From: Greg Maxwell <gmaxwell@gmail.com>
To: Josh Doman <joshsdoman@gmail.com>
Cc: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Does GCC preclude a soft fork to handle timestamp overflow?
Date: Sun, 14 Dec 2025 20:43:24 +0000 [thread overview]
Message-ID: <CAAS2fgSTqeE-vxPsn3hfH0d096ycM-rmTVqGSVH3nELeiG885g@mail.gmail.com> (raw)
In-Reply-To: <2ac708f3-8e73-4cd5-ba62-be64a2acea04n@googlegroups.com>
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> nodes require that the block hash meets the difficulty target determined
in the coinbase (in addition to the artificially low legacy difficulty
target).
I don't think that's a softfork in the same sense as any that have
historically been deployed, as it leaves the older participants entirely
insecure and even easily DOS attacked. You're also left with the locktime
functionality totally destroyed leaving bitcoin hobbled-- so it's not
really fixing the timestamp issue.
The long time assumption I know in the community is that normative
unwrapping behavior could be adopted decades ahead of 2106--
compatibility would be fine until 2106 and after that point unupdated
software would be safely stuck. That seems better to me.
On Sun, Dec 14, 2025 at 8:33 PM Josh Doman <joshsdoman@gmail.com> wrote:
> *TLDR:* The "timewarp attack" could enable a future soft fork that fixes
> the timestamp overflow bug.
>
> I saw there is a discussion about a hard fork to handle the timestamp
> overflow bug, by migrating from u32 to u64 timestamps.[1] I considered
> making this post in that thread, but as it has more to do with the Great
> Consensus Cleanup [2], I thought it better to make this its own post.
>
> My question is: *does BIP54 inadvertently preclude the possibility of a
> soft fork to handle timestamp overflow?*
>
> Conceptually, I think you could implement a soft fork that resolves the
> timestamp overflow bug, by using the "timewarp attack" [3] to intentionally
> minimize the timestamp and reduce the legacy difficulty, while
> simultaneously using a u64 timestamp in the coinbase transaction to enforce
> the real difficulty target.
>
> In short, the "timewarp attack" makes it possible to increment the u32
> timestamp by 1 second each block, ensuring the chain will practically never
> halt (provided the soft fork is adopted sufficiently in advance).
>
> Formally, given a block of height N and a timestamp T at activation height
> H:
> - if N % 2016 < 2015: miners set the legacy timestamp to T + (N - H).
> - if N % 2016 = 2015, miners set the legacy timestamp to min(2^32 - 1,
> timestamp in the coinbase transaction).
> - nodes require that the block hash meets the difficulty target determined
> in the coinbase (in addition to the artificially low legacy difficulty
> target).
>
> This solution, of course, doesn't work if the Great Consensus Cleanup is
> adopted and the "timewarp attack" gets fixed. Also, it will make header and
> SPV validation more complex, as nodes will need the coinbase transaction
> and a merkle proof in order to validate the header chain. Perhaps worst of
> all, it could confiscate coins that are locked to a timestamp, rather than
> a block height.
>
> The upside is that this is a soft fork, rather than a hard fork, which has
> its own advantages. Meanwhile, confiscation concerns could potentially be
> mitigated by signaling activation several decades in advance.
>
> Is the possibility of a soft fork worth forgoing the timewarp fix? I'm not
> sure. A compromise could be to expire the timewarp fix after a certain
> block height, but that introduces a new set of tradeoffs.
>
> Josh
>
> [1] https://groups.google.com/g/bitcoindev/c/PHZEIRb04RY
> [2] https://github.com/bitcoin/bips/blob/master/bip-0054.md
> [3]
> https://bitcoin.stackexchange.com/questions/75831/what-is-time-warp-attack-and-how-does-it-work-in-general/75834#75834
>
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next prev parent reply other threads:[~2025-12-14 20:45 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-14 19:45 Josh Doman
2025-12-14 20:43 ` Greg Maxwell [this message]
2025-12-14 21:53 ` Josh Doman
2025-12-15 1:44 ` Antoine Riard
2025-12-15 16:31 ` 'Russell O'Connor' via Bitcoin Development Mailing List
2025-12-15 17:27 ` Josh Doman
2025-12-16 6:04 ` Henry Romp
2025-12-17 14:55 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
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