Great Consensus Cleanup Revival

[Antoine Poinsot]

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

[Antoine Riard]

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

[Antoine Poinsot]

Hi Poinsot,

Hi Riard,

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

I would not qualify this hack of "beneficial". Besides the centralization pressure of an increased block frequency, leveraging the timewarp to achieve it would put the network constantly on the Brink of being seriously (fatally?) harmed. And this sets pernicious incentives too. Every individual user has a short-term incentive to get lower fees by the increased block space, at the expense of all users longer term. And every individual miner has an incentive to get more block reward at the expense of future miners. (And of course bigger miners benefit from an increased block frequency.)

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.

Note in my writeup i suggest we do not introduce a minimum transaction, but we instead only make 64 bytes transactions invalid. See https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710#can-we-come-up-with-a-better-fix-10:

However the BIP proposes to also make less-than-64-bytes transactions invalid. Although they are of no (or little) use, such transactions are not harmful. I believe considering a type of transaction useless is not sufficient motivation for making them invalid through a soft fork.

Making (exactly) 64 bytes long transactions invalid is also what AJ implemented in his pull request to Bitcoin-inquisition.

I doubt `MIN_STANDARD_TX_NON_WITNESS_SIZE` is implemented correctly by all transaction-relay backends and it's a mess in this area.

What type of backend are you referring to here? Bitcoin full nodes reimplementations? These transactions have been non-standard in Bitcoin Core for the past 6 years (commit 7485488e907e236133a016ba7064c89bf9ab6da3).

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 ?

This restriction is on the size of the transaction serialized without witness. So this particular instance would not be affected and whatever the witness is isn't relevant.

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.

Could you clarify? Are you suggesting something else than to set the nLockTime in the coinbase transaction to the height of the block? If so, what exactly are you referring to by "monotonic counter in the past"?

At any rate in my writeup i suggested making the coinbase commitment mandatory (even when empty) instead for compatibility reasons.

That said, since we could make this rule kick in in 25 years from now, we might want to just do the Obvious Thing and just require the height in nLockTime.

 and b) it's already a lot of careful consensus

code to get right :)

Definitely. I just want to make sure we are not missing anything important if a soft fork gets proposed along these lines in the future.

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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[Antoine Riard]

Hi Darosior,

> I would not qualify this hack of "beneficial". Besides the centralization pressure of an increased block frequency, leveraging the timewarp to achieve it would put the network constantly on the Brink of being seriously (fatally?) harmed. And this sets pernicious incentives too. Every individual user has a short-term incentive to get lower fees by the increased block space, at the expense of all users longer term. And every individual miner has an incentive to get more block reward at the expense of future miners. (And of course bigger miners benefit from an increased block frequency.)

I'm not saying the hack is beneficial either. The "forward block" paper is just good to provide more context around timewarp.

> Note in my writeup i suggest we do not introduce a minimum transaction, but we instead only make 64 bytes transactions invalid

I think it's easier for the sake of analysis.

See this mailing list issue for 60-byte example transaction use-case: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-May/017883.html

Only I'm aware of to the best of my knowledge.

> What type of backend are you referring to here?

I can't find where `MIN_STANDARD_TX_NON_WITNESS_SIZE` is checked in btcd's `maybeAcceptTransaction()`.

> This restriction is on the size of the transaction serialized without witness. 

Oky.

> Could you clarify? Are you suggesting something else than to set the nLockTime in the coinbase transaction to the height of the block? If so, what exactly are you referring to by "monotonic counter in the past"?

Thinking more, I believe it's okay to use the nLocktime in the coinbase transaction, as the wtxid of the coinbase is assumed to be 0x00.

To be checked if it doesn't break anything w.rt Stratum V2 / mining job distribution.

Best,

Antoine

  

[Mark F]

On Wednesday, March 27, 2024 at 4:00:34 AM UTC-7 Antoine Poinsot wrote:

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

I would not qualify this hack of "beneficial". Besides the centralization pressure of an increased block frequency, leveraging the timewarp to achieve it would put the network constantly on the Brink of being seriously (fatally?) harmed. And this sets pernicious incentives too. Every individual user has a short-term incentive to get lower fees by the increased block space, at the expense of all users longer term. And every individual miner has an incentive to get more block reward at the expense of future miners. (And of course bigger miners benefit from an increased block frequency.)

 

Every single concern mentioned here is addressed prominently in the paper/presentation for Forward Blocks:

* Increased block frequency is only on the compatibility chain, where the content of blocks is deterministic anyway. There is no centralization pressure from the frequency of blocks on the compatibility chain, as the content of the blocks is not miner-editable in economically meaningful ways. Only the block frequency of the forward block chain matters, and here the block frequency is actually *reduced*, thereby decreasing centralization pressure.

* The elastic block size adjustment mechanism proposed in the paper is purposefully constructed so that users or miners wanting to increase the block size beyond what is currently provided for will have to pay significantly (multiple orders of magnitude) more than they could possibly acquire from larger blocks, and the block size would re-adjust downward shortly after the cessation of that artificial fee pressure.

* Increased block frequency of compatibility blocks has no effect on the total issuance, so miners are not rewarded by faster blocks.

You are free to criticize Forward Blocks, but please do so by actually addressing the content of the proposal. Let's please hold a standard of intellectual excellence on this mailing list in which ideas are debated based on content-level arguments rather than repeating inaccurate takes from Reddit/Twitter.

To the topic of the thread, disabling time-warp will close off an unlikely and difficult to pull off subsidy draining attack that to activate would necessarily require weeks of forewarning and could be easily countered in other ways, with the tradeoff of removing the only known mechanism for upgrading the bitcoin protocol to larger effective block sizes while staying 100% compatible with un-upgraded nodes (all nodes see all transactions).

I think we should keep our options open.

-Mark

[Antoine Poinsot]

You are free to criticize Forward Blocks, but please do so by actually addressing the content of the proposal. Let's please hold a standard of intellectual excellence on this mailing list in which ideas are debated based on content-level arguments rather than repeating inaccurate takes from Reddit/Twitter.

You are the one being dishonest here. Look, i understand you came up with a fun hack exploiting bugs in Bitcoin and you are biased against fixing them. Yet, the cost of not fixing timewarp objectively far exceeds the cost of making "forward blocks" impossible.

As already addressed in the DelvingBitcoin post:

1. The timewarp bug significantly changes the 51% attacker threat model. Without exploiting it a censoring miner needs to continuously keep more hashrate than the rest of the network combined for as long as he wants to prevent some people from using Bitcoin. By exploiting timewarp the attacker can prevent everybody from using Bitcoin within 40 days.
2. The timewarp bug allows an attacking miner to force on full nodes more block data than they agreed to. This is actually the attack leveraged by your proposal. I believe this variant of the attack is more likely to happen, simply for the reason that all participants of the system have a short term incentive to exploit this (yay lower fees! yay more block subsidy!), at the expense of the long term health of the system. As the block subsidy exponentially decreases miners are likely to start playing more games and that's a particularly attractive one. Given the level of mining centralization we are witnessing [0] i believe this is particularly worrisome.
3. I'm very skeptical of arguments about how "we" can stop an attack which requires "weeks of forewarning". Who's we? How do we proceed, all Bitcoin users coordinate and arbitrarily decide of the validity of a block? A few weeks is very little time if this is at all achievable. If you add on top of that the political implications of the previous point it gets particularly messy.

I've got better things to do than to play "you are being dishonest! -no it's you -no you" games. So unless you bring something new to the table this will be my last reply to your accusations.

Antoine

[0] https://x.com/0xB10C/status/1780611768081121700

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[Antoine Riard]

Hi Maaku,

> Every single concern mentioned here is addressed prominently in the paper/presentation for Forward Blocks:

>

> * Increased block frequency is only on the compatibility chain, where the content of blocks is deterministic anyway. There is no centralization pressure from the frequency > of blocks on the compatibility chain, as the content of the blocks is not miner-editable in economically meaningful ways. Only the block frequency of the forward block > chain matters, and here the block frequency is actually *reduced*, thereby decreasing centralization pressure.

>

> * The elastic block size adjustment mechanism proposed in the paper is purposefully constructed so that users or miners wanting to increase the block size beyond what > is currently provided for will have to pay significantly (multiple orders of magnitude) more than they could possibly acquire from larger blocks, and the block size would re-> adjust downward shortly after the cessation of that artificial fee pressure.

> * Increased block frequency of compatibility blocks has no effect on the total issuance, so miners are not rewarded by faster blocks.

> You are free to criticize Forward Blocks, but please do so by actually addressing the content of the proposal. Let's please hold a standard of intellectual excellence on this > mailing list in which ideas are debated based on content-level arguments rather than repeating inaccurate takes from Reddit/Twitter.

> To the topic of the thread, disabling time-warp will close off an unlikely and difficult to pull off subsidy draining attack that to activate would necessarily require weeks of > forewarning and could be easily countered in other ways, with the tradeoff of removing the only known mechanism for upgrading the bitcoin protocol to larger effective > block sizes while staying 100% compatible with un-upgraded nodes (all nodes see all transactions).

> I think we should keep our options open.

Somehow, I'm sharing your concerns on preserving the long-term evolvability w.r.t scalability options

of bitcoin under the security model as very roughly describer in the paper. Yet, from my understanding

of the forwarding block proposal as described in your paper, I wonder if the forward block chain could

be re-pegged to the main bitcoin chain using the BIP141 extensible commitment structure (assuming

a future hypothetical soft-fork).

From my understanding, it's like doubly linked-list in C, you just need a pointer in the BIP141 extensible

commitment structure referencing back the forward chain headers. If one wishes no logically authoritative

cross-chain commitment, one could leverage some dynamic-membership multi-party signature. This

DMMS could even be backup by proof-of-work based schemes.

The forward block chain can have higher block-rate frequency and the number of block headers be

compressed in a merkle tree committed in the BIP141 extensible commitment structure. Compression

structure can only be defined by the forward chain consensus algorithm to allow more efficient accumulator

than merkle tree to be used".

The forward block chain can have elastic block size consensus-bounded by miners fees on long period

of time. Transaction elements can be just committed in the block headers themselves, so no centralization

pressure on the main chain. Increased block frequency or block size on the forward block chain have not

effect on the total issuance (modulo the game-theory limits of the known empirical effects of colored coins

on miners incentives).

I think the time-warp issues opens the door to economically non-null exploitation under some scenarios

over some considered time periods. If one can think to other ways to mitigate the issue in minimal and

non-invasive way w.r.t current Bitcoin consensus rules and respecting un-upgraded node ressources

consumption, I would say you're free to share them.

I can only share your take on maintaining a standard of intellectual excellence on the mailing list,

and avoid faltering in Reddit / Twitter-style "madness of the crowd"-like conversations.

Best,

Antoine

[Mark F]

Hi Antoine,

That's a reasonable suggestion, and one which has been discussed in the past under various names. Concrete ideas for a pegged extension-block side chain go back to 2014 at the very least. However there is one concrete way in which these proposals differ from forward blocks: the replay of transactions to the compatibility block chain. With forward blocks, even ancient versions of bitcoind that have been running since 2013 (picked as a cutoff because of the probabilistic fork caused by v0.8) will see all blocks, and have a complete listing of all UTXOs, and the content of transactions as they appear.

Does this matter? In principle you can just upgrade all nodes to understand the extension block, but in practice for a system as diverse as bitcoin support of older node versions is often required in critical infrastructure. Think of all the block explorer and mempool websites out there, for example, and various network monitoring and charting tools. Many of which are poorly maintained and probably running on two or three year old versions of Bitcoin Core.

The forward blocks proposal uses the timewarp bug to enable (1) a proof-of-work change, (2) sharding, (3) subsidy schedule smoothing, and (4) a flexible block size, all without forcing any non-mining nodes to *have* to upgrade in order to regain visibility into the network. Yes it's an everything-and-the-kitchen-sink straw man proposal, but that was on purpose to show that all these so-called “hard-fork” changes can in fact be done as a soft-fork on vanilla bitcoin, while supporting even the oldest still-running nodes.

That changes if we "fix" the timewarp bug though. At the very least, the flexible block size and subsidy schedule smoothing can't be accomplished without exploiting the timewarp bug, as far as anyone can tell. Therefore fixing the timewarp bug will _permanently_ cutoff the bitcoin community from ever having the ability to scale on-chain in a backwards-compatible way, now or decades or centuries into the future.

Once thrown, this fuse switch can't be undone. We should be damn sure we will never, ever need that capability before giving it up.

Mark

[Antoine Riard]

Hi Maaku,

From reading back the "forward block" paper, while it effectively guarantees an on-chain settlment throughput increases without the
necessity to upgrade old clients, one could argue the proof-of-work change on the forward chain (unless it's a no-op double-sha256)
coupled with the subsidy schedule smoothing, constitutes a substantial change of the already-mined UTXO security model. You can
use a lot of hash functions as proof-of-work primitive, though it doesn't mean they are relying on as strong assumptions or level
of cryptanalysis.

In fine, you could have poorly picked up hash function for the forward chain resulting in a lowering of everyone coins security
(the > 100 TH/s of today is securing years old coins from block mined under < 1 TH/s). I hold the opinion that fundamental changes
affecting the security of everyone coins should be better to be opted-in by the super-economic majority of nodes, including non-mining
nodes. At the contrary, the "forward block" proposal sounds to make the point it's okay to update proof-of-work algorithm by a
combined set of mining nodes and upgraded non-mining nodes, which could hypothetically lead to a "security downgrade" due to weaker
proof-of-work algorithm used on the forward chain.

While your papers introduce formalization of both full-node cost of validation and censorship resistance concepts, one could also
add "hardness to change" as a property of the Bitcoin network we all cherishes. If tomorrow, 10% of the hahrate was able to enforce
proof-of-work upgrade to the broken SHA-1, I think we would all consider as a security downgrade.

Beyond, this is corect we have a diversity of old nodes used in the ecosystem, probably for block explorer and mempool websites.
Yet in practice, they're more certainly vectors of weakness for their end-users, as Bitcoin Core has sadly a limited security fixes
backport policy, which doesn't go as far as v0.8 for sure. That we can all deplore the lack of half-decade old LTS release policy for
Bitcoin Core, like done by the Linux kernel is a legitimate conversation to have (and it would be indeed make it easier with
libbitcoinkernel progress). I think we shall rather invite operators of oldest still-running nodes to upgrade to more recent
versions, before to ask them to go through the analytical process of weighting all the security / scalability trade-offs of a
proposal like "forward block".

Finally, on letting options open to bump block inter-val as a soft-fork on the compatibility chain, I think one could still have
a multi-stage "forward block" deployment, where a) a new difficutly adjustment algoritm with parameters is introduced bumping block
inter-val for upgraded mining nodes e.g a block every 400 s in average and the b) re-use this block inter-val capacity increase for
the forward chain flexible block size. Now why a miner would opt-in in such block-interval constraining soft-fork is a good question,
in a paradigm where they still get the same block subsidy distribution.

This is just a thought experiment aiming to invalidate the "as far as anyone can tell" statement on forclosing forever on-chain
settlement throughput increase, if we fix the timewarp bug.

Best,
Antoine