From: Antoine Riard <antoine.riard@gmail•com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Re: Great Consensus Cleanup Revival
Date: Thu, 18 Jul 2024 10:39:06 -0700 (PDT) [thread overview]
Message-ID: <ac6cc3b8-43e5-4cd6-aabe-f5ffc4672812n@googlegroups.com> (raw)
In-Reply-To: <a76b8dc5-d37f-4059-882b-207004874887n@googlegroups.com>
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Hi Eric,
> While 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 "null point") and any other input, is not
nullptr vs. !nullptr. A "null point" is a 36 byte value, 32 0x00 byes
followed by 4 0xff bytes. In his infinite wisdom Satoshi decided it was
better (or easier) 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 "null point", and of course cannot be pointed to by a
"null pointer". The coinbase must be identified by comparing those 36 bytes
to the well-known null point value (and if this does not match the Merkle
hash cannot have been type64 malleated).
Good for the clarification here, I had in mind the core's `CheckBlock` path
where the first block transaction pointer is dereferenced to verify if the
transaction is a coinbase (i.e a "null point" where the prevout is null).
Zooming out and back to my remark, I think this is correct that adding a
new 64 byte size check on all block transactions to detect block hash
invalidity could be a low memory overhead (implementation dependant),
rather than making that 64 byte check alone on the coinbase transaction as
in my understanding you're proposing.
> We call this type64 malleability (or malleation where it is not only
possible but occurs).
Yes, the problem which has been described as the lack of "domain
separation".
> The second one is the bip141 wtxid commitment in one of the coinbase
transaction `scriptpubkey` output, which is itself covered by a txid in the
merkle tree.
> While 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 correct it is computationally infeasible for the witness
commitment to be malleated, 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 necessary to validate the witness commitment to
ensure that the correct witness data has been provided in the block message.
>
> 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 blocks, as a function of the number of txs contained.
To precise more your statement in describing source of malleability. The
witness stack can be malleated altering the wtxid and yet still valid. I
think you can still have the case where you're feeded a block header with a
merkle root commitment deserializing to a valid coinbase transaction with
an invalid witness commitment. This is the case of a "block message with
valid header but malleatead committed valid tx data". Validation of the
witness commitment to ensure the correct witness data has been provided in
the block message is indeed necessary.
>> Background: A fully-validated block has established identity in its
block hash. However an invalid block message may include the same block
header, producing the same hash, but with any kind of nonsense following
the header. The purpose of the transaction and witness commitments is of
course to establish this identity, so these two checks are therefore
necessary even under checkpoint/milestone. And then of course the two
Merkle tree issues complicate the tx commitment (the integrity of the
witness commitment is assured by that of the tx commitment).
>>
>> So what does it mean to speak of a block hash derived from:
>> (1) a block message with an unparseable header?
>> (2) a block message with parseable but invalid header?
>> (3) a block message with valid header but unparseable tx data?
>> (4) a block message with valid header but parseable invalid uncommitted
tx data?
>> (5) a block message with valid header but parseable invalid malleated
committed tx data?
>> (6) a block message with valid header but parseable invalid unmalleated
committed tx data?
>> (7) a block message with valid header but uncommitted valid tx data?
>> (8) a block message with valid header but malleated committed valid tx
data?
>> (9) a block message with valid header but unmalleated committed valid tx
data?
>>
>> Note that only the #9 p2p block message contains an actual Bitcoin
block, the others are bogus messages. In all cases the message can be
sha256 hashed to establish the identity of the *message*. And if one's
objective is to reject repeating bogus messages, this might be a useful
strategy. It's already part of the p2p protocol, is orders of magnitude
cheaper to produce than a Merkle root, and has no identity issues.
> I think I mostly agree with the identity issue as laid out so far, there
is one caveat to add if you're considering identity caching as the problem
solved. A validation node might have to consider differently block messages
processed if they connect on the longest most PoW valid chain for which all
blocks have been validated. Or alternatively if they have to be added on a
candidate longest most PoW valid chain.
> Certainly an important consideration. We store both types. Once there is
a stronger candidate header chain we store the headers and proceed to
obtaining the blocks (if we don't already have them). The blocks are stored
in the same table; the confirmed vs. candidate indexes simply point to them
as applicable. It is feasible (and has happened twice) 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 simplicity). This
remains only because two competing branches can have blocks at the same
height, and bip34 requires only height in the coinbase input script. This
therefore implies the same transaction but distinct blocks. It is however
infeasible for one block to exist in multiple distinct chains. In 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 they either
(1) have distinct identity due to another header property deviation, or (2)
are the same block with the same parent and are therefore in just one
chain. So I don't see an actual caveat. I'm not certain if this is the
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 the
future consensus rule, I cannot see how one block with no header property
deviation can exist in multiple distinct chains. The ambiguity I was
referring was about a different angle, if the design goal of introducing a
64 byte size check is to "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", in my thinking we shall consider the
whole block headers caching strategy and be sure we don't get situations
where an attacker can attach a chain of low-pow block headers with
malleated committed valid tx data yielding a block invalidity at the 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.
> We don't do this and I don't see how it would be relevant. If a peer
provides any invalid message or otherwise violates the protocol it is
simply dropped.
>
> The "problematic" that I'm referring to is the reliance on the block hash
as a message identifier, because it does not identify the message and
cannot be useful in an effectively unlimited number of zero-cost cases.
Historically, it was to isolate transaction-relay from block-relay to
optimistically harden in face of network partition, as this is easy to
infer transaction-relay topology with a lot of heuristics.
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
pointing that bitcoin core partially mitigate that with discouraging
connections to block-relay peers servicing block messages
(`MaybePunishNodeForBlocks`).
> #4 and #5 refer to "uncommitted" and "malleated committed". It may not be
clear, but "uncommitted" means that the tx commitment is not valid (Merkle
root doesn't match the header's value) and "malleated committed" means that
the (matching) commitment cannot be relied upon because the txs represent
malleation, invalidating the identifier. So neither of these are usable
identifiers.
>
> It seems you may be referring to "unconfirmed" txs as opposed to
"uncommitted" txs. This doesn't pertain to tx storage or identifiers.
Neither #7 nor #8 are usable for the same reasons.
>
> I'm making no reference to tx malleability. This concerns only Merkle
tree (block hash) malleability, the two types described in detail in the
paper I referenced earlier, here again:
>
>
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20190225/a27d8837/attachment-0001.pdf
I believe somehow the bottleneck we're circling around is computationally
definining what are the "usable" identifiers for block messages.
The most straightforward answer to this question is the full block in one
single peer message, at least in my perspective.
Reality since headers first synchronization (`getheaders`), block
validation has been dissociated in steps for performance reasons, among
others.
> Again, this has no relation to tx hashes/identifiers. Libbitcoin has a tx
pool, we just don't store them in RAM (memory).
> I don't follow this. An invalid 64 byte tx consensus rule would
definitely not make it harder to exploit block message invalidity. In fact
it would just slow down validation by adding a redundant rule. Furthermore,
as I have detailed in a previous message, caching invalidity does
absolutely nothing to increase protection. In fact it makes the situation
materially worse.
Just to recall, in my understanding the proposal we're discussing is about
outlawing 64 bytes size transactions at the consensus-level to minimize
denial-of-service vectors during block validation. I think we're talking
about each other because the mempool already introduce a layer of caching
in bitcoin core, of which the result are re-used at block validation, such
as signature verification results. I'm not sure we can fully waive apart
performance considerations, though I agree implementation architecture
subsystems like mempool should only be a sideline considerations.
> No, this is not the case. As I detailed in my previous message, there is
no possible scenario where invalidation caching does anything but make the
situation materially worse.
I think this can be correct that invalidation caching make the situation
materially worse, or is denial-of-service neutral, as I believe a full node
is only trading space for time resources in matters of block messages
validation. I still believe such analysis, as detailed in your previous
message, would benefit to be more detailed.
> On the other hand, just dealing with parse failure on the spot by
introducing a leading pattern in the stream just inflates the size of p2p
messages, and the transaction-relay bandwidth cost.
> I think you misunderstood me. I am suggesting no change to serialization.
I can see how it might be unclear, but I said, "nothing precludes
incorporating a requirement for a necessary leading pattern in the stream."
I meant that the parser can simply incorporate the *requirement* that the
byte stream starts with a null input point. That identifies the malleation
or invalidity without a single hash operation and while only reading a
handful of bytes. No change to any messages.
Indeed, this is clearer with the re-explanation above about what you meant
by the "null point". 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
If I'm understanding correctly, the last rule has for effect to constraint
the transaction space that can be used to brute-force and mount a Merkle
root forgery with a 64-byte coinbase transaction.
As described in the 3.1.1 of the paper:
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20190225/a27d8837/attachment-0001.pdf
> I'm referring to DoS mitigation (the only relevant security consideration
here). I'm pointing out that invalidity caching is pointless in all cases,
and in this case is the most pointless as type64 malleation is the cheapest
of all invalidity to detect. I would prefer that all bogus blocks sent to
my node are of this type. The worst types of invalidity detection have no
mitigation and from a security standpoint are counterproductive to cache.
I'm describing what overall is actually not a tradeoff. It's all negative
and no positive.
I think we're both discussing the same issue about DoS mitigation for sure.
Again, I think that saying the "invalidity caching" is pointless in all
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 and if
any miner engage in self-fish mining like strategy.
About (a), I'll maintain my point I think it's a classic time-space
trade-off to ponder in function of the internal cache layouts. About (b) I
think we''ll be back to the headers synchronization strategy as implemented
by a full node to discuss if they're exploitable asymmetries for self-fish
mining like strategies.
If you can give a pseudo-code example of the "null point" validation
implementation in libbitcoin code (?) I think this can make the
conversation more concrete on the caching aspect.
> Rust has its own set of problems. No need to get into a language Jihad
here. 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
abstraction.
Thanks for the additional comments on libbitcoin usage of dependencies, yes
I don't think there is a need to get into a language jihad here. It's just
like all languages have their memory model (stack, dynamic alloc, smart
pointers, etc) and when you're talking about performance it's useful to
have their minds, imho.
Best,
Antoine
ots hash: 058d7b3adb154a3e64d5f8ccf1944903bcd0c49dbb525f7212adf4f7ac7f8c55
Le mardi 9 juillet 2024 à 02:16:20 UTC+1, Eric Voskuil a écrit :
> > This is why we don't use C - unsafe, unclear, unnecessary.
>
> Actually, I think libbitcoin is using its own maintained fork of
> secp256k1, which is written in C.
>
>
> We do not maintain secp256k1 code. For years that library carried the same
> version, despite regular breaking changes to its API. This compelled us to
> place these different versions on distinct git branches. When it finally
> became versioned we started phasing this unfortunate practice out.
>
> Out of the 10 repositories and at least half million lines of code, apart
> from an embedded copy of qrencode that we don’t independently maintain, I
> believe there is only one .c file in use in the entire project - the
> database mmap.c implementation for msvc builds. This includes hash
> functions, with vectorization optimizations, etc.
>
>
> For sure, I wouldn't recommend using C across a whole codebase as it's not
> memory-safe (euphemism) though it's still un-match if you wish to
> understand low-level memory management in hot paths.
>
>
> This is a commonly held misperception.
>
> It can be easier to use C++ or Rust, though it doesn't mean it will be as
> (a) perf optimal and (b) hardened against side-channels.
>
>
> Rust has its own set of problems. No need to get into a language Jihad
> here. 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
> abstraction.
>
> e
>
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next prev parent reply other threads:[~2024-07-18 18:07 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-24 18:10 [bitcoindev] " 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-03-26 19:11 ` [bitcoindev] " Antoine Riard
2024-03-27 10:35 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-03-27 18:57 ` Antoine Riard
2024-04-18 0:46 ` Mark F
2024-04-18 10:04 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-04-25 6:08 ` Antoine Riard
2024-04-30 22:20 ` Mark F
2024-05-06 1:10 ` Antoine Riard
2024-07-20 21:39 ` Murad Ali
2024-06-17 22:15 ` Eric Voskuil
2024-06-18 8:13 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-06-18 13:02 ` Eric Voskuil
2024-06-21 13:09 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-06-24 0:35 ` Eric Voskuil
2024-06-27 9:35 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-06-28 17:14 ` Eric Voskuil
2024-06-29 1:06 ` Antoine Riard
2024-06-29 1:31 ` Eric Voskuil
2024-06-29 1:53 ` Antoine Riard
2024-06-29 20:29 ` Eric Voskuil
2024-06-29 20:40 ` Eric Voskuil
2024-07-02 2:36 ` Antoine Riard
2024-07-03 1:07 ` Larry Ruane
2024-07-03 23:29 ` Eric Voskuil
2024-07-04 13:20 ` Antoine Riard
2024-07-04 14:45 ` Eric Voskuil
2024-07-18 17:39 ` Antoine Riard [this message]
2024-07-20 20:29 ` Eric Voskuil
2024-07-03 1:13 ` Eric Voskuil
2024-07-02 10:23 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-07-02 15:57 ` Eric Voskuil
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