Subject: [bitcoindev] Re: Great Consensus Cleanup Revival

From: Antoine Riard <[email-masked]>
Date: Tue, 26 Mar 2024 12:11:28 -0700 (PDT)

Hi Poinsot,

I think fixing the timewarp attack is a good idea, especially w.r.t safety 
implications of long-term timelocks usage.

The only beneficial case I can remember about the timewarp issue is 
"forwarding blocks" by maaku for on-chain scaling:
http://freico.in/forward-blocks-scalingbitcoin-paper.pdf

Shall we as a community completely disregard this approach for on-chain 
settlement throughput scaling ?
Personally, I think you can still design extension-block / side-chains like 
protocols by using other today available
Bitcoin Script mechanisms and get roughly (?) the same security / 
scalability trade-offs. Shall be fine to me to fix timewarp.

Worst-block validation time is concerning. I bet you can do worst than your 
examples if you're playing with other vectors like
low-level ECC tricks and micro-architectural layout of modern processors.

Consensus invalidation of old legacy scripts was quite controversial last 
time a consensus cleanup was proposed:
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-March/016714.html

Only making scripts invalid after a given block height (let's say the 
consensus cleanup activation height) is obviously a
way to solve the concern and any remaining sleeping DoSy unspent coins can 
be handled with  newly crafted and dedicated
transaction-relay rules (e.g at max 1000 DoSy coins can be spent per block 
for a given IBT span).

I think any consensus boundaries on the minimal transaction size would need 
to be done carefully and have all lightweight
clients update their own transaction acceptance logic to enforce the check 
to avoid years-long transitory massive double-spend
due to software incoordination. I doubt `MIN_STANDARD_TX_NON_WITNESS_SIZE` 
is implemented correctly by all transaction-relay
backends and it's a mess in this area. Quid if we have  < 64 bytes 
transaction where the only witness is enforced to be a minimal 1-byte
as witness elements are only used for higher layers protocols semantics  ? 
Shall get its own "only-after-height-X" exemption, I think.

Making coinbase unique by requesting the block height to be enforced in 
nLocktime, sounds more robust to take a monotonic counter
in the past in case of accidental or provoked shallow reorgs. I can see of 
you would have to re-compute a block template, loss a round-trip
compare to your mining competitors. Better if it doesn't introduce a new 
DoS vector at mining job distribution and control.

Beyond, I don't deny other mentioned issues (e.g UTXO entries growth limit) 
could be source of denial-of-service but a) I think it's hard
to tell if they're economically neutral on modern Bitcoin use-cases and 
their plausible evolvability and b) it's already a lot of careful consensus
code to get right :)

Best,
Antoine

Le dimanche 24 mars 2024 à 19:06:57 UTC, Antoine Poinsot a écrit :

> Hey all,
>
> I've recently posted about the Great Consensus Cleanup there: 
> https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710.
>
> I'm starting a thread on the mailing list as well to get comments and 
> opinions from people who are not on Delving.
>
> TL;DR:
> - i think the worst block validation time is concerning. The mitigations 
> proposed by Matt are effective, but i think we should also limit the 
> maximum size of legacy transactions for an additional safety margin;
> - i believe it's more important to fix the timewarp bug than people 
> usually think;
> - it would be nice to include a fix to make coinbase transactions unique 
> once and for all, to avoid having to resort back to doing BIP30 validation 
> after block 1,983,702;
> - 64 bytes transactions should definitely be made invalid, but i don't 
> think there is a strong case for making less than 64 bytes transactions 
> invalid.
>
> Anything in there that people disagree with conceptually?
> Anything in there that people think shouldn't (or don't need to) be fixed?
> Anything in there which can be improved (a simpler, or better fix)?
> Anything NOT in there that people think should be fixed?
>
>
> Antoine Poinsot
>

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