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

From: "'Antoine Poinsot' via Bitcoin Development Mailing List" <[email-masked]>
Date: Sun, 23 Feb 2025 22:35:59 +0000

> A 40x decrease to a validation time of 30 seconds probably is worth a bit of risk for a further improvement.

Depends on what hardware. I don't think a worst case of 30 seconds for end users is worth more risks. A worst case of 30 seconds for miners would probably be concerning.

In addition, although the worst case is important to limit the damage an attacker can do without being economically rational, what we care about for miners attacking each other is economically viable attacks. At the moment the optimal "validation time / cost to attacker" ratio is not the worst case, by a large margin [0].

I believe we should take into account the worst case to miners even if not economically viable for an attacker as a safety margin. But we should keep in mind this is already overestimating the attack risk since in this scenario what we should look at is the worst case validation time of blocks that may have positive returns to an attacker.

> Can you put numbers to this?

Sure. Using my functional test which runs the worst case today and the worst case under various mitigations [1] on a Dell XPS 15 9520 laptop (the model with an i5-12500H) i get 120 seconds for the worst case today and 10 seconds for the worst block with a limited of 2500 (potentially) executed sigops per transaction. To (maybe, they could be running more powerful machines than my laptop) impose such a validation time to other miners an attacker would have to invest 89 (!!) preparation blocks. Sure, with low fees the opportunity cost of mining preparation transactions is not as high as it could be. But still.

[0] [https://delvingbitcoin.org/t/worst-block-validation-time-inquiry/711/70](https://delvingbitcoin.org/t/worst-block-validation-time-inquiry/711/70?u=antoinep)
[1] [https://delvingbitcoin.org/t/worst-block-validation-time-inquiry/711/61](https://delvingbitcoin.org/t/worst-block-validation-time-inquiry/711/61?u=antoinep)
On Thursday, February 20th, 2025 at 8:22 PM, Matt Corallo <[email-masked]> wrote:

> In the delving post you said “This provides a 40x decrease in the worst case validation time with a straightforward and flexible rule minimizing the confiscatory surface. A further 7x decrease is possible by combining it with another rule, which is in my opinion not worth the additional confiscatory surface.”
>
> Can you put numbers to this? How long does it take to validate a full block with this 40x decrease and how long would it take with the further 7x decrease?
>
> A 40x decrease to a validation time of 30 seconds probably is worth a bit of risk for a further improvement. A 40x decrease to 1 second is obviously fine :).
>
>> On Feb 5, 2025, at 19:57, 'Antoine Poinsot' via Bitcoin Development Mailing List <[email-masked]> 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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