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boundary="----=_Part_159329_1526928484.1721392866788" ------=_Part_159329_1526928484.1721392866788 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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 > --=20 You received this message because you are subscribed to the Google Groups "= Bitcoin Development Mailing List" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to bitcoindev+unsubscribe@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/= bitcoindev/18a5e5a2-92b3-4345-853d-5a63b71d848bn%40googlegroups.com. ------=_Part_159329_1526928484.1721392866788 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi Peter,

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

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

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

/dev/fd0
floppy disk guy

<= /div>
On T= hursday, 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 disclo= sed to
the bitcoin-security mailing list. It's an easy vulnerability to fi= x. Although
as with other "free" relay attacks I've disclosed, I didn= 't get a substantive
response from Bitcoin Core, other than Core closing the my pull-req ena= bling
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 not
as simple as Core refusing to fix a vulnerability. Also, I've inclu= ding a fun
homework problem at the end: figure out how TRUC/V3 transactions itself= creates
a "free" relay attack.


# Background

This is just one of a few "free" relay attacks that I have re= cently disclosed,
including, but not limited to:

"A Free-Relay Attack Exploiting RBF Rule #6" - Mar 18th 2= 024
https://groups.google.com/g/bitcoindev/c/EJYoeNTPVhg

"A Free-Relay Attack Exploiting Min-Relay-Fee Differences"= ; - Mar 31st 2024
https://groups.google.com/g/bitcoindev/c/3XqfIOYzXqo

The term "free relay attack" simply refers to any mechanism w= here transaction
data can be broadcast at unusually low cost; the "free" in &q= uot;free relay" is a
misnomer as all these attacks do in fact have some cost.

This particular attack isn't significantly different than the other= attacks
I've disclosed. With one important exception: unlike those other at= tacks,
fixing this particular attack would be quite easy, by enabling full-rbf= by
default. So I disclosed it to the bitcoin-security mailing list as a te= st: does
Bitcoin Core actually care about free relay attacks? My hypothesis is t= hat Core
does not, as they know full well that "free" relay is an unav= oidable problem;
I've received absolutely no feedback from any Bitcoin Core members = for the
other disclosed attacks, beyond achow using my disclosure of the RBF Ru= le #6
attack as an excuse to remove me from the bitcoin-security mailing list= .

The fact that Core doesn't actually care about "free" rel= ay attacks is relevant
to TRUC/V3 Transactions. As per BIP-431:

"The primary problem with [RBFR proposals] is the potential fo= r free relay and DDoS attacks.

Removing Rule 3 and 4 in general would allow free relay [27]."
https://github.com/bitcoin/bips= /blob/812907c2b00b92ee31e2b638622a4fe14a428aee/bip-0431.mediawiki#user-cont= ent-Alternatives_replace_by_feerate

I believe the authors of that BIP are fully aware of the fact that &quo= t;free" relay
is an unavoidable problem, making their rational for TRUC/V3 bogus, and= don't
want to admit that they've wasted a large amount of engineering tim= e on a bad
proposal. I will be submitting a pull-req to get BIP-431 corrected, as = the many
"free" relay attacks I've disclosed clearly show that cla= iming RBFR would
"allow" free relay is simply not true.

Notably, full-RBF is _itself_ a transaction pinning fix for many use-ca= ses;
part of the TRUC/V3 standard is to force full-RBF behavior for V3 trans= actions.
So Core closing my full-RF pull-req is doubling down on TRUC/V3 in a se= cond
way, and TRUC/V3 proponents were the ones who tried to get the full-RBF= 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 defau= lt would
have been merged many months ago, as almost all hashpower has adopted f= ull-RBF
making objections based on "zeroconf" absurd.


# The Attack

If you're a competent Bitcoin engineer, familiar with how mempools = work, you've
probably figured it out already based on the title: obviously, if a hig= h
percentage of miners are adopting a policy that Bitcoin Core nodes are = not, you
can cheaply consume transaction relay bandwidth by simply relaying tran= sations
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 = 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 prop= agating
B.
4. (Optional) Double-spend B repeatedly. Again, Bitcoin Core nodes will= 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 i= f 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. Sinc= e
Bitcoin Core defaults to a fairly large mempool, the minimum relay fee-= rate is
typically well below the economic fee-rate required for miners to actua= lly mine
a transaction; Core accepts transactions that are uneconomical for mine= rs to
mine for the forseeable future.

For example, at the moment typical mempools require transactions to pay= at
least 1sat/vB, while there are hundreds of MvB worth of transactions pa= ying
4sat/vB, the minimum economical fee-rate. Thus, transactions paying les= s than
4sat/VB are extremely unlikely to get mined in the nearish future.

Concretely, broadcasting B transactions at 1sat/vB, 2sat/vB, and 3sat/v= B would
have almost zero cost as the probability of those transactions getting = mined is
nearly zero. This is true _regardless_ of what % of miners are mining f= ull-RBF!
As long as you can get at least one miner to mine the A double-spend, t= he
attack only costs what it cost to get A mined.

If B's are broadcast at a higher fee-rate than the minimum economic= al fee-rate,
then the % of full-RBF miners matters. For example, if only 99% of mine= rs mine
full-RBF, the chance of a B transaction getting mined per block is abou= t 1%, so
the amortized cost of broadcasting B is about 1% of whatever total fee = the
highest fee-rate variant of B pays.

For an attacker who does not need any B to be broadcast, the cost savin= gs to
use of relay bandwidth is approximately the ratio of the difference in = size
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 = total
number of B variants broadcast, and the % chance of each B being mined;= it's a
few orders of magnitude.

Of course, as mentioned above, this is just one of *many* "free&qu= ot; relay 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 ex= change
who needs to do large consolidation transactions. They could use this a= ttack
(and some attacks like it) as a way to goad users and miners into minin= g
consolidation transactions for them at low cost. In this variant of the= attack,
the attacker would pad the size of B with consolidation spends that the= y needed
to do anyway. Someone who tried to stop the attack by getting B mined (= eg via
mempool.space's transaction accellerator) woul= d simply be paying the attacker's
fees for them.

Obviously, this strategy is only relevant for B's below the economi= c fee-rate.
However, the weaker version of this strategy is to parallize the attack= , and do
your consolidation with the _A_ double-spends to reduce the # of bytes = 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 atta= ck 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 adv= antage
of this difference in mining policy.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

--
You received this message because you are subscribed to the Google Groups &= quot;Bitcoin Development Mailing List" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to bitcoind= ev+unsubscribe@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msg= id/bitcoindev/18a5e5a2-92b3-4345-853d-5a63b71d848bn%40googlegroups.com.=
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