From: Eric Voskuil <eric@voskuil•org>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Re: Great Consensus Cleanup Revival
Date: Tue, 2 Jul 2024 18:13:20 -0700 (PDT) [thread overview]
Message-ID: <d9834ad5-f803-4a39-a854-95b2439738f5n@googlegroups.com> (raw)
In-Reply-To: <301c64c7-0f0f-476a-90c4-913659477276n@googlegroups.com>
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Hi Antoine R,
>> Ok, thanks for clarifying. I'm still not making the connection to
"checking a non-null [C] pointer" but that's prob on me.
> A C pointer, which is a language idiome assigning to a memory address A
the value o memory address B can be 0 (or NULL a standard macro defined in
stddef.h).
> Here a snippet example of linked list code checking the pointer
(`*begin_list`) is non null before the comparison operation to find the
target element list.
> ...
> While both libbitcoin and bitcoin core are both written in c++, you still
have underlying pointer derefencing playing out to access the coinbase
transaction, and all underlying implications in terms of memory management.
I'm familiar with pointers ;).
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).
> I think it's interesting to point out the two types of malleation that a
bitcoin consensus validation logic should respect w.r.t block validity
checks. Like you said the first one on the merkle root committed in the
headers's `hashMerkleRoot` due to the lack of domain separation between
leaf and merkle tree nodes.
We call this type64 malleability (or malleation where it is not only
possible but occurs).
> 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.
>> Caching identity in the case of invalidity is more interesting question
than it might seem.
>> 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.
>> The concept of Bitcoin block hash as unique identifier for invalid p2p
block messages is problematic. Apart from the malleation question, what is
the Bitcoin block hash for a message with unparseable data (#1 and #3)?
Such messages are trivial to produce and have no block hash.
> For reasons, bitcoin core has the concept of outbound `BLOCK_RELAY` (in
`src/node/connection_types.h`) where some preferential peering policy is
applied in matters of block messages download.
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.
>> What is the useful identifier for a block with malleated commitments (#5
and #8) or invalid commitments (#4 and #7) - valid txs or otherwise?
> The block header, as it commits to the transaction identifier tree can be
useful as much for #4 and #5.
#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.
> On the bitcoin core side, about #7 the uncommitted valid tx data can be
already present in the validation cache from mempool acceptance. About #8,
the malleaed committed valid transactions shall be also committed in the
merkle root in headers.
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.
>> This seems reasonable at first glance, but given the list of scenarios
above, which does it apply to?
>> This seems reasonable at first glance, but given the list of scenarios
above, which does it apply to? Presumably the invalid header (#2) doesn't
get this far because of headers-first.
>> That leaves just invalid blocks with useful block hash identifiers (#6).
In all other cases the message is simply discarded. In this case the
attempt is to move category #5 into category #6 by prohibiting 64 byte txs.
> Yes, it's moving from the category #5 to the category #6. Note,
transaction malleability can be a distinct issue than lack of domain
separation.
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
>> The requirement to "avoid re-downloading and re-validating it" is about
performance, presumably minimizing initial block download/catch-up time.
There is a > computational cost to producing 64 byte malleations and none
for any of the other bogus block message categories above, including the
other form of malleation. > Furthermore, 64 byte malleation has almost zero
cost to preclude. No hashing and not even true header or tx parsing are
required. Only a handful of bytes must be read > from the raw message
before it can be discarded presently.
>> That's actually far cheaper than any of the other scenarios that again,
have no cost to produce. The other type of malleation requires parsing all
of the txs in the block and > hashing and comparing some or all of them. In
other words, if there is an attack scenario, that must be addressed before
this can be meaningful. In fact all of the other bogus message scenarios
(with tx data) will remain more expensive to discard than this one.
> In practice on the bitcoin core side, the bogus block message categories
from #4 to #6 are already mitigated by validation caching for transactions
that have been received early. While libbitcoin has no mempool (at least in
earlier versions) transactions buffering can be done by bip152's
HeadersAndShortIds message.
Again, this has no relation to tx hashes/identifiers. Libbitcoin has a tx
pool, we just don't store them in RAM (memory).
> About #7 and #8, introducing a domain separation where 64 bytes
transactions are rejected and making it harder to exploit #7 and #8
categories of bogus block messages. This is correct that bitcoin core might
accept valid transaction data before the merkle tree commitment has been
verified.
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.
>> The problem arises from trying to optimize dismissal by storing an
identifier. Just *producing* the identifier is orders of magnitude more
costly than simply dismissing this > bogus message. I can't imagine why any
implementation would want to compute and store and retrieve and recompute
and compare hashes when the alterative is just dismissing the bogus
messages with no hashing at all.
>> Bogus messages will arrive, they do not even have to be requested. The
simplest are dealt with by parse failure. What defines a parse is entirely
subjective. Generally it's
>> "structural" but nothing precludes incorporating a requirement for a
necessary leading pattern in the stream, sort of like how the witness
pattern is identified. If we were
>> going to prioritize early dismissal this is where we would put it.
> I don't think this is that simple - While producing an identifier comes
with a computational cost (e.g fixed 64-byte structured coinbase
transaction), if the full node have a hierarchy of validation cache like
bitcoin core has already, the cost of bogus block messages can be slashed
down.
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.
> 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.
>> However, there is a tradeoff in terms of early dismissal. Looking up
invalid hashes is a costly tradeoff, which becomes multiplied by every
block validated. For example, expending 1 millisecond in hash/lookup to
save 1 second of validation time in the failure case seems like a
reasonable tradeoff, until you multiply across the whole chain. > 1 ms
becomes 14 minutes across the chain, just to save a second for each mallied
block encountered. That means you need to have encountered 840 such mallied
blocks > just to break even. Early dismissing the block for non-null
coinbase point (without hashing anything) would be on the order of 1000x
faster than that (breakeven at 1 > encounter). So why the block hash cache
requirement? It cannot be applied to many scenarios, and cannot be optimal
in this one.
> I think what you're describing is more a classic time-space tradeoff
which is well-known in classic computer science litterature. In my
reasonable opinion, one should more reason under what is the security
paradigm we wish for bitcoin block-relay network and perduring
decentralization, i.e one where it's easy to verify block messages proofs
which could have been generated on specialized hardware with an asymmetric
cost. Obviously encountering 840 such malliead blocks to make it break even
doesn't make the math up to save on hash lookup, unless you can reduce the
attack scenario in terms of adversaries capabilities.
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.
Best,
Eric
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next prev parent reply other threads:[~2024-07-03 1:31 UTC|newest]
Thread overview: 26+ 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
2024-06-17 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 1:13 ` Eric Voskuil [this message]
2024-07-02 10:23 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
2024-07-02 15:57 ` Eric Voskuil
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