From: Josh Doman <joshsdoman@gmail.com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Does GCC preclude a soft fork to handle timestamp overflow?
Date: Mon, 15 Dec 2025 09:27:58 -0800 (PST) [thread overview]
Message-ID: <e7a70843-a304-4d04-9365-08b8b4259caen@googlegroups.com> (raw)
In-Reply-To: <CAMZUoK=KqPTXCuBhy9O5YDXrLhRQMZJ25qAvTtRrE78s8SZniQ@mail.gmail.com>
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> your idea is to have the header nTime used for difficulty adjustment
enforced in the coinbase tx.
Correct. As written, BIP54 makes that soft fork impossible, leaving a hard
fork as the only option to resolve nTime overflow.
> I was about to write this email myself, but then I realized that since
BIP 113, timelocks are based on MTP time, and any soft-fork mechanism that
messes with MTP time will destroy existing transaction's timelock semantics.
Yes, it's unfortunate. There is certainly a tradeoff. On the one hand,
there is a risk of coin confiscation, if the soft fork isn't signaled early
enough (a few decades in advance is probably sufficient). On the other
hand, there are material benefits to avoiding a hard fork (i.e. you get a
smooth and secure upgrade path, developers can write immutable programs
that verify the chain, etc).
I think it's presumptive to assume which option a future generation would
prefer, in the year 2070, 2080, 2090, 2100, etc, given the tradeoffs
involved. I'm not suggesting we decide today, but I am suggesting that
BIP54 may be unnecessarily restrictive.
The following modification to BIP54 would resolve the timewarp attack while
leaving open the possibility of an nTime soft fork:
1) Add a u64 timestamp to the coinbase and enforce BIP54 there (in addition
to other timestamp rules)
2) Given a block of height N, where N % 2016 = 2015, the difference between
the nTime and the nTime at height (N - 2015) must be the same as in the
coinbase.
On Monday, December 15, 2025 at 11:36:31 AM UTC-5 Russell O'Connor wrote:
I was about to write this email myself, but then I realized that since BIP
113, timelocks are based on MTP time, and any soft-fork mechanism that
messes with MTP time will destroy existing transaction's timelock
semantics. Now I think the best is to have a hardfork.
On Sun, Dec 14, 2025 at 3:33 PM Josh Doman <joshs...@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:[~2025-12-15 19:30 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
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 [this message]
2025-12-16 6:04 ` Henry Romp
2025-12-17 14:55 ` 'Antoine Poinsot' via Bitcoin Development Mailing List
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