and the story is=
=20
> not
> as simple as Core refusing to fix a vulnerability. Also, I've including a=
=20
> fun
> homework problem at the end: figure out how TRUC/V3 transactions itself=
=20
> creates
> a "free" relay attack.
>
>
> # Background
>
> This is just one of a few "free" relay attacks that I have recently=20
> disclosed,
> including, but not limited to:
>
> "A Free-Relay Attack Exploiting RBF Rule #6" - Mar 18th 2024
> https://groups.google.com/g/bitcoindev/c/EJYoeNTPVhg
>
> "A Free-Relay Attack Exploiting Min-Relay-Fee Differences" - Mar 31st 202=
4
> https://groups.google.com/g/bitcoindev/c/3XqfIOYzXqo
>
> The term "free relay attack" simply refers to any mechanism where=20
> transaction
> data can be broadcast at unusually low cost; the "free" in "free relay" i=
s=20
> a
> misnomer as all these attacks do in fact have some cost.
>
> This particular attack isn't significantly different than the other attac=
ks
> I've disclosed. With one important exception: unlike those other attacks,
> fixing this particular attack would be quite easy, by enabling full-rbf b=
y
> default. So I disclosed it to the bitcoin-security mailing list as a test=
:=20
> does
> Bitcoin Core actually care about free relay attacks? My hypothesis is tha=
t=20
> Core
> does not, as they know full well that "free" relay is an unavoidable=20
> problem;
> I've received absolutely no feedback from any Bitcoin Core members for th=
e
> other disclosed attacks, beyond achow using my disclosure of the RBF Rule=
=20
> #6
> attack as an excuse to remove me from the bitcoin-security mailing list.
>
> The fact that Core doesn't actually care about "free" relay attacks is=20
> relevant
> to TRUC/V3 Transactions. As per BIP-431:
>
> "The primary problem with [RBFR proposals] is the potential for free rela=
y=20
> and DDoS attacks.
>
> Removing Rule 3 and 4 in general would allow free relay [27]."
>
> https://github.com/bitcoin/bips/blob/812907c2b00b92ee31e2b638622a4fe14a42=
8aee/bip-0431.mediawiki#user-content-Alternatives_replace_by_feerate
>
> I believe the authors of that BIP are fully aware of the fact that "free"=
=20
> relay
> is an unavoidable problem, making their rational for TRUC/V3 bogus, and=
=20
> don't
> want to admit that they've wasted a large amount of engineering time on a=
=20
> bad
> proposal. I will be submitting a pull-req to get BIP-431 corrected, as th=
e=20
> many
> "free" relay attacks I've disclosed clearly show that claiming RBFR would
> "allow" free relay is simply not true.
>
> Notably, full-RBF is _itself_ a transaction pinning fix for many use-case=
s;
> part of the TRUC/V3 standard is to force full-RBF behavior for V3=20
> transactions.
> So Core closing my full-RF pull-req is doubling down on TRUC/V3 in a seco=
nd
> way, and TRUC/V3 proponents were the ones who tried to get the full-RBF=
=20
> option
> removed from Core in the first place. If not for this dumb bit of Core
> politics, I'm sure my year-old pull-req to enable full-RBF by default wou=
ld
> have been merged many months ago, as almost all hashpower has adopted=20
> full-RBF
> making objections based on "zeroconf" absurd.
>
>
> # The Attack
>
> If you're a competent Bitcoin engineer, familiar with how mempools work,=
=20
> you've
> probably figured it out already based on the title: obviously, if a high
> percentage of miners are adopting a policy that Bitcoin Core nodes are=20
> not, you
> can cheaply consume transaction relay bandwidth by simply relaying=20
> transations
> that miners are rejecting.
>
> Specifically, do the following:
>
> 1. Broadcast a small, low-fee-rate, tx A with BIP-125 opt-in disabled.
> 2. Broadcast a full-RBF double-spend of A, A2, with a higher fee-rate.
> 3. Spend the outputs of A in a large, low fee-rate, transaction B with=20
> BIP-125
> opt-in enabled. ~100% of miners will reject B, as it spends an input not =
in
> their mempools. However Bitcoin Core nodes will waste bandwidth propagati=
ng
> B.
> 4. (Optional) Double-spend B repeatedly. Again, Bitcoin Core nodes will=
=20
> waste
> bandwidth propagating Bn's that ~100% of miners are ignoring.
> 5. Double-spend A2 to recover your funds and do it all over again (or if=
=20
> A2 had
> a high enough fee-rate, just wait for it to be mined).
>
> The cost to relay each B transaction depends on the fee-rate of B. Since
> Bitcoin Core defaults to a fairly large mempool, the minimum relay=20
> fee-rate is
> typically well below the economic fee-rate required for miners to actuall=
y=20
> mine
> a transaction; Core accepts transactions that are uneconomical for miners=
=20
> to
> mine for the forseeable future.
>
> For example, at the moment typical mempools require transactions to pay a=
t
> least 1sat/vB, while there are hundreds of MvB worth of transactions payi=
ng
> 4sat/vB, the minimum economical fee-rate. Thus, transactions paying less=
=20
> than
> 4sat/VB are extremely unlikely to get mined in the nearish future.
>
> Concretely, broadcasting B transactions at 1sat/vB, 2sat/vB, and 3sat/vB=
=20
> would
> have almost zero cost as the probability of those transactions getting=20
> mined is
> nearly zero. This is true _regardless_ of what % of miners are mining=20
> full-RBF!
> As long as you can get at least one miner to mine the A double-spend, the
> attack only costs what it cost to get A mined.
>
> If B's are broadcast at a higher fee-rate than the minimum economical=20
> fee-rate,
> then the % of full-RBF miners matters. For example, if only 99% of miners=
=20
> mine
> full-RBF, the chance of a B transaction getting mined per block is about=
=20
> 1%, so
> the amortized cost of broadcasting B is about 1% of whatever total fee th=
e
> highest fee-rate variant of B pays.
>
> For an attacker who does not need any B to be broadcast, the cost savings=
=20
> to
> use of relay bandwidth is approximately the ratio of the difference in si=
ze
> between B and and A. With a maximum standard transaction size of 100KvB, =
or
> 400KB serialized size, this ratio is on the order of 5000:1, times the=20
> total
> number of B variants broadcast, and the % chance of each B being mined;=
=20
> it's a
> few orders of magnitude.
>
> Of course, as mentioned above, this is just one of *many* "free" relay=20
> attacks,
> so fixing this particular issue doesn't change much.
>
>
> # Attackers Who Benefit From B Getting Mined
>
> Some attackers actually need B to get mined. For example, imagine an=20
> exchange
> who needs to do large consolidation transactions. They could use this=20
> attack
> (and some attacks like it) as a way to goad users and miners into mining
> consolidation transactions for them at low cost. In this variant of the=
=20
> attack,
> the attacker would pad the size of B with consolidation spends that they=
=20
> needed
> to do anyway. Someone who tried to stop the attack by getting B mined (eg=
=20
> via
> mempool.space's transaction accellerator) would simply be paying the=20
> attacker's
> fees for them.
>
> Obviously, this strategy is only relevant for B's below the economic=20
> fee-rate.
> However, the weaker version of this strategy is to parallize the attack,=
=20
> and do
> your consolidation with the _A_ double-spends to reduce the # of bytes=20
> used per
> full-rbf double-spend.
>
>
> # TRUC/V3 Creates a Free Relay Attack
>
> I'll leave the details of this as a homework problem. But obviously, the
> introduction of TRUC/V3 transactions *itself* creates a free relay attack=
=20
> very
> similar to the above! Just like full-RBF, not all miners will mine V3
> transactions. So you can do the exact same type of attack by taking=20
> advantage
> of this difference in mining policy.
>
> --=20
> https://petertodd.org 'peter'[:-1]@petertodd.org
>

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Hi Peter,<br /><br />&gt;=C2=A0I didn't get a substantive<br />&gt; respons=
e from Bitcoin Core, other than Core closing the my pull-req enabling<br />=
&gt; full-RBF by default that would fix this specific vulnerability.<br /><=
br />

The last comment in the pull request suggests opening a new pull request to=
 enable full RBF by default, referencing the one closed due to off-topic co=
mments.

<div><br /></div><div>&gt;=C2=A0But read on, this is quite an odd case of C=
ore politics, and the story is not<br />&gt; as simple as Core refusing to =
fix a vulnerability.</div><div><br /></div><div>

It seems that you are the one trying to politicize this issue.

</div><div><br /></div><div>/dev/fd0</div><div>floppy disk guy<br /><br /><=
/div><div class=3D"gmail_quote"><div dir=3D"auto" class=3D"gmail_attr">On T=
hursday, July 18, 2024 at 4:04:26=E2=80=AFPM UTC Peter Todd wrote:<br/></di=
v><blockquote class=3D"gmail_quote" style=3D"margin: 0 0 0 0.8ex; border-le=
ft: 1px solid rgb(204, 204, 204); padding-left: 1ex;"># Summary
<br>
<br>This is a public disclosure of a vulnerability that I previously disclo=
sed to
<br>the bitcoin-security mailing list. It&#39;s an easy vulnerability to fi=
x. Although
<br>as with other &quot;free&quot; relay attacks I&#39;ve disclosed, I didn=
&#39;t get a substantive
<br>response from Bitcoin Core, other than Core closing the my pull-req ena=
bling
<br>full-RBF by default that would fix this specific vulnerability.
<br>
<br>But read on, this is quite an odd case of Core politics, and the story =
is not
<br>as simple as Core refusing to fix a vulnerability. Also, I&#39;ve inclu=
ding a fun
<br>homework problem at the end: figure out how TRUC/V3 transactions itself=
 creates
<br>a &quot;free&quot; relay attack.
<br>
<br>
<br># Background
<br>
<br>This is just one of a few &quot;free&quot; relay attacks that I have re=
cently disclosed,
<br>including, but not limited to:
<br>
<br>    &quot;A Free-Relay Attack Exploiting RBF Rule #6&quot; - Mar 18th 2=
024
<br>    <a href=3D"https://groups.google.com/g/bitcoindev/c/EJYoeNTPVhg" ta=
rget=3D"_blank" rel=3D"nofollow" data-saferedirecturl=3D"https://www.google=
.com/url?hl=3Den&amp;q=3Dhttps://groups.google.com/g/bitcoindev/c/EJYoeNTPV=
hg&amp;source=3Dgmail&amp;ust=3D1721478911283000&amp;usg=3DAOvVaw2tGCUebRo6=
vjKF8SKftMMH">https://groups.google.com/g/bitcoindev/c/EJYoeNTPVhg</a>
<br>
<br>    &quot;A Free-Relay Attack Exploiting Min-Relay-Fee Differences&quot=
; - Mar 31st 2024
<br>    <a href=3D"https://groups.google.com/g/bitcoindev/c/3XqfIOYzXqo" ta=
rget=3D"_blank" rel=3D"nofollow" data-saferedirecturl=3D"https://www.google=
.com/url?hl=3Den&amp;q=3Dhttps://groups.google.com/g/bitcoindev/c/3XqfIOYzX=
qo&amp;source=3Dgmail&amp;ust=3D1721478911284000&amp;usg=3DAOvVaw3TOPkBQ_Nd=
JKtzd8KxxdVy">https://groups.google.com/g/bitcoindev/c/3XqfIOYzXqo</a>
<br>
<br>The term &quot;free relay attack&quot; simply refers to any mechanism w=
here transaction
<br>data can be broadcast at unusually low cost; the &quot;free&quot; in &q=
uot;free relay&quot; is a
<br>misnomer as all these attacks do in fact have some cost.
<br>
<br>This particular attack isn&#39;t significantly different than the other=
 attacks
<br>I&#39;ve disclosed. With one important exception: unlike those other at=
tacks,
<br>fixing this particular attack would be quite easy, by enabling full-rbf=
 by
<br>default. So I disclosed it to the bitcoin-security mailing list as a te=
st: does
<br>Bitcoin Core actually care about free relay attacks? My hypothesis is t=
hat Core
<br>does not, as they know full well that &quot;free&quot; relay is an unav=
oidable problem;
<br>I&#39;ve received absolutely no feedback from any Bitcoin Core members =
for the
<br>other disclosed attacks, beyond achow using my disclosure of the RBF Ru=
le #6
<br>attack as an excuse to remove me from the bitcoin-security mailing list=
.
<br>
<br>The fact that Core doesn&#39;t actually care about &quot;free&quot; rel=
ay attacks is relevant
<br>to TRUC/V3 Transactions. As per BIP-431:
<br>
<br>    &quot;The primary problem with [RBFR proposals] is the potential fo=
r free relay and DDoS attacks.
<br>
<br>    Removing Rule 3 and 4 in general would allow free relay [27].&quot;
<br>    <a href=3D"https://github.com/bitcoin/bips/blob/812907c2b00b92ee31e=
2b638622a4fe14a428aee/bip-0431.mediawiki#user-content-Alternatives_replace_=
by_feerate" target=3D"_blank" rel=3D"nofollow" data-saferedirecturl=3D"http=
s://www.google.com/url?hl=3Den&amp;q=3Dhttps://github.com/bitcoin/bips/blob=
/812907c2b00b92ee31e2b638622a4fe14a428aee/bip-0431.mediawiki%23user-content=
-Alternatives_replace_by_feerate&amp;source=3Dgmail&amp;ust=3D1721478911284=
000&amp;usg=3DAOvVaw3fh7gqD6KbjyJF9u5jDGnE">https://github.com/bitcoin/bips=
/blob/812907c2b00b92ee31e2b638622a4fe14a428aee/bip-0431.mediawiki#user-cont=
ent-Alternatives_replace_by_feerate</a>
<br>
<br>I believe the authors of that BIP are fully aware of the fact that &quo=
t;free&quot; relay
<br>is an unavoidable problem, making their rational for TRUC/V3 bogus, and=
 don&#39;t
<br>want to admit that they&#39;ve wasted a large amount of engineering tim=
e on a bad
<br>proposal. I will be submitting a pull-req to get BIP-431 corrected, as =
the many
<br>&quot;free&quot; relay attacks I&#39;ve disclosed clearly show that cla=
iming RBFR would
<br>&quot;allow&quot; free relay is simply not true.
<br>
<br>Notably, full-RBF is _itself_ a transaction pinning fix for many use-ca=
ses;
<br>part of the TRUC/V3 standard is to force full-RBF behavior for V3 trans=
actions.
<br>So Core closing my full-RF pull-req is doubling down on TRUC/V3 in a se=
cond
<br>way, and TRUC/V3 proponents were the ones who tried to get the full-RBF=
 option
<br>removed from Core in the first place. If not for this dumb bit of Core
<br>politics, I&#39;m sure my year-old pull-req to enable full-RBF by defau=
lt would
<br>have been merged many months ago, as almost all hashpower has adopted f=
ull-RBF
<br>making objections based on &quot;zeroconf&quot; absurd.
<br>
<br>
<br># The Attack
<br>
<br>If you&#39;re a competent Bitcoin engineer, familiar with how mempools =
work, you&#39;ve
<br>probably figured it out already based on the title: obviously, if a hig=
h
<br>percentage of miners are adopting a policy that Bitcoin Core nodes are =
not, you
<br>can cheaply consume transaction relay bandwidth by simply relaying tran=
sations
<br>that miners are rejecting.
<br>
<br>Specifically, do the following:
<br>
<br>1. Broadcast a small, low-fee-rate, tx A with BIP-125 opt-in disabled.
<br>2. Broadcast a full-RBF double-spend of A, A2, with a higher fee-rate.
<br>3. Spend the outputs of A in a large, low fee-rate, transaction B with =
BIP-125
<br>   opt-in enabled. ~100% of miners will reject B, as it spends an input=
 not in
<br>   their mempools. However Bitcoin Core nodes will waste bandwidth prop=
agating
<br>   B.
<br>4. (Optional) Double-spend B repeatedly. Again, Bitcoin Core nodes will=
 waste
<br>   bandwidth propagating Bn&#39;s that ~100% of miners are ignoring.
<br>5. Double-spend A2 to recover your funds and do it all over again (or i=
f A2 had
<br>   a high enough fee-rate, just wait for it to be mined).
<br>
<br>The cost to relay each B transaction depends on the fee-rate of B. Sinc=
e
<br>Bitcoin Core defaults to a fairly large mempool, the minimum relay fee-=
rate is
<br>typically well below the economic fee-rate required for miners to actua=
lly mine
<br>a transaction; Core accepts transactions that are uneconomical for mine=
rs to
<br>mine for the forseeable future.
<br>
<br>For example, at the moment typical mempools require transactions to pay=
 at
<br>least 1sat/vB, while there are hundreds of MvB worth of transactions pa=
ying
<br>4sat/vB, the minimum economical fee-rate. Thus, transactions paying les=
s than
<br>4sat/VB are extremely unlikely to get mined in the nearish future.
<br>
<br>Concretely, broadcasting B transactions at 1sat/vB, 2sat/vB, and 3sat/v=
B would
<br>have almost zero cost as the probability of those transactions getting =
mined is
<br>nearly zero. This is true _regardless_ of what % of miners are mining f=
ull-RBF!
<br>As long as you can get at least one miner to mine the A double-spend, t=
he
<br>attack only costs what it cost to get A mined.
<br>
<br>If B&#39;s are broadcast at a higher fee-rate than the minimum economic=
al fee-rate,
<br>then the % of full-RBF miners matters. For example, if only 99% of mine=
rs mine
<br>full-RBF, the chance of a B transaction getting mined per block is abou=
t 1%, so
<br>the amortized cost of broadcasting B is about 1% of whatever total fee =
the
<br>highest fee-rate variant of B pays.
<br>
<br>For an attacker who does not need any B to be broadcast, the cost savin=
gs to
<br>use of relay bandwidth is approximately the ratio of the difference in =
size
<br>between B and and A. With a maximum standard transaction size of 100KvB=
, or
<br>400KB serialized size, this ratio is on the order of 5000:1, times the =
total
<br>number of B variants broadcast, and the % chance of each B being mined;=
 it&#39;s a
<br>few orders of magnitude.
<br>
<br>Of course, as mentioned above, this is just one of *many* &quot;free&qu=
ot; relay attacks,
<br>so fixing this particular issue doesn&#39;t change much.
<br>
<br>
<br># Attackers Who Benefit From B Getting Mined
<br>
<br>Some attackers actually need B to get mined. For example, imagine an ex=
change
<br>who needs to do large consolidation transactions. They could use this a=
ttack
<br>(and some attacks like it) as a way to goad users and miners into minin=
g
<br>consolidation transactions for them at low cost. In this variant of the=
 attack,
<br>the attacker would pad the size of B with consolidation spends that the=
y needed
<br>to do anyway. Someone who tried to stop the attack by getting B mined (=
eg via
<br><a href=3D"http://mempool.space" target=3D"_blank" rel=3D"nofollow" dat=
a-saferedirecturl=3D"https://www.google.com/url?hl=3Den&amp;q=3Dhttp://memp=
ool.space&amp;source=3Dgmail&amp;ust=3D1721478911284000&amp;usg=3DAOvVaw3LA=
8yoWHtUO8-O7KhNWIld">mempool.space</a>&#39;s transaction accellerator) woul=
d simply be paying the attacker&#39;s
<br>fees for them.
<br>
<br>Obviously, this strategy is only relevant for B&#39;s below the economi=
c fee-rate.
<br>However, the weaker version of this strategy is to parallize the attack=
, and do
<br>your consolidation with the _A_ double-spends to reduce the # of bytes =
used per
<br>full-rbf double-spend.
<br>
<br>
<br># TRUC/V3 Creates a Free Relay Attack
<br>
<br>I&#39;ll leave the details of this as a homework problem. But obviously=
, the
<br>introduction of TRUC/V3 transactions *itself* creates a free relay atta=
ck very
<br>similar to the above! Just like full-RBF, not all miners will mine V3
<br>transactions. So you can do the exact same type of attack by taking adv=
antage
<br>of this difference in mining policy.
<br>
<br>--=20
<br><a href=3D"https://petertodd.org" target=3D"_blank" rel=3D"nofollow" da=
ta-saferedirecturl=3D"https://www.google.com/url?hl=3Den&amp;q=3Dhttps://pe=
tertodd.org&amp;source=3Dgmail&amp;ust=3D1721478911284000&amp;usg=3DAOvVaw3=
Oozvov6XJ1qRfXOK6i_I7">https://petertodd.org</a> &#39;peter&#39;[:-1]@<a hr=
ef=3D"http://petertodd.org" target=3D"_blank" rel=3D"nofollow" data-safered=
irecturl=3D"https://www.google.com/url?hl=3Den&amp;q=3Dhttp://petertodd.org=
&amp;source=3Dgmail&amp;ust=3D1721478911284000&amp;usg=3DAOvVaw17SWdmJvOinr=
lFD8MBev1B">petertodd.org</a>
<br></blockquote></div>

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