[bitcoindev] Update on the Great Consensus Cleanup Revival

[bitcoindev] Update on the Great Consensus Cleanup Revival

['Antoine Poinsot' via Bitcoin Development Mailing List]

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

[Murch]

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

['Antoine Poinsot' via Bitcoin Development Mailing List]

I laid out my reasoning for increasing the grace period to 7200 on the Consensus Cleanup Delving
thread [0]. TL;DR: there is marginal safety benefits to doing so and virtually no cost (it only
increases the worst case block rate from ~0.1% to ~0.65%). So on balance i concluded it was
preferable to err on the safe side.

I chose to go with mandating nLockTime be set in coinbase transactions to the height of the block
they are included in minus 1 because it has marginal benefits in addition to ensuring coinbase
transactions can't be duplicate (retrieving / proving the block height more efficiently), and the
feedback i got from miners both publicly [1] and privately was that none of the options presented
significantly more challenge for them.

Antoine

[0] https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/66
[1] https://groups.google.com/g/bitcoinminingdev/c/qyrPzU1WKSI/m/uzxS5jG0AwAJ



On Thursday, February 6th, 2025 at 4:34 PM, Murch <murch@murch.one> wrote:

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

[Antoine Riard]

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

['Antoine Poinsot' via Bitcoin Development Mailing List]

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Hi "evil" Ariard,

I believe it is important to bundle all fixes together to make up for the substantial fixed cost of deploying a soft fork. It also seems absurd to deploy a soft fork aimed at patching security bugs, but only fix some of them and leave the protocol partly vulnerable. While it is technically possible it is not something i want to encourage.

Regarding the confiscation surface, please note the specific concerns raised about the 2019 proposal do not apply to the fix proposed here. The new approach to mitigating the worst case validation time is extremely conservative in this regard: no opcodes or other Script functionality get disabled. Only a limit is introduced at the transaction level, which allows to pinpoint exactly the harmful behaviour without rendering any innocent transaction invalid.

Best,
Antoine (the non-evil-one?)

On Sunday, February 9th, 2025 at 7:15 PM, Antoine Riard <antoine.riard@gmail.com> wrote:

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

[Chris Stewart]

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Hi everyone! Excited to see this work moving forward. I've taken the
liberty of carving off the 64 byte transaction portion of this proposal and
drafted a BIP. You can view a rendered draft with references here:
https://github.com/Christewart/bips/blob/2024-12-20-64bytetxs/bip-XXXX.mediawiki

<pre>
  BIP: ?
  Layer: Consensus (soft fork)
  Title: Disallow 64 byte transactions
  Author: Chris Stewart <stewart.chris1234@gmail.com>
  Status: Draft
  Type: Specification
  License: BSD-3-Clause
  Created: ?
</pre>

==Abstract==

This BIP describes the rationale for disallowing transactions that are
serialized to 64 bytes without the transaction's witness.
We describe the weaknesses to the merkle tree included in bitcoin block
headers, various exploits for those weaknesses.

==Motivation==

Bitcoin block headers include a commitment to the set of transactions in a
given
block, which is implemented by constructing a Merkle tree of transaction
id’s
(double-SHA256 hash of a transaction) and including the root of the tree in
the
block header. This in turn allows for proving to a Bitcoin light client
that a
given transaction is in a given block by providing a path through the tree
to the
transaction. However, Bitcoin’s particular construction of the Merkle tree
has
several security weaknesses, including at least two forms of block
malleability
that have an impact on the consensus logic of Bitcoin Core, and an attack on
light clients, where an invalid transaction could be ”proven” to appear in
a block
by doing substantially less work than a SHA256 hash collision would require.
This has been prevented by relay policy since 2018<ref>[
https://github.com/bitcoin/bitcoin/pull/11423/commits/7485488e907e236133a016ba7064c89bf9ab6da3
PR #11423 disallows 64 byte transactions in bitcoin core relay]</ref>

==Specification==

This BIP disallows bitcoin transactions that are serialized to 64 bytes in
length without it's witness.

==Rationale==

=== Block malleability ===

64 byte transactions introduce block malleability. Malicious peers can
construct consensus valid and invalid 64 byte
transactions that have the same serialization as the concatenation of 2
nodes in the merkle tree.

Assume we have a valid bitcoin block with 2 transactions in it -
T<sub>0</sub> and T<sub>1</sub>.
The merkle root for this block is H(T<sub>0</sub>||T<sub>1</sub>).
A user could find a malicious 64 byte transaction T<sub>m</sub> that
serializes to T<sub>0</sub>||T<sub>1</sub>.
Next the malicious user relays the block containing the malicious
T<sub>m</sub> rather than the
valid bitcoin transactions T<sub>0</sub> and T<sub>1</sub>.

==== Block malleability with consensus INVALID transactions ====

The peer receiving the malicious block marks the block as invalid as
T<sub>m</sub>
is not a valid transaction according to network consensus rules.
Other peers on the network receive the valid block containing T<sub>0</sub>
and T<sub>1</sub>
add the block to their blockchain. Peers that receive the invalid block
before the valid block
will never come to consensus with their peers due to the malicious user
finding a collision
within the block's merkle root. Finding this collision approximately 22
bits worth of work<ref>[
https://github.com/Christewart/bips/blob/2024-12-20-64bytetxs/bip-XXXX/2-BitcoinMerkle.pdf
to produce a block that has a Merkle
root which is a hash of a 64-byte quantity that deserializes validly, it’s
enough
to just do 8 bits of work to find a workable coinbase (which will hash to
the first
32 bytes), plus another ≈22 bits of work ((1/5) ∗224, so slightly less) to
find
a workable second transaction which will hash to the second 32 bytes) – a
very
small amount of computation.]</ref>

This attack vector was fixed in 0.6.2<ref>[
https://bitcoin.org/en/alert/2012-05-14-dos#risks CVE-2012-2459]</ref>,
re-introduced in 0.13.x<ref>[https://github.com/bitcoin/bitcoin/pull/7225
#7225]</ref> and patched again in
0.14<ref>[https://github.com/bitcoin/bitcoin/pull/9765 #9765]</ref> of
bitcoin core.

==== Block malleability with consensus VALID transactions ====

Producing a valid bitcoin transaction T<sub>m</sub> that adheres to network
consesnsus
rules requires 224 bits of work<ref>[
https://github.com/Christewart/bips/blob/2024-12-20-64bytetxs/bip-XXXX/2-BitcoinMerkle.pdf
Note that the first transaction in a block must be a coinbase, and as
discussed
above, that largely constrains the first 32 bytes of the first transaction:
only
the 4 version bytes are unconstrained. So it would take at least 28*8= 224
bits
of work to find the first node in a given row of the tree that would match
the
first half of a coinbase, in addition to the amount of work required to
grind the
second half of the transaction to something meaningful (which is much
easier –
only 16 bytes or so are constrained, so approximately 128 bits of work to
find a collision). Of course, any of the rows in the Merkle tree could be
used, but it nevertheless seems clear that this should be computationally
infeasible.]</ref>.
This is computationally and financially expensive but theoretically
possible. This can lead to a persistent chain split on the network.

=== Attack on SPV clients ===

BIP37<ref>[https://github.com/bitcoin/bips/blob/master/bip-0037.mediawiki
BIP37]</ref>provides a partial merkle tree format<ref>[
https://github.com/bitcoin/bips/blob/master/bip-0037.mediawiki#user-content-Partial_Merkle_branch_format
Partial Merkle Tree Format]</ref>
that allows you to verify your bitcoin transaction is included in a merkle
root embedded in a bitcoin block header.
Notably this format does not commit to the height of the merkle tree.

Suppose a (valid) 64-byte transaction T is included in a block with the
property that the second 32 bytes (which
are less constrained than the first 32 bytes) are constructed so that they
collide
with the hash of some other fake, invalid transaction F. The attacker can
fool the SPV client into believing that F
was included in a bitcoin block rather than T with 81 bits<ref>[
https://github.com/Christewart/bips/blob/2024-12-20-64bytetxs/bip-XXXX/2-BitcoinMerkle.pdf
An attacker who can do 81 bits of work (followed by another 40 bits of
work, to
construct the funding transaction whose coins will be spent by this one) is
able
to fool an SPV client in this way.]</ref> of work. This also reduces
implementation complexity of SPV wallets<ref>[
https://delvingbitcoin.org/t/great-consensus-cleanup-revival/710/43 The
steps needed to make sure a merkle proof for a transaction is
secure.]</ref>.

This could be mitigated by knowing the depth of the merkle tree. Requiring
SPV clients to request both the coinbase transaction could mitigate this
attack.
To produce a valid coinbase transaction at the same depth that our fake
transaction F occurs at would require 224 bits of work.
As mentioned above, this is computionally and financially expensive, but
theoretically possible.

==Backward compatibility==

There have been 5 64 byte transactions that have occcurred in the bitcoin
blockchain as of this
writing <ref>[
https://github.com/Christewart/bips/blob/2024-12-20-64bytetxs/64byte-tx-mainnet.txt
64 byte transactions in the bitcoin blockchain]</ref>
With the last transaction
7f2efc6546011ad3227b2da678be0d30c7f4b08e2ce57b5edadd437f9e27a612<ref>[
https://mempool.space/tx/7f2efc6546011ad3227b2da678be0d30c7f4b08e2ce57b5edadd437f9e27a612
Last 64 byte transaction in the bitcoin blockchain]</ref>
occurring at block height 419,606<ref>[
https://mempool.space/block/000000000000000000308f1efc24419f34a3bafcc2b53c32dd57e4502865fd84
Block 419,606]</ref>.

TODO

==Reference implementation==

<source lang="cpp">
/**
 * We want to enforce certain rules (specifically the 64-byte transaction
check)
 * before we call CheckBlock to check the merkle root. This allows us to
enforce
 * malleability checks which may interact with other CheckBlock checks.
 * This is currently called both in AcceptBlock prior to writing the block
to
 * disk and in ConnectBlock.
 * Note that as this is called before merkle-tree checks so must never
return a
 * non-malleable error condition.
 */
static bool ContextualBlockPreCheck(const CBlock& block,
BlockValidationState& state, const ChainstateManager& chainman, const
CBlockIndex* pindexPrev)
{
    if (DeploymentActiveAfter(pindexPrev, chainman,
Consensus::DEPLOYMENT_64BYTETX)) {
      for (const auto& tx : block.vtx) {
            if (::GetSerializeSize(TX_NO_WITNESS(tx)) == 64) {
                return state.Invalid(BlockValidationResult::BLOCK_MUTATED,
"64-byte-transaction", strprintf("size of tx %s without witness is 64
bytes", tx->GetHash().ToString()));
            }
        }
    }

    return true;
}
</source>

https://github.com/bitcoin-inquisition/bitcoin/pull/24/files

== Rationale ==

<references />

==Copyright==
This BIP is licensed under the [https://opensource.org/license/BSD-3-Clause
BSD-3-Clause License].

==Acknowledgements==

Suhas Daftuar, AJ Towns, Sergio Demian Lerner, Greg Maxwell, Matt Corallo,
Antoine Poinsont, Dave Harding and Erik Voskuil

On Wed, Feb 5, 2025 at 6:57 PM 'Antoine Poinsot' via Bitcoin Development
Mailing List <bitcoindev@googlegroups.com> 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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> .
>

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

[Antoine Riard]

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Hi Darosior,

> I believe it is important to bundle all fixes together to make up for the 
substantial fixed cost of deploying a soft fork. It also seems absurd to 
deploy a soft fork aimed at patching security bugs, but only fix some of 
them and lea> ve the protocol partly vulnerable. While it is technically 
possible it is not something i want to encourage.

I don't wish to be dogmatic here, though I believe we have 2 distinct 
things. There is (a) to have 4 distinct BIPs for each fix 
("timewarp"/"worst-block-time"/"merkle-tree-weakness"/"enhanced-duplicated-txn") 
and there is (b) to bundle all the fixes together as of course there is 
substantial ecosystem coordination cost to deploy a soft fork, even with 
BIP9. Getting 4 distinct BIPs, it's of course a bit more work for the soft 
fork authors and champions,  though I think this avoid too-beefy 
ill-written BIP as we already had and undocumented future rules in the 
script interpreter code like the SIGPUSHONLY check I was pointing out.

Apart of this editorial good practice motivation, this also indeed make it 
simpler to deploy the fix one by one (of course you can always hack-in 
activation code, even if it's single BIP), in the pessimistic scenario in 
the future there are no consensus on all the fixes, though only a subset of 
the 4 ones. In that optic, yes if we can get in 3 of the 4 fixes in a 
bundled soft fork and the remaining one at a latter time, it would be 
already a win to reduce the protocol vulnerability exposure.

It's unlike the Taproot patchset here, each proposed soft-fork fix is 
supposed to be addressing a vulnerability of its own, and there is almost 
no technical coupling between them (well one could go to argue there is one 
among the timewarp fix and the worst-block-validation time, if you go to 
consider the maximum DoS surface of a full-node under wall clock time). I 
don't think it's a question of wishing what we wished to encourage as the 
"ideal" deployment of this set of consensus rules changes, as if there are 
good technical reasons to object on 1 fix and no consensus, this shouldn't 
prevent to be realistic and go to deploy all the remaining ones for which 
there is consensus.

> Regarding the confiscation surface, please note the specific concerns 
raised about the 2019 proposal do not apply to the fix proposed here. The 
new approach to mitigating the worst case validation time is extremely 
conservative in this regard: no opcodes or other Script functionality get 
disabled. Only a limit is introduced at the transaction level, which allows 
to pinpoint exactly the harmful behaviour without rendering any innocent 
transaction invalid.

I'm not sure if there is already code, and even a BIP on the 
"worst-block-validation-time", though it's unclear to me if the limit aims 
to apply on UTXO spends spending lecacy scripts ex post to the activation 
(as I think the proposal of few months ago was laying out) or ex ante to 
the activation. If the latter one, with a "retro-active" confiscation 
surface, I think it's akin to the specific concerns raised in 2019, and if 
it's something like that we should be
very careful in the design.

> 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. 

There is another point - I'm sharing the practice on not exposing all 
experimentation on the worst-block-validation-time, as you never know it's 
the wild Internet and there can be always script kiddies at the corner of 
the block for unpatched vulnerabilities. On the other hand, we're proposing 
to change what is the "consensus" definition of people money, so a bit more 
of publicity in rationalizing why the changes are proposed could be 
welcome. For clarity I have access on the thread and there is bunch of 
other devs. It's always a (hard) question how much info you share on 
vulnerabilities in the process of fixing them.

Anyway, I'll go to review code+BIP(s) for all the fixes, those raised 
points are just relevant to keep in mind imho.

Best,
Antoine R. (nah you're evil, i'm evil, we're all evil, btc is beyond good 
and evil)
OTS hash: 0334fb9d557426c4f7d71d3e99bb9badbfd87903

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