Subject: Re: [bitcoindev] Update on the Great Consensus Cleanup Revival

From: Antoine Riard <[email-masked]>
Date: Fri, 7 Feb 2025 05:02:46 -0800 (PST)

Hi Darosior,

Thanks for the work on reviving the Great Consensus Cleanup.

> Now i would like to update the broader Bitcoin development community on 
the outcome of this effort.
> I believe a Consensus Cleanup proposal should include the following.
> - A fix for vulnerabilities surrounding the use of timestamps in the 
difficulty adjustment
> algorithm. In particular, a fix for the timewarp attack with a 7200 
seconds grace period as well
> as a fix for the Murch-Zawy attack [4] by making invalid any difficulty 
adjustment period with a
> negative duration.
> - A fix for long block validation times with a minimal "confiscation 
surface", by introducing a
> per-transaction limit on the number of legacy sigops in the inputs.
> - A fix for merkle tree weaknesses by making transactions which serialize 
to exactly 64 bytes
> invalid.
> - A fix for duplicate transactions to supplement BIP34 in order to avoid 
resuming unnecessary BIP30
> validation in the future. This is achieved by mandating the nLockTime 
field of coinbase
> transaction to be set to the height of their block minus 1.
> 
> I have started drafting a BIP draft with the detailed specs for this.

So assuming some hypothetical future BIP-9-based deployment, there can be 
multiple soft-forks
activation at the same time (up to 30), as a soft-fork can be assigned 
distinct block nVersion 
bit. While BIP-9 recommends a 95% activation threshold on mainnet, it's one 
line change to
adjust the `nThreshold` variable to another value. For the fix about 
timewarp vulnerabilities,
as it's an additional constraint on the validity of mined blocks allowed 
the current reward
schedule, there could be some reluctance in adopting the new consensus 
rules, and this fix
could deserve a specific threshold of its own - imho.

Additionally, the proposed soft-fork fixes are very different than the 3 
set of rules than
have been activated under the DEPLOYMENT_TAPROOT flag. While BIP340, BIP341 
and BIP342 are
building on top of each other in a modular fashion, this is not the case 
here with the 4
proposed fixes 
("timewarp"/"worst-block-time"/"merkle-tree-weakness"/"enhanced-duplicated-txn",
as adoption of one fix is not necessitated to adopt the other fixes. There 
could be some
community consensus on 
"timewarp"/"merke-tree-weakness"/"enhanced-duplicated-txn", while
the minimal "confiscation surface" (which was very controversial when the 
GCC was proposed
the 1st time in 2019), not suiting a wide majority of folks, or even people 
who have use-cases
potentially affected.

For those reasons, I think it's wiser to spread each fix in its own BIP and 
patchset of
code changes to not only have discussions of each fix in parallel, though 
also eventually
enable separate activation of each consensus fix, in the optic that each 
fix might gather
different level of consensus, whatever the reasons.

This might be a stylistic note, though I could point in bitcoin core code 
today implemented
check in the script interpreter right in the crux of consensus code paths 
that is just stale
due to a never-activated BIP (-- yes I'm starring at you SIGPUSHONLY).

Best,
Antoine (the "evil" one)

OTS hash: 6c809fde007a53f380af41f0e22f3b9e95c83da24c2718ac2de0004570f94990

Le jeudi 6 février 2025 à 21:46:42 UTC, Murch a écrit :

> Thank you for the update and your work on the Great Consensus Cleanup. I 
> am looking forward to reading your BIP, and would hope that you could 
> share here or in the BIP’s Rationale what convinced you to change the 
> grace period from 600 seconds to 7200 seconds and how the nLockTime of 
> height-1 won out.
>
> Cheers,
> Murch
>
> On 2025-02-05 13:09, 'Antoine Poinsot' via Bitcoin Development Mailing 
> List wrote:
> > Hi everyone,
> > 
> > A bit over a year ago i started working on revisiting the 2019 Great 
> Consensus Cleanup proposal from
> > Matt Corallo [0]. His proposal included:
> > - making <=64 bytes transactions invalid to fix merkle tree weaknesses;
> > - making non-pushonly scriptSigs, FindAndDelete matches, 
> OP_CODESEPARATOR and non-standard sighash
> > types fail script validation to mitigate the worst case block validation 
> time;
> > - restrict the nTime field of the first block in each difficulty 
> adjustment interval to be no less
> > than 600 seconds lower than the previous block's;
> > 
> > I set out to research the impact of each of the vulnerabilities this 
> intended to patch, the
> > alternative fixes possible for each and finally if there was any other 
> protocol bug fix we'd want to
> > include if we went through the considerable effort of soft forking 
> Bitcoin already.
> > 
> > Later in March i shared some first findings on Delving [1] and 
> advertized the effort on this mailing
> > list [2]. I also created a companion thread on Delving, kept private, to 
> discuss the details of the
> > worst case block validation time [3]. As one would expect due to the 
> larger design space available
> > to fix this issue, this private thread is where most of the discussion 
> would happen. Thank you to
> > everyone who contributed feedback, insights, ideas and argumented 
> opinions on the different issues
> > all along the process.
> > 
> > Now i would like to update the broader Bitcoin development community on 
> the outcome of this effort.
> > I believe a Consensus Cleanup proposal should include the following.
> > - A fix for vulnerabilities surrounding the use of timestamps in the 
> difficulty adjustment
> > algorithm. In particular, a fix for the timewarp attack with a 7200 
> seconds grace period as well
> > as a fix for the Murch-Zawy attack [4] by making invalid any difficulty 
> adjustment period with a
> > negative duration.
> > - A fix for long block validation times with a minimal "confiscation 
> surface", by introducing a
> > per-transaction limit on the number of legacy sigops in the inputs.
> > - A fix for merkle tree weaknesses by making transactions which 
> serialize to exactly 64 bytes
> > invalid.
> > - A fix for duplicate transactions to supplement BIP34 in order to avoid 
> resuming unnecessary BIP30
> > validation in the future. This is achieved by mandating the nLockTime 
> field of coinbase
> > transaction to be set to the height of their block minus 1.
> > 
> > I have started drafting a BIP draft with the detailed specs for this.
> > 
> > Antoine Poinsot
> > 
> > 
> > [0] 
> https://github.com/TheBlueMatt/bips/blob/7f9670b643b7c943a0cc6d2197d3eabe661050c2/bip-XXXX.mediawiki
> > [1] https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710
> > [2] https://groups.google.com/g/bitcoindev/c/CAfm7D5ppjo/m/bYJ3BiOuAAAJ
> > [3] https://delvingbitcoin.org/t/worst-block-validation-time-inquiry/711
> > [4] 
> https://delvingbitcoin.org/t/zawy-s-alternating-timestamp-attack/1062#variant-on-zawys-attack-2
> > 
>
>

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