Subject: Re: [bitcoindev] Re: Great Consensus Cleanup Revival

From: Eric Voskuil <[email-masked]>
Date: Sat, 29 Jun 2024 13:40:39 -0700 (PDT)

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.

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. 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 stated objective for a consensus rule to invalidate all 64 byte txs is:

> being able to cache the hash of a (non-malleated) invalid block as 
permanently invalid to avoid re-downloading and re-validating it.

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.

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.

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.

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.

Eric

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