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Date: Wed, 27 Nov 2024 21:18:37 -0800 (PST)
From: Antoine Riard <antoine.riard@gmail.com>
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Subject: Re: [bitcoindev] Re: Great Consensus Cleanup Revival
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Hi Eric,

Going back to this thread with a bit of delay...

tl;dr: See specifically comment on the lack of proof that invalidating=20
64-byte transactions are actually solving merkle root weaknesses that could=
=20
lead to a fork or unravel SPV clients.

> I'm not sure what you mean by stating that a new consensus rule, "could=
=20
be a low memory overhead". Checking all tx sizes is far more overhead than=
=20
validating the coinbase for a null point. As AntoineP agreed, it cannot be=
=20
done earlier, and I have shown that it is *significantly* more=20
computationally intensive. It makes the determination much more costly and=
=20
in all other cases by adding an additional check that serves no purpose.

I think for any (new) consensus rule, we shall be able to evalute its=20
implication in term of at least 2 dimensions a) memory overhead (e.g do a=
=20
full-node needs more memory to validate post-segwit blocks now that witness=
=20
fields are discounted ?) and b) computational overhead (e.g do a full-node=
=20
needs more CPU cycles to validate confidential transactions perdersen=20
commitments ?). A same consensus rule can achieve the same effect e.g=20
reducing headers merkle tree ambiguities, with completely different memory=
=20
or computational cost. For the checking all tx sizes vs validating the=20
coinbase for a null point, indeed I agree with you that the latter is=20
intuitively better on the 2 dimensions.

> I think you misunderstood me. Of course the witness commitment must be=20
validated (as I said, "Yet it remains necessary to validate the witness=20
commitment..."), as otherwise the witnesses within a block can be anything=
=20
without affecting the block hash. And of course the witness commitment is=
=20
computed in the same manner as the tx commitment and is therefore subject=
=20
to the same malleations. However, because the coinbase tx is committed to=
=20
the block hash, there is no need to guard the witness commitment for=20
malleation. And to my knowledge nobody has proposed doing so.

Yes, we misunderstood each other here.

> It cannot, that was my point: "(1) have distinct identity due to another=
=20
header property deviation, or (2) are the same block..."

Ok.

> This was already the presumption.

Ok.

> I'm not seeing the connection here. Are you suggesting that tx and block=
=20
hashes may collide with each other? Or that that a block message may be=20
confused with a transaction message?

This was about how to deal with types of invalid block messages in bitcoin=
=20
core that could sources of denial-of-service. E.g invalid bitcoin block=20
hash for a message with unparsable data (#1 and #3 in your typology.
My point was bitcoin core is making some assumption at block download to=20
fetch them from outbound peers rather than inbound. Outboud peers are=20
assumed to be more reliable, as the connection is attempted from the=20
outside.
The point in my point was that outbound-block-relay connections where=20
initially introduce to alleviate those types of concerns i.e tx probe to=20
infer the topology, see https://arxiv.org/pdf/1812.00942

> This does not mitigate the issue. It's essentially dead code. It's=20
exactly like saying, "there's an arbitrary number of holes in the bucket,=
=20
but we can plug a subset of those holes." Infinite minus any number is=20
still infinite.

I disagree with you here - If the fundamental problem is efficiently=20
caching identity in the case of block invalidity, one cannot ignore a=20
robust peeering policy, i.e you pick up peers allowed a scarce connection=
=20
slot.
This is indeed useless if you don't have first an efficient verification=20
algorithm to determine the block invalidity, though it's part of the=20
overall equation.
While infinite minus any number is of course still infinite, thinking=20
security by layers it's the base you can have a secure signature=20
verification algorithm still on a hot computing host.

> I don't follow this statement. The term "usable" was specifically=20
addressing the proposal - that a header hash must uniquely identify a block=
=20
(a header and committed set of txs) as valid or otherwise. As I have=20
pointed out, this will still not be the case if 64 byte blocks are=20
invalidated. It is also not the case that detection of type64 malleated=20
blocks can be made more performant if 64 byte txs are globally invalid. In=
=20
fact the opposite is true, it becomes more costly (and complex) and is=20
therefore just dead code.

Okay, in my statement, the term "usable" was to be understood as any=20
meaningful bit of information that can lead to=20
computationally-hard-to-forge progress in the determination problem you=20
laid out here:
https://groups.google.com/g/bitcoindev/c/CAfm7D5ppjo/m/T1-HKqSLAAAJ

> Headers first only defers malleation checks. The same checks are=20
necessary whether you perform blocks first or headers first sync (we=20
support both protocol levels). The only difference is that for headers=20
first, a stored header might later become invalidated. However, this is the=
=20
case with and without the possibility of malleation.

Yes, I agree with you here, a stored header might become invalidated, e.g a=
=20
reorg-out tx committed in the header's merkle tree after the header=20
reception.

> I have not suggested that anything is waived or ignored here. I'm stating=
=20
that there is no "mempool" performance benefit whatsoever to invalidating=
=20
64 byte txs. Mempool caching could only rely on tx identifiers, not block=
=20
identifiers. Tx identifiers are not at issue.

Once again, if the goal is an efficient algorithm making progress to=20
determinate a block invalidity, and as such reducing the denial-of-service=
=20
surface, caching signatures which are committed in the wtixd tree or in the=
=20
txid tree is a plus.
Though yes, I agree there is no "mempool" performance benefit to invalidate=
=20
the 64 byte tx.

> I don't know how to add any more detail than I already have. There are=20
three relevant considerations:
>=20
> (1) block hashes will not become unique identifiers for block messages.
> (2) the earliest point at which type64 malleation can be detected will=20
not be reduced.
> (3) the necessary cost of type64 malleated determination will not be=20
reduced.
> (4) the additional consensus rule will increase validation cost and code=
=20
complexity.
> (5) invalid blocks can still be produced at no cost that require full=20
double tx hashing/Merkle root computations.
>=20
> Which of these statements are not evident at this point?

That's five statements, not three. Minding implementation-dependent=20
considerations, I'm leaning to agree with up to (4) included.
About (5), I don't see how it makes sense that invalid blocks can be still=
=20
produced at not cost, at least pow should be the first thing first to be=20
verified.
Like this statement could be clarified what you mean by this.

> No, no part of this thread has any bearing on p2p transaction messages -=
=20
nor are coinbase transactions relayed as transaction messages. You could=20
restate it as:
>=20
> - receive block p2p messages
> - if the first tx's first input does not have a null point, reject the=20
block

I don't believe we can fully dissociate bearing on p2p blocks /=20
transactions messages, from the overall goal of reducing denial-of-service=
=20
arising from invalid blocks. How can you be sure the block is invalid until=
=20
you validate all txn ? Though let's waiwe this observation for the present=
=20
point.

The idea of exploiting block malleability is to grind one transaction T0=20
for a block B such that H(T0) =3D=3D H(H(T1) || H(T2)) =3D=3D B's Root. I.e=
 to have=20
T0 =3D=3D H(T1) || H(T2). T0 can be consensus valid or invalid to provoke a=
=20
consensus fork (it's the collision in the deserialization which is the=20
source of the merkle tree root weakness). The first transaction in the=20
block is necessarily the coinbase per current consensus rules. Checking=20
that T0 is a valid coinbase transaction is sufficient to reject the block.=
=20
Grinding 64-byte transactions that all deserialize as valid transactions,=
=20
including the null point requirement is computationally infeasible.

I'm not sure that even if we get ride of 64-byte transactions, we would=20
remove the merkle root weaknesses. Back to the previous example, one could=
=20
find T3 and T4 such that H(H(T3) || H(T4)) is equivalent to H(H(T1) ||=20
H(T2)). Of course, it would consist in breaking SHA256, which is deemed=20
computationally infeasible. I'm not even sure the header verifcation=20
algorithms gains second-preimage resistance from forbidding 64-byte=20
transaction.

So I think more that the minimal sufficient check to reject a block should=
=20
be more carefully analyzed, rather than advocating that forbidding some=20
magic value obviously fix an issue, in the present the bitcoin's merkle=20
root weaknesses.

> The above approach makes this malleation computationally infeasible.

I'm intuitively leaning so, though see comments above that it should be=20
more carefully thought about.

> It has nothing to do with internal cache layout and nothing to do with=20
mining resources. Not having a cache is clearly more efficient than having=
=20
a cache that provides no advantage, regardless of how the cache is laid=20
out. There is no cost to forcing a node to perform far more block=20
validation computations than can be precluded by invalidity caching. The=20
caching simply increases the overall computational cost (as would another=
=20
redundant rule to try and make it more efficient). Discarding invalid=20
blocks after the minimal amount of work is the most efficient resolution.=
=20
What one does with the peer at that point is orthogonal (e.g. drop, ban).

I disagree here - If the goal is an efficient algorithm making progress to=
=20
determinate a block invalidity, and then being able to re-use a run of this=
=20
algorithm when a blocks occurs again, having a cache widens the range of=20
algorithmsone can design. Same with the mining ressources, if it's to=20
consider denial-of-services and an attacker could be able to fully forge=20
blocks. If such invalidity caching strategy was efficient it would actually=
=20
minimize or erase the cost for a node to perform more block validation=20
computations. Where yes I share you opinion is that an ill-designed caching=
=20
could increase the overall computational cost, and that discarding invalid=
=20
blocks after the minimal amount of work is the most efficient resolution=20
for the 1st seen, though it doesn't say on the next N seen. Having the=20
signatures already validated could be obviously a win, even with a blind,=
=20
decaying cache, it's all about the memory space of an
average full-node.

> An attacker can throw a nearly infinite number of distinct invalid blocks=
=20
at your node (and all will connect to the chain and show proper PoW). As=20
such you will encounter zero cache hits and therefore nothing but overhead=
=20
from the cache. Please explain to me in detail how "cache layout" is going=
=20
to make any difference at all.

Going back to your typology from (1) to (9), e.g for the step 9 to=20
determine if a block message with valid header but unmalleated committed=20
valid tx data.
If you have seen the block message a first-time, though it wasn't yet on=20
the longest pow-chain and you disregarded its validation.

> I don't see this as a related/relevant topic. There are zero mining=20
resources required to overflow the invalidity cache. Just as Core recently=
=20
published regarding overflowing to its "ban" store, resulting in process=20
termination, this then introduces another attack vector that must be=20
mitigated.

Depends if your invalidity cache is safeguarded by a minimal valid=20
proof-of-work. I'm certaintly not going to defend that all bitcoin core=20
internal caches and stores are well-designed for adversarials environments.

> pseudo-code , not from libbitcoin...
>=20
> ```
> bool malleated64(block)
> {
>     segregated =3D ((block[80 + 4] =3D=3D 0) and (block[80 + 4 + 1] =3D=
=3D 1))
>     return block[segregated ? 86 : 85] !=3D=20
0xffffffff0000000000000000000000000000000000000000000000000000000000000000
> }
> ```
>=20
> Obviously there is no error handling (e.g. block too small, too many=20
inputs, etc.) but that is not relevant to the particular question. The=20
block.header is fixed size, always 80 bytes. The tx.version is also fixed,=
=20
always 4 bytes. A following 0 implies a segregated witness (otherwise it's=
=20
the input count), assuming there is a following 1. The first and only input=
=20
for the coinbase tx, which must be the first block tx, follows. If it does=
=20
not match=20
0xffffffff0000000000000000000000000000000000000000000000000000000000000000=
=20
then the block is invalid. If it does match, it is computationally=20
infeasible that the merkle root is type64 malleated. That's it, absolutely=
=20
trivial and with no prerequisites. The only thing that even makes it=20
interesting is the segwit bifurcation.

Thanks for the example with the segwit bifurcation for the marker. By the=
=20
way, the segwit marker is documented in BIP144, which is incorrectly=20
labeled as "Peer Services", though obviously misimplementing the=20
transaction ser / deser algorithm for segwit blocks would lead to consensus=
=20
divergence (what if you expect the "flag" to be 0xff and not 0x01 ?).=20
Personally, I think it's a good example of how tedious consensus changes=20
can be, when even documents for inter-compatibility about consensus changes=
=20
are not drawing a clear line between what is consensus and what are p2p=20
rules...

> Sure, but no language difference that I'm aware of could have any bearing=
=20
on this particular question.

Same, I don't see language difference that could have bearing on this=20
question, at that level of granularity.

Best,
Antoine R
ots hash: 3d5ed1718683ce1e864751a2eccf21908ed3b11079f183cdf863729d71ae3f36
Le samedi 20 juillet 2024 =C3=A0 21:51:27 UTC+1, Eric Voskuil a =C3=A9crit =
:

> Hi Antoine R,
>
> >> While at some level the block message buffer would generally be=20
> referenced by one or more C pointers, the difference between a valid=20
> coinbase input (i.e. with a "null point") and any other input, is not=20
> nullptr vs. !nullptr. A "null point" is a 36 byte value, 32 0x00 byes=20
> followed by 4 0xff bytes. In his infinite wisdom Satoshi decided it was=
=20
> better (or easier) to serialize a first block tx (coinbase) with an input=
=20
> containing an unusable script and pointing to an invalid [tx:index] tuple=
=20
> (input point) as opposed to just not having any input. That invalid input=
=20
> point is called a "null point", and of course cannot be pointed to by a=
=20
> "null pointer". The coinbase must be identified by comparing those 36 byt=
es=20
> to the well-known null point value (and if this does not match the Merkle=
=20
> hash cannot have been type64 malleated).
>
> > Good for the clarification here, I had in mind the core's `CheckBlock`=
=20
> path where the first block transaction pointer is dereferenced to verify =
if=20
> the transaction is a coinbase (i.e a "null point" where the prevout is=20
> null). Zooming out and back to my remark, I think this is correct that=20
> adding a new 64 byte size check on all block transactions to detect block=
=20
> hash invalidity could be a low memory overhead (implementation dependant)=
,=20
> rather than making that 64 byte check alone on the coinbase transaction a=
s=20
> in my understanding you're proposing.
>
> I'm not sure what you mean by stating that a new consensus rule, "could b=
e=20
> a low memory overhead". Checking all tx sizes is far more overhead than=
=20
> validating the coinbase for a null point. As AntoineP agreed, it cannot b=
e=20
> done earlier, and I have shown that it is *significantly* more=20
> computationally intensive. It makes the determination much more costly an=
d=20
> in all other cases by adding an additional check that serves no purpose.
>
> >>> The second one is the bip141 wtxid commitment in one of the coinbase=
=20
> transaction `scriptpubkey` output, which is itself covered by a txid in t=
he=20
> merkle tree.
>
> >> While symmetry seems to imply that the witness commitment would be=20
> malleable, just as the txs commitment, this is not the case. If the tx=20
> commitment is correct it is computationally infeasible for the witness=20
> commitment to be malleated, as the witness commitment incorporates each=
=20
> full tx (with witness, sentinel, and marker). As such the block identifie=
r,=20
> which relies only on the header and tx commitment, is a sufficient=20
> identifier. Yet it remains necessary to validate the witness commitment t=
o=20
> ensure that the correct witness data has been provided in the block messa=
ge.
> >>
> >> The second type of malleability, in addition to type64, is what we cal=
l=20
> type32. This is the consequence of duplicated trailing sets of txs (and=
=20
> therefore tx hashes) in a block message. This is applicable to some but n=
ot=20
> all blocks, as a function of the number of txs contained.
>
> > To precise more your statement in describing source of malleability. Th=
e=20
> witness stack can be malleated altering the wtxid and yet still valid. I=
=20
> think you can still have the case where you're feeded a block header with=
 a=20
> merkle root commitment deserializing to a valid coinbase transaction with=
=20
> an invalid witness commitment. This is the case of a "block message with=
=20
> valid header but malleatead committed valid tx data". Validation of the=
=20
> witness commitment to ensure the correct witness data has been provided i=
n=20
> the block message is indeed necessary.
>
> I think you misunderstood me. Of course the witness commitment must be=20
> validated (as I said, "Yet it remains necessary to validate the witness=
=20
> commitment..."), as otherwise the witnesses within a block can be anythin=
g=20
> without affecting the block hash. And of course the witness commitment is=
=20
> computed in the same manner as the tx commitment and is therefore subject=
=20
> to the same malleations. However, because the coinbase tx is committed to=
=20
> the block hash, there is no need to guard the witness commitment for=20
> malleation. And to my knowledge nobody has proposed doing so.
>
> >>> I think I mostly agree with the identity issue as laid out so far,=20
> there is one caveat to add if you're considering identity caching as the=
=20
> problem solved. A validation node might have to consider differently bloc=
k=20
> messages processed if they connect on the longest most PoW valid chain fo=
r=20
> which all blocks have been validated. Or alternatively if they have to be=
=20
> added on a candidate longest most PoW valid chain.
>
> >> Certainly an important consideration. We store both types. Once there=
=20
> is a stronger candidate header chain we store the headers and proceed to=
=20
> obtaining the blocks (if we don't already have them). The blocks are stor=
ed=20
> in the same table; the confirmed vs. candidate indexes simply point to th=
em=20
> as applicable. It is feasible (and has happened twice) for two blocks to=
=20
> share the very same coinbase tx, even with either/all bip30/34/90 active=
=20
> (and setting aside future issues here for the sake of simplicity). This=
=20
> remains only because two competing branches can have blocks at the same=
=20
> height, and bip34 requires only height in the coinbase input script. This=
=20
> therefore implies the same transaction but distinct blocks. It is however=
=20
> infeasible for one block to exist in multiple distinct chains. In order f=
or=20
> this to happen two blocks at the same height must have the same coinbase=
=20
> (ok), and also the same parent (ok). But this then means that they either=
=20
> (1) have distinct identity due to another header property deviation, or (=
2)=20
> are the same block with the same parent and are therefore in just one=20
> chain. So I don't see an actual caveat. I'm not certain if this is the=20
> ambiguity that you were referring to. If not please feel free to clarify.
>
> > If you assume no network partition and the no blocks more than 2h in th=
e=20
> future consensus rule, I cannot see how one block with no header property=
=20
> deviation can exist in multiple distinct chains.
>
> It cannot, that was my point: "(1) have distinct identity due to another=
=20
> header property deviation, or (2) are the same block..."
>
> > The ambiguity I was referring was about a different angle, if the desig=
n=20
> goal of introducing a 64 byte size check is to "it was about being able t=
o=20
> cache the hash of a (non-malleated) invalid block as permanently invalid =
to=20
> avoid re-downloading and re-validating it", in my thinking we shall=20
> consider the whole block headers caching strategy and be sure we don't ge=
t=20
> situations where an attacker can attach a chain of low-pow block headers=
=20
> with malleated committed valid tx data yielding a block invalidity at the=
=20
> end, provoking as a side-effect a network-wide data download blowup. So I=
=20
> think any implementation of the validation of a block validity, of which=
=20
> identity is a sub-problem, should be strictly ordered by adequate=20
> proof-of-work checks.
>
> This was already the presumption.
>
> >> We don't do this and I don't see how it would be relevant. If a peer=
=20
> provides any invalid message or otherwise violates the protocol it is=20
> simply dropped.
> >>
> >> The "problematic" that I'm referring to is the reliance on the block=
=20
> hash as a message identifier, because it does not identify the message an=
d=20
> cannot be useful in an effectively unlimited number of zero-cost cases.
>
> > Historically, it was to isolate transaction-relay from block-relay to=
=20
> optimistically harden in face of network partition, as this is easy to=20
> infer transaction-relay topology with a lot of heuristics.
>
> I'm not seeing the connection here. Are you suggesting that tx and block=
=20
> hashes may collide with each other? Or that that a block message may be=
=20
> confused with a transaction message?
>
> > I think this is correct that block hash message cannot be relied on as=
=20
> it cannot be useful in an unlimited number of zero-cost cases, as I was=
=20
> pointing that bitcoin core partially mitigate that with discouraging=20
> connections to block-relay peers servicing block messages=20
> (`MaybePunishNodeForBlocks`).
>
> This does not mitigate the issue. It's essentially dead code. It's exactl=
y=20
> like saying, "there's an arbitrary number of holes in the bucket, but we=
=20
> can plug a subset of those holes." Infinite minus any number is still=20
> infinite.
>
> > I believe somehow the bottleneck we're circling around is=20
> computationally definining what are the "usable" identifiers for block=20
> messages. The most straightforward answer to this question is the full=20
> block in one single peer message, at least in my perspective.
>
> I don't follow this statement. The term "usable" was specifically=20
> addressing the proposal - that a header hash must uniquely identify a blo=
ck=20
> (a header and committed set of txs) as valid or otherwise. As I have=20
> pointed out, this will still not be the case if 64 byte blocks are=20
> invalidated. It is also not the case that detection of type64 malleated=
=20
> blocks can be made more performant if 64 byte txs are globally invalid. I=
n=20
> fact the opposite is true, it becomes more costly (and complex) and is=20
> therefore just dead code.
>
> > Reality since headers first synchronization (`getheaders`), block=20
> validation has been dissociated in steps for performance reasons, among=
=20
> others.
>
> Headers first only defers malleation checks. The same checks are necessar=
y=20
> whether you perform blocks first or headers first sync (we support both=
=20
> protocol levels). The only difference is that for headers first, a stored=
=20
> header might later become invalidated. However, this is the case with and=
=20
> without the possibility of malleation.
>
> >> Again, this has no relation to tx hashes/identifiers. Libbitcoin has a=
=20
> tx pool, we just don't store them in RAM (memory).
> >>
> >> I don't follow this. An invalid 64 byte tx consensus rule would=20
> definitely not make it harder to exploit block message invalidity. In fac=
t=20
> it would just slow down validation by adding a redundant rule. Furthermor=
e,=20
> as I have detailed in a previous message, caching invalidity does=20
> absolutely nothing to increase protection. In fact it makes the situation=
=20
> materially worse.
>
> > Just to recall, in my understanding the proposal we're discussing is=20
> about outlawing 64 bytes size transactions at the consensus-level to=20
> minimize denial-of-service vectors during block validation. I think we're=
=20
> talking about each other because the mempool already introduce a layer of=
=20
> caching in bitcoin core, of which the result are re-used at block=20
> validation, such as signature verification results. I'm not sure we can=
=20
> fully waive apart performance considerations, though I agree implementati=
on=20
> architecture subsystems like mempool should only be a sideline=20
> considerations.
>
> I have not suggested that anything is waived or ignored here. I'm stating=
=20
> that there is no "mempool" performance benefit whatsoever to invalidating=
=20
> 64 byte txs. Mempool caching could only rely on tx identifiers, not block=
=20
> identifiers. Tx identifiers are not at issue.
>
> >> No, this is not the case. As I detailed in my previous message, there=
=20
> is no possible scenario where invalidation caching does anything but make=
=20
> the situation materially worse.
>
> > I think this can be correct that invalidation caching make the situatio=
n=20
> materially worse, or is denial-of-service neutral, as I believe a full no=
de=20
> is only trading space for time resources in matters of block messages=20
> validation. I still believe such analysis, as detailed in your previous=
=20
> message, would benefit to be more detailed.
>
> I don't know how to add any more detail than I already have. There are=20
> three relevant considerations:
>
> (1) block hashes will not become unique identifiers for block messages.
> (2) the earliest point at which type64 malleation can be detected will no=
t=20
> be reduced.
> (3) the necessary cost of type64 malleated determination will not be=20
> reduced.
> (4) the additional consensus rule will increase validation cost and code=
=20
> complexity.
> (5) invalid blocks can still be produced at no cost that require full=20
> double tx hashing/Merkle root computations.
>
> Which of these statements are not evident at this point?
>
> >> On the other hand, just dealing with parse failure on the spot by=20
> introducing a leading pattern in the stream just inflates the size of p2p=
=20
> messages, and the transaction-relay bandwidth cost.
> >>
> >> I think you misunderstood me. I am suggesting no change to=20
> serialization. I can see how it might be unclear, but I said, "nothing=20
> precludes incorporating a requirement for a necessary leading pattern in=
=20
> the stream." I meant that the parser can simply incorporate the=20
> *requirement* that the byte stream starts with a null input point. That=
=20
> identifies the malleation or invalidity without a single hash operation a=
nd=20
> while only reading a handful of bytes. No change to any messages.
>
> > Indeed, this is clearer with the re-explanation above about what you=20
> meant by the "null point".
>
> Ok
>
> > In my understanding, you're suggesting the following algorithm:
> > - receive transaction p2p messages
> > - deserialize transaction p2p messages
> > - if the transaction is a coinbase candidate, verify null input point
> > - if null input point pattern invalid, reject the transaction
>
> No, no part of this thread has any bearing on p2p transaction messages -=
=20
> nor are coinbase transactions relayed as transaction messages. You could=
=20
> restate it as:
>
> - receive block p2p messages
> - if the first tx's first input does not have a null point, reject the=20
> block
>
> > If I'm understanding correctly, the last rule has for effect to=20
> constraint the transaction space that can be used to brute-force and moun=
t=20
> a Merkle root forgery with a 64-byte coinbase transaction.
> >
> > As described in the 3.1.1 of the paper:=20
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20190=
225/a27d8837/attachment-0001.pdf
>
> The above approach makes this malleation computationally infeasible.
>
> >> I'm referring to DoS mitigation (the only relevant security=20
> consideration here). I'm pointing out that invalidity caching is pointles=
s=20
> in all cases, and in this case is the most pointless as type64 malleation=
=20
> is the cheapest of all invalidity to detect. I would prefer that all bogu=
s=20
> blocks sent to my node are of this type. The worst types of invalidity=20
> detection have no mitigation and from a security standpoint are=20
> counterproductive to cache. I'm describing what overall is actually not a=
=20
> tradeoff. It's all negative and no positive.
>
> > I think we're both discussing the same issue about DoS mitigation for=
=20
> sure. Again, I think that saying the "invalidity caching" is pointless in=
=20
> all cases cannot be fully grounded as a statement without precising (a)=
=20
> what is the internal cache(s) layout of the full node processing block=20
> messages and (b) the sha256 mining resources available during N difficult=
y=20
> period and if any miner engage in self-fish mining like strategy.
>
> It has nothing to do with internal cache layout and nothing to do with=20
> mining resources. Not having a cache is clearly more efficient than havin=
g=20
> a cache that provides no advantage, regardless of how the cache is laid=
=20
> out. There is no cost to forcing a node to perform far more block=20
> validation computations than can be precluded by invalidity caching. The=
=20
> caching simply increases the overall computational cost (as would another=
=20
> redundant rule to try and make it more efficient). Discarding invalid=20
> blocks after the minimal amount of work is the most efficient resolution.=
=20
> What one does with the peer at that point is orthogonal (e.g. drop, ban).
>
> > About (a), I'll maintain my point I think it's a classic time-space=20
> trade-off to ponder in function of the internal cache layouts.
>
> An attacker can throw a nearly infinite number of distinct invalid blocks=
=20
> at your node (and all will connect to the chain and show proper PoW). As=
=20
> such you will encounter zero cache hits and therefore nothing but overhea=
d=20
> from the cache. Please explain to me in detail how "cache layout" is goin=
g=20
> to make any difference at all.
>
> > About (b) I think we''ll be back to the headers synchronization strateg=
y=20
> as implemented by a full node to discuss if they're exploitable asymmetri=
es=20
> for self-fish mining like strategies.
>
> I don't see this as a related/relevant topic. There are zero mining=20
> resources required to overflow the invalidity cache. Just as Core recentl=
y=20
> published regarding overflowing to its "ban" store, resulting in process=
=20
> termination, this then introduces another attack vector that must be=20
> mitigated.
>
> > If you can give a pseudo-code example of the "null point" validation=20
> implementation in libbitcoin code (?) I think this can make the=20
> conversation more concrete on the caching aspect.
>
> pseudo-code , not from libbitcoin...
>
> ```
> bool malleated64(block)
> {
>     segregated =3D ((block[80 + 4] =3D=3D 0) and (block[80 + 4 + 1] =3D=
=3D 1))
>     return block[segregated ? 86 : 85] !=3D=20
> 0xffffffff000000000000000000000000000000000000000000000000000000000000000=
0
> }
> ```
>
> Obviously there is no error handling (e.g. block too small, too many=20
> inputs, etc.) but that is not relevant to the particular question. The=20
> block.header is fixed size, always 80 bytes. The tx.version is also fixed=
,=20
> always 4 bytes. A following 0 implies a segregated witness (otherwise it'=
s=20
> the input count), assuming there is a following 1. The first and only inp=
ut=20
> for the coinbase tx, which must be the first block tx, follows. If it doe=
s=20
> not match=20
> 0xffffffff000000000000000000000000000000000000000000000000000000000000000=
0=20
> then the block is invalid. If it does match, it is computationally=20
> infeasible that the merkle root is type64 malleated. That's it, absolutel=
y=20
> trivial and with no prerequisites. The only thing that even makes it=20
> interesting is the segwit bifurcation.
>
> >> Rust has its own set of problems. No need to get into a language Jihad=
=20
> here. My point was to clarify that the particular question was not about =
a=20
> C (or C++) null pointer value, either on the surface or underneath an=20
> abstraction.
>
> > Thanks for the additional comments on libbitcoin usage of dependencies,=
=20
> yes I don't think there is a need to get into a language jihad here. It's=
=20
> just like all languages have their memory model (stack, dynamic alloc,=20
> smart pointers, etc) and when you're talking about performance it's usefu=
l=20
> to have their minds, imho.
>
> Sure, but no language difference that I'm aware of could have any bearing=
=20
> on this particular question.
>
> Best,
> Eric
>

--=20
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------=_Part_150667_1323905878.1732771117097
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

Hi Eric,<br /><br />Going back to this thread with a bit of delay...<br /><=
br />tl;dr: See specifically comment on the lack of proof that invalidating=
 64-byte transactions are actually solving merkle root weaknesses that coul=
d lead to a fork or unravel SPV clients.<br /><br />&gt; I'm not sure what =
you mean by stating that a new consensus rule, "could be a low memory overh=
ead". Checking all tx sizes is far more overhead than validating the coinba=
se for a null point. As AntoineP agreed, it cannot be done earlier, and I h=
ave shown that it is *significantly* more computationally intensive. It mak=
es the determination much more costly and in all other cases by adding an a=
dditional check that serves no purpose.<br /><br />I think for any (new) co=
nsensus rule, we shall be able to evalute its implication in term of at lea=
st 2 dimensions a) memory overhead (e.g do a full-node needs more memory to=
 validate post-segwit blocks now that witness fields are discounted ?) and =
b) computational overhead (e.g do a full-node needs more CPU cycles to vali=
date confidential transactions perdersen commitments ?). A same consensus r=
ule can achieve the same effect e.g reducing headers merkle tree ambiguitie=
s, with completely different memory or computational cost. For the checking=
 all tx sizes vs validating the coinbase for a null point, indeed I agree w=
ith you that the latter is intuitively better on the 2 dimensions.<br /><br=
 />&gt; I think you misunderstood me. Of course the witness commitment must=
 be validated (as I said, "Yet it remains necessary to validate the witness=
 commitment..."), as otherwise the witnesses within a block can be anything=
 without affecting the block hash. And of course the witness commitment is =
computed in the same manner as the tx commitment and is therefore subject t=
o the same malleations. However, because the coinbase tx is committed to th=
e block hash, there is no need to guard the witness commitment for malleati=
on. And to my knowledge nobody has proposed doing so.<br /><br />Yes, we mi=
sunderstood each other here.<br /><br />&gt; It cannot, that was my point: =
"(1) have distinct identity due to another header property deviation, or (2=
) are the same block..."<br /><br />Ok.<br /><br />&gt; This was already th=
e presumption.<br /><br />Ok.<br /><br />&gt; I'm not seeing the connection=
 here. Are you suggesting that tx and block hashes may collide with each ot=
her? Or that that a block message may be confused with a transaction messag=
e?<br /><br />This was about how to deal with types of invalid block messag=
es in bitcoin core that could sources of denial-of-service. E.g invalid bit=
coin block hash for a message with unparsable data (#1 and #3 in your typol=
ogy.<br />My point was bitcoin core is making some assumption at block down=
load to fetch them from outbound peers rather than inbound. Outboud peers a=
re assumed to be more reliable, as the connection is attempted from the out=
side.<br />The point in my point was that outbound-block-relay connections =
where initially introduce to alleviate those types of concerns i.e tx probe=
 to infer the topology, see https://arxiv.org/pdf/1812.00942<br /><br />&gt=
; This does not mitigate the issue. It's essentially dead code. It's exactl=
y like saying, "there's an arbitrary number of holes in the bucket, but we =
can plug a subset of those holes." Infinite minus any number is still infin=
ite.<br /><br />I disagree with you here - If the fundamental problem is ef=
ficiently caching identity in the case of block invalidity, one cannot igno=
re a robust peeering policy, i.e you pick up peers allowed a scarce connect=
ion slot.<br />This is indeed useless if you don't have first an efficient =
verification algorithm to determine the block invalidity, though it's part =
of the overall equation.<br />While infinite minus any number is of course =
still infinite, thinking security by layers it's the base you can have a se=
cure signature verification algorithm still on a hot computing host.<br /><=
br />&gt; I don't follow this statement. The term "usable" was specifically=
 addressing the proposal - that a header hash must uniquely identify a bloc=
k (a header and committed set of txs) as valid or otherwise. As I have poin=
ted out, this will still not be the case if 64 byte blocks are invalidated.=
 It is also not the case that detection of type64 malleated blocks can be m=
ade more performant if 64 byte txs are globally invalid. In fact the opposi=
te is true, it becomes more costly (and complex) and is therefore just dead=
 code.<br /><br />Okay, in my statement, the term "usable" was to be unders=
tood as any meaningful bit of information that can lead to computationally-=
hard-to-forge progress in the determination problem you laid out here:<br /=
>https://groups.google.com/g/bitcoindev/c/CAfm7D5ppjo/m/T1-HKqSLAAAJ<br /><=
br />&gt; Headers first only defers malleation checks. The same checks are =
necessary whether you perform blocks first or headers first sync (we suppor=
t both protocol levels). The only difference is that for headers first, a s=
tored header might later become invalidated. However, this is the case with=
 and without the possibility of malleation.<br /><br />Yes, I agree with yo=
u here, a stored header might become invalidated, e.g a reorg-out tx commit=
ted in the header's merkle tree after the header reception.<br /><br />&gt;=
 I have not suggested that anything is waived or ignored here. I'm stating =
that there is no "mempool" performance benefit whatsoever to invalidating 6=
4 byte txs. Mempool caching could only rely on tx identifiers, not block id=
entifiers. Tx identifiers are not at issue.<br /><br />Once again, if the g=
oal is an efficient algorithm making progress to determinate a block invali=
dity, and as such reducing the denial-of-service surface, caching signature=
s which are committed in the wtixd tree or in the txid tree is a plus.<br /=
>Though yes, I agree there is no "mempool" performance benefit to invalidat=
e the 64 byte tx.<br /><br />&gt; I don't know how to add any more detail t=
han I already have. There are three relevant considerations:<br />&gt; <br =
/>&gt; (1) block hashes will not become unique identifiers for block messag=
es.<br />&gt; (2) the earliest point at which type64 malleation can be dete=
cted will not be reduced.<br />&gt; (3) the necessary cost of type64 mallea=
ted determination will not be reduced.<br />&gt; (4) the additional consens=
us rule will increase validation cost and code complexity.<br />&gt; (5) in=
valid blocks can still be produced at no cost that require full double tx h=
ashing/Merkle root computations.<br />&gt; <br />&gt; Which of these statem=
ents are not evident at this point?<br /><br />That's five statements, not =
three. Minding implementation-dependent considerations, I'm leaning to agre=
e with up to (4) included.<br />About (5), I don't see how it makes sense t=
hat invalid blocks can be still produced at not cost, at least pow should b=
e the first thing first to be verified.<br />Like this statement could be c=
larified what you mean by this.<br /><br />&gt; No, no part of this thread =
has any bearing on p2p transaction messages - nor are coinbase transactions=
 relayed as transaction messages. You could restate it as:<br />&gt; <br />=
&gt; - receive block p2p messages<br />&gt; - if the first tx's first input=
 does not have a null point, reject the block<br /><br />I don't believe we=
 can fully dissociate bearing on p2p blocks / transactions messages, from t=
he overall goal of reducing denial-of-service arising from invalid blocks. =
How can you be sure the block is invalid until you validate all txn ? Thoug=
h let's waiwe this observation for the present point.<br /><br />The idea o=
f exploiting block malleability is to grind one transaction T0 for a block =
B such that H(T0) =3D=3D H(H(T1) || H(T2)) =3D=3D B's Root. I.e to have T0 =
=3D=3D H(T1) || H(T2). T0 can be consensus valid or invalid to provoke a co=
nsensus fork (it's the collision in the deserialization which is the source=
 of the merkle tree root weakness). The first transaction in the block is n=
ecessarily the coinbase per current consensus rules. Checking that T0 is a =
valid coinbase transaction is sufficient to reject the block. Grinding 64-b=
yte transactions that all deserialize as valid transactions, including the =
null point requirement is computationally infeasible.<br /><br />I'm not su=
re that even if we get ride of 64-byte transactions, we would remove the me=
rkle root weaknesses. Back to the previous example, one could find T3 and T=
4 such that H(H(T3) || H(T4)) is equivalent to H(H(T1) || H(T2)). Of course=
, it would consist in breaking SHA256, which is deemed computationally infe=
asible. I'm not even sure the header verifcation algorithms gains second-pr=
eimage resistance from forbidding 64-byte transaction.<br /><br />So I thin=
k more that the minimal sufficient check to reject a block should be more c=
arefully analyzed, rather than advocating that forbidding some magic value =
obviously fix an issue, in the present the bitcoin's merkle root weaknesses=
.<br /><br />&gt; The above approach makes this malleation computationally =
infeasible.<br /><br />I'm intuitively leaning so, though see comments abov=
e that it should be more carefully thought about.<br /><br />&gt; It has no=
thing to do with internal cache layout and nothing to do with mining resour=
ces. Not having a cache is clearly more efficient than having a cache that =
provides no advantage, regardless of how the cache is laid out. There is no=
 cost to forcing a node to perform far more block validation computations t=
han can be precluded by invalidity caching. The caching simply increases th=
e overall computational cost (as would another redundant rule to try and ma=
ke it more efficient). Discarding invalid blocks after the minimal amount o=
f work is the most efficient resolution. What one does with the peer at tha=
t point is orthogonal (e.g. drop, ban).<br /><br />I disagree here - If the=
 goal is an efficient algorithm making progress to determinate a block inva=
lidity, and then being able to re-use a run of this algorithm when a blocks=
 occurs again, having a cache widens the range of algorithmsone can design.=
 Same with the mining ressources, if it's to consider denial-of-services an=
d an attacker could be able to fully forge blocks. If such invalidity cachi=
ng strategy was efficient it would actually minimize or erase the cost for =
a node to perform more block validation computations. Where yes I share you=
 opinion is that an ill-designed caching could increase the overall computa=
tional cost, and that discarding invalid blocks after the minimal amount of=
 work is the most efficient resolution for the 1st seen, though it doesn't =
say on the next N seen. Having the signatures already validated could be ob=
viously a win, even with a blind, decaying cache, it's all about the memory=
 space of an<br />average full-node.<br /><br />&gt; An attacker can throw =
a nearly infinite number of distinct invalid blocks at your node (and all w=
ill connect to the chain and show proper PoW). As such you will encounter z=
ero cache hits and therefore nothing but overhead from the cache. Please ex=
plain to me in detail how "cache layout" is going to make any difference at=
 all.<br /><br />Going back to your typology from (1) to (9), e.g for the s=
tep 9 to determine if a block message with valid header but unmalleated com=
mitted valid tx data.<br />If you have seen the block message a first-time,=
 though it wasn't yet on the longest pow-chain and you disregarded its vali=
dation.<br /><br />&gt; I don't see this as a related/relevant topic. There=
 are zero mining resources required to overflow the invalidity cache. Just =
as Core recently published regarding overflowing to its "ban" store, result=
ing in process termination, this then introduces another attack vector that=
 must be mitigated.<br /><br />Depends if your invalidity cache is safeguar=
ded by a minimal valid proof-of-work. I'm certaintly not going to defend th=
at all bitcoin core internal caches and stores are well-designed for advers=
arials environments.<br /><br />&gt; pseudo-code , not from libbitcoin...<b=
r />&gt; <br />&gt; ```<br />&gt; bool malleated64(block)<br />&gt; {<br />=
&gt; =C2=A0 =C2=A0 segregated =3D ((block[80 + 4] =3D=3D 0) and (block[80 +=
 4 + 1] =3D=3D 1))<br />&gt; =C2=A0 =C2=A0 return block[segregated ? 86 : 8=
5] !=3D 0xffffffff000000000000000000000000000000000000000000000000000000000=
0000000<br />&gt; }<br />&gt; ```<br />&gt; <br />&gt; Obviously there is n=
o error handling (e.g. block too small, too many inputs, etc.) but that is =
not relevant to the particular question. The block.header is fixed size, al=
ways 80 bytes. The tx.version is also fixed, always 4 bytes. A following 0 =
implies a segregated witness (otherwise it's the input count), assuming the=
re is a following 1. The first and only input for the coinbase tx, which mu=
st be the first block tx, follows. If it does not match 0xffffffff000000000=
0000000000000000000000000000000000000000000000000000000 then the block is i=
nvalid. If it does match, it is computationally infeasible that the merkle =
root is type64 malleated. That's it, absolutely trivial and with no prerequ=
isites. The only thing that even makes it interesting is the segwit bifurca=
tion.<br /><br />Thanks for the example with the segwit bifurcation for the=
 marker. By the way, the segwit marker is documented in BIP144, which is in=
correctly labeled as "Peer Services", though obviously misimplementing the =
transaction ser / deser algorithm for segwit blocks would lead to consensus=
 divergence (what if you expect the "flag" to be 0xff and not 0x01 ?). Pers=
onally, I think it's a good example of how tedious consensus changes can be=
, when even documents for inter-compatibility about consensus changes are n=
ot drawing a clear line between what is consensus and what are p2p rules...=
<br /><br />&gt; Sure, but no language difference that I'm aware of could h=
ave any bearing on this particular question.<br /><br />Same, I don't see l=
anguage difference that could have bearing on this question, at that level =
of granularity.<br /><br />Best,<br />Antoine R<br />ots hash: 3d5ed1718683=
ce1e864751a2eccf21908ed3b11079f183cdf863729d71ae3f36<br /><div class=3D"gma=
il_quote"><div dir=3D"auto" class=3D"gmail_attr">Le samedi 20 juillet 2024 =
=C3=A0 21:51:27 UTC+1, Eric Voskuil a =C3=A9crit=C2=A0:<br/></div><blockquo=
te class=3D"gmail_quote" style=3D"margin: 0 0 0 0.8ex; border-left: 1px sol=
id rgb(204, 204, 204); padding-left: 1ex;">Hi Antoine R,<br><br>&gt;&gt; Wh=
ile at some level the block message buffer would generally be referenced by=
 one or more C pointers, the difference between a valid coinbase input (i.e=
. with a &quot;null point&quot;) and any other input, is not nullptr vs. !n=
ullptr. A &quot;null point&quot; is a 36 byte value, 32 0x00 byes followed =
by 4 0xff bytes. In his infinite wisdom Satoshi decided it was better (or e=
asier) to serialize a first block tx (coinbase) with an input containing an=
 unusable script and pointing to an invalid [tx:index] tuple (input point) =
as opposed to just not having any input. That invalid input point is called=
 a &quot;null point&quot;, and of course cannot be pointed to by a &quot;nu=
ll pointer&quot;. The coinbase must be identified by comparing those 36 byt=
es to the well-known null point value (and if this does not match the Merkl=
e hash cannot have been type64 malleated).<br><br>&gt; Good for the clarifi=
cation here, I had in mind the core&#39;s `CheckBlock` path where the first=
 block transaction pointer is dereferenced to verify if the transaction is =
a coinbase (i.e a &quot;null point&quot; where the prevout is null). Zoomin=
g out and back to my remark, I think this is correct that adding a new 64 b=
yte size check on all block transactions to detect block hash invalidity co=
uld be a low memory overhead (implementation dependant), rather than making=
 that 64 byte check alone on the coinbase transaction as in my understandin=
g you&#39;re proposing.<br><br>I&#39;m not sure what you mean by stating th=
at a new consensus rule, &quot;could be a low memory overhead&quot;. Checki=
ng all tx sizes is far more overhead than validating the coinbase for a nul=
l point. As AntoineP agreed, it cannot be done earlier, and I have shown th=
at it is *significantly* more computationally intensive. It makes the deter=
mination much more costly and in all other cases by adding an additional ch=
eck that serves no purpose.<br><br>&gt;&gt;&gt; The second one is the bip14=
1 wtxid commitment in one of the coinbase transaction `scriptpubkey` output=
, which is itself covered by a txid in the merkle tree.<br><br>&gt;&gt; Whi=
le symmetry seems to imply that the witness commitment would be malleable, =
just as the txs commitment, this is not the case. If the tx commitment is c=
orrect it is computationally infeasible for the witness commitment to be ma=
lleated, as the witness commitment incorporates each full tx (with witness,=
 sentinel, and marker). As such the block identifier, which relies only on =
the header and tx commitment, is a sufficient identifier. Yet it remains ne=
cessary to validate the witness commitment to ensure that the correct witne=
ss data has been provided in the block message.<br>&gt;&gt;<br>&gt;&gt; The=
 second type of malleability, in addition to type64, is what we call type32=
. This is the consequence of duplicated trailing sets of txs (and therefore=
 tx hashes) in a block message. This is applicable to some but not all bloc=
ks, as a function of the number of txs contained.<br><br>&gt; To precise mo=
re your statement in describing source of malleability. The witness stack c=
an be malleated altering the wtxid and yet still valid. I think you can sti=
ll have the case where you&#39;re feeded a block header with a merkle root =
commitment deserializing to a valid coinbase transaction with an invalid wi=
tness commitment. This is the case of a &quot;block message with valid head=
er but malleatead committed valid tx data&quot;. Validation of the witness =
commitment to ensure the correct witness data has been provided in the bloc=
k message is indeed necessary.<br><br>I think you misunderstood me. Of cour=
se the witness commitment must be validated (as I said, &quot;Yet it remain=
s necessary to validate the witness commitment...&quot;), as otherwise the =
witnesses within a block can be anything without affecting the block hash. =
And of course the witness commitment is computed in the same manner as the =
tx commitment and is therefore subject to the same malleations. However, be=
cause the coinbase tx is committed to the block hash, there is no need to g=
uard the witness commitment for malleation. And to my knowledge nobody has =
proposed doing so.<br><br>&gt;&gt;&gt; I think I mostly agree with the iden=
tity issue as laid out so far, there is one caveat to add if you&#39;re con=
sidering identity caching as the problem solved. A validation node might ha=
ve to consider differently block messages processed if they connect on the =
longest most PoW valid chain for which all blocks have been validated. Or a=
lternatively if they have to be added on a candidate longest most PoW valid=
 chain.<br><br>&gt;&gt; Certainly an important consideration. We store both=
 types. Once there is a stronger candidate header chain we store the header=
s and proceed to obtaining the blocks (if we don&#39;t already have them). =
The blocks are stored in the same table; the confirmed vs. candidate indexe=
s simply point to them as applicable. It is feasible (and has happened twic=
e) for two blocks to share the very same coinbase tx, even with either/all =
bip30/34/90 active (and setting aside future issues here for the sake of si=
mplicity). This remains only because two competing branches can have blocks=
 at the same height, and bip34 requires only height in the coinbase input s=
cript. This therefore implies the same transaction but distinct blocks. It =
is however infeasible for one block to exist in multiple distinct chains. I=
n order for this to happen two blocks at the same height must have the same=
 coinbase (ok), and also the same parent (ok). But this then means that the=
y either (1) have distinct identity due to another header property deviatio=
n, or (2) are the same block with the same parent and are therefore in just=
 one chain. So I don&#39;t see an actual caveat. I&#39;m not certain if thi=
s is the ambiguity that you were referring to. If not please feel free to c=
larify.<br><br>&gt; If you assume no network partition and the no blocks mo=
re than 2h in the future consensus rule, I cannot see how one block with no=
 header property deviation can exist in multiple distinct chains.<br><br>It=
 cannot, that was my point: &quot;(1) have distinct identity due to another=
 header property deviation, or (2) are the same block...&quot;<br><br>&gt; =
The ambiguity I was referring was about a different angle, if the design go=
al of introducing a 64 byte size check is to &quot;it was about being able =
to cache the hash of a (non-malleated) invalid block as permanently invalid=
 to avoid re-downloading and re-validating it&quot;, in my thinking we shal=
l consider the whole block headers caching strategy and be sure we don&#39;=
t get situations where an attacker can attach a chain of low-pow block head=
ers with malleated committed valid tx data yielding a block invalidity at t=
he end, provoking as a side-effect a network-wide data download blowup. So =
I think any implementation of the validation of a block validity, of which =
identity is a sub-problem, should be strictly ordered by adequate proof-of-=
work checks.<br><br>This was already the presumption.<br><br>&gt;&gt; We do=
n&#39;t do this and I don&#39;t see how it would be relevant. If a peer pro=
vides any invalid message or otherwise violates the protocol it is simply d=
ropped.<br>&gt;&gt;<br>&gt;&gt; The &quot;problematic&quot; that I&#39;m re=
ferring to is the reliance on the block hash as a message identifier, becau=
se it does not identify the message and cannot be useful in an effectively =
unlimited number of zero-cost cases.<br><br>&gt; Historically, it was to is=
olate transaction-relay from block-relay to optimistically harden in face o=
f network partition, as this is easy to infer transaction-relay topology wi=
th a lot of heuristics.<br><br>I&#39;m not seeing the connection here. Are =
you suggesting that tx and block hashes may collide with each other? Or tha=
t that a block message may be confused with a transaction message?<br><br>&=
gt; I think this is correct that block hash message cannot be relied on as =
it cannot be useful in an unlimited number of zero-cost cases, as I was poi=
nting that bitcoin core partially mitigate that with discouraging connectio=
ns to block-relay peers servicing block messages (`MaybePunishNodeForBlocks=
`).<br><br>This does not mitigate the issue. It&#39;s essentially dead code=
. It&#39;s exactly like saying, &quot;there&#39;s an arbitrary number of ho=
les in the bucket, but we can plug a subset of those holes.&quot; Infinite =
minus any number is still infinite.<br><br>&gt; I believe somehow the bottl=
eneck we&#39;re circling around is computationally definining what are the =
&quot;usable&quot; identifiers for block messages. The most straightforward=
 answer to this question is the full block in one single peer message, at l=
east in my perspective.<br><br>I don&#39;t follow this statement. The term =
&quot;usable&quot; was specifically addressing the proposal - that a header=
 hash must uniquely identify a block (a header and committed set of txs) as=
 valid or otherwise. As I have pointed out, this will still not be the case=
 if 64 byte blocks are invalidated. It is also not the case that detection =
of type64 malleated blocks can be made more performant if 64 byte txs are g=
lobally invalid. In fact the opposite is true, it becomes more costly (and =
complex) and is therefore just dead code.<br><br>&gt; Reality since headers=
 first synchronization (`getheaders`), block validation has been dissociate=
d in steps for performance reasons, among others.<br><br>Headers first only=
 defers malleation checks. The same checks are necessary whether you perfor=
m blocks first or headers first sync (we support both protocol levels). The=
 only difference is that for headers first, a stored header might later bec=
ome invalidated. However, this is the case with and without the possibility=
 of malleation.<br><br>&gt;&gt; Again, this has no relation to tx hashes/id=
entifiers. Libbitcoin has a tx pool, we just don&#39;t store them in RAM (m=
emory).<br>&gt;&gt;<br>&gt;&gt; I don&#39;t follow this. An invalid 64 byte=
 tx consensus rule would definitely not make it harder to exploit block mes=
sage invalidity. In fact it would just slow down validation by adding a red=
undant rule. Furthermore, as I have detailed in a previous message, caching=
 invalidity does absolutely nothing to increase protection. In fact it make=
s the situation materially worse.<br><br>&gt; Just to recall, in my underst=
anding the proposal we&#39;re discussing is about outlawing 64 bytes size t=
ransactions at the consensus-level to minimize denial-of-service vectors du=
ring block validation. I think we&#39;re talking about each other because t=
he mempool already introduce a layer of caching in bitcoin core, of which t=
he result are re-used at block validation, such as signature verification r=
esults. I&#39;m not sure we can fully waive apart performance consideration=
s, though I agree implementation architecture subsystems like mempool shoul=
d only be a sideline considerations.<br><br>I have not suggested that anyth=
ing is waived or ignored here. I&#39;m stating that there is no &quot;mempo=
ol&quot; performance benefit whatsoever to invalidating 64 byte txs. Mempoo=
l caching could only rely on tx identifiers, not block identifiers. Tx iden=
tifiers are not at issue.<br><br>&gt;&gt; No, this is not the case. As I de=
tailed in my previous message, there is no possible scenario where invalida=
tion caching does anything but make the situation materially worse.<br><br>=
&gt; I think this can be correct that invalidation caching make the situati=
on materially worse, or is denial-of-service neutral, as I believe a full n=
ode is only trading space for time resources in matters of block messages v=
alidation. I still believe such analysis, as detailed in your previous mess=
age, would benefit to be more detailed.<br><br>I don&#39;t know how to add =
any more detail than I already have. There are three relevant consideration=
s:<br><br>(1) block hashes will not become unique identifiers for block mes=
sages.<br>(2) the earliest point at which type64 malleation can be detected=
 will not be reduced.<br>(3) the necessary cost of type64=C2=A0malleated=C2=
=A0determination will not be reduced.<br>(4) the additional consensus rule =
will increase validation cost and code complexity.<br>(5) invalid blocks ca=
n still be produced at no cost that require full double tx hashing/Merkle r=
oot computations.<br><br>Which of these statements are not evident at this =
point?<br><br>&gt;&gt; On the other hand, just dealing with parse failure o=
n the spot by introducing a leading pattern in the stream just inflates the=
 size of p2p messages, and the transaction-relay bandwidth cost.<br>&gt;&gt=
;<br>&gt;&gt; I think you misunderstood me. I am suggesting no change to se=
rialization. I can see how it might be unclear, but I said, &quot;nothing p=
recludes incorporating a requirement for a necessary leading pattern in the=
 stream.&quot; I meant that the parser can simply incorporate the *requirem=
ent* that the byte stream starts with a null input point. That identifies t=
he malleation or invalidity without a single hash operation and while only =
reading a handful of bytes. No change to any messages.<br><br>&gt; Indeed, =
this is clearer with the re-explanation above about what you meant by the &=
quot;null point&quot;.<br><br>Ok<br><br>&gt; In my understanding, you&#39;r=
e suggesting the following algorithm:<br>&gt; - receive transaction p2p mes=
sages<br>&gt; - deserialize transaction p2p messages<br>&gt; - if the trans=
action is a coinbase candidate, verify null input point<br>&gt; - if null i=
nput point pattern invalid, reject the transaction<br><br>No, no part of th=
is thread has any bearing on p2p transaction messages - nor are coinbase tr=
ansactions relayed as transaction messages. You could restate it as:<br><br=
>- receive block p2p messages<br>- if the first tx&#39;s first input does n=
ot have a null point, reject the block<br><br>&gt; If I&#39;m understanding=
 correctly, the last rule has for effect to constraint the transaction spac=
e that can be used to brute-force and mount a Merkle root forgery with a 64=
-byte coinbase transaction.<br>&gt;<br>&gt; As described in the 3.1.1 of th=
e paper: <a href=3D"https://lists.linuxfoundation.org/pipermail/bitcoin-dev=
/attachments/20190225/a27d8837/attachment-0001.pdf" target=3D"_blank" rel=
=3D"nofollow" data-saferedirecturl=3D"https://www.google.com/url?hl=3Dfr&am=
p;q=3Dhttps://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/2=
0190225/a27d8837/attachment-0001.pdf&amp;source=3Dgmail&amp;ust=3D173285743=
2040000&amp;usg=3DAOvVaw04td23bqKDQYDkWTx_hMhq">https://lists.linuxfoundati=
on.org/pipermail/bitcoin-dev/attachments/20190225/a27d8837/attachment-0001.=
pdf</a><br><br>The above approach makes this malleation computationally inf=
easible.<br><br>&gt;&gt; I&#39;m referring to DoS mitigation (the only rele=
vant security consideration here). I&#39;m pointing out that invalidity cac=
hing is pointless in all cases, and in this case is the most pointless as t=
ype64 malleation is the cheapest of all invalidity to detect. I would prefe=
r that all bogus blocks sent to my node are of this type. The worst types o=
f invalidity detection have no mitigation and from a security standpoint ar=
e counterproductive to cache. I&#39;m describing what overall is actually n=
ot a tradeoff. It&#39;s all negative and no positive.<br><br>&gt; I think w=
e&#39;re both discussing the same issue about DoS mitigation for sure. Agai=
n, I think that saying the &quot;invalidity caching&quot; is pointless in a=
ll cases cannot be fully grounded as a statement without precising (a) what=
 is the internal cache(s) layout of the full node processing block messages=
 and (b) the sha256 mining resources available during N difficulty period a=
nd if any miner engage in self-fish mining like strategy.<br><br>It has not=
hing to do with internal cache layout and nothing to do with mining resourc=
es. Not having a cache is clearly more efficient than having a cache that p=
rovides no advantage, regardless of how the cache is laid out. There is no =
cost to forcing a node to perform far more block validation computations th=
an can be precluded by invalidity caching. The caching simply increases the=
 overall computational cost (as would another redundant rule to try and mak=
e it more efficient). Discarding invalid blocks after the minimal amount of=
 work is the most efficient resolution. What one does with the peer at that=
 point is orthogonal (e.g. drop, ban).<br><br>&gt; About (a), I&#39;ll main=
tain my point I think it&#39;s a classic time-space trade-off to ponder in =
function of the internal cache layouts.<br><br>An attacker can throw a near=
ly infinite number of distinct invalid blocks at your node (and all will co=
nnect to the chain and show proper PoW). As such you will encounter zero ca=
che hits and therefore nothing but overhead from the cache. Please explain =
to me in detail how &quot;cache layout&quot; is going to make any differenc=
e at all.<br><br>&gt; About (b) I think we&#39;&#39;ll be back to the heade=
rs synchronization strategy as implemented by a full node to discuss if the=
y&#39;re exploitable asymmetries for self-fish mining like strategies.<br><=
br>I don&#39;t see this as a related/relevant topic. There are zero mining =
resources required to overflow the invalidity cache. Just as Core recently =
published regarding overflowing to its &quot;ban&quot; store, resulting in =
process termination, this then introduces another attack vector that must b=
e mitigated.<br><br>&gt; If you can give a pseudo-code example of the &quot=
;null point&quot; validation implementation in libbitcoin code (?) I think =
this can make the conversation more concrete on the caching aspect.<br><br>=
pseudo-code=C2=A0, not from libbitcoin...<br><br>```<br>bool malleated64(bl=
ock)<br>{<br>=C2=A0 =C2=A0 segregated =3D ((block[80 + 4] =3D=3D 0) and (bl=
ock[80 + 4 + 1] =3D=3D 1))<br>=C2=A0 =C2=A0 return block[segregated ? 86 : =
85] !=3D 0xffffffff00000000000000000000000000000000000000000000000000000000=
00000000<br>}<br>```<br><br>Obviously there is no error handling (e.g. bloc=
k too small, too many inputs, etc.) but that is not relevant to the particu=
lar question. The block.header is fixed size, always 80 bytes. The tx.versi=
on is also fixed, always 4 bytes. A following 0 implies a segregated witnes=
s (otherwise it&#39;s the input count), assuming there is a following 1. Th=
e first and only input for the coinbase tx, which must be the first block t=
x, follows. If it does not match 0xffffffff00000000000000000000000000000000=
00000000000000000000000000000000 then the block is invalid. If it does matc=
h, it is computationally infeasible that the merkle root is type64 malleate=
d. That&#39;s it, absolutely trivial and with no prerequisites. The only th=
ing that even makes it interesting is the segwit bifurcation.<br><br>&gt;&g=
t; Rust has its own set of problems. No need to get into a language Jihad h=
ere. My point was to clarify that the particular question was not about a C=
 (or C++) null pointer value, either on the surface or underneath an abstra=
ction.<br><br>&gt; Thanks for the additional comments on libbitcoin usage o=
f dependencies, yes I don&#39;t think there is a need to get into a languag=
e jihad here. It&#39;s just like all languages have their memory model (sta=
ck, dynamic alloc, smart pointers, etc) and when you&#39;re talking about p=
erformance it&#39;s useful to have their minds, imho.<br><br>Sure, but no l=
anguage difference that I&#39;m aware of could have any bearing on this par=
ticular question.<br><br>Best,<br>Eric<br></blockquote></div>

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