BIP 54: Consensus Cleanup
The soft fork that fixes timewarp, slow blocks, 64-byte transactions and duplicate coinbases, and what it asks of pools.
Four long-standing weaknesses, fixed in one soft fork so the cost of a deployment is paid once. Two of them are about mining directly.
What BIP 54 fixes
- Timewarp
- A majority-hashrate attacker could lower difficulty by bending timestamps around retarget periods and so speed up blocks, taking future miners' subsidy. New rule: the first block of a period (height divisible by 2,016) may not be more than 7,200 seconds (2 hours) earlier than the block before it, and the last block of a period may not be earlier than the period's first block.
- Slow-to-validate blocks
- Specially crafted blocks can take several minutes to validate on high-end hardware and hours on weaker devices, which a miner could use against competitors. New rule: a transaction's legacy signature operations (CHECKSIG and CHECKMULTISIG in scriptSig, scriptPubKey and P2SH redeemScript) may not exceed 2,500.
- 64-byte transactions
- A transaction of exactly 64 bytes (without witness) can pass for an inner node of the Merkle tree and fool a light client's inclusion proof. New rule: such transactions are invalid.
- Duplicate coinbases
- Since BIP 34, explicit BIP 30 checks were not needed until block 1,983,702. New rule: the coinbase nLockTime is set to height minus 1 and nSequence may not be 0xffffffff, so a new coinbase can never repeat an old one.
What it means for miners
- Pool software and firmware that build the coinbase must set nLockTime to height − 1 and avoid nSequence 0xffffffff once the rules are active.
- Timestamps at the first and last block of each 2,016-block period get bounds. Honest miners are already inside them.
- Blocks that would take minutes to validate become invalid, which removes a way to slow down rival miners.
- Difficulty can no longer be pushed down through timestamps, so a majority miner cannot take future subsidy that way.
| BIP | Title | Authors | Status | Assigned |
|---|---|---|---|---|
| 54 | Consensus Cleanup | Antoine Poinsot, Matt Corallo | Complete | 2025-04-11 |
Questions
- What is BIP 54?
- BIP 54, Consensus Cleanup, is a proposed Bitcoin soft fork by Antoine Poinsot and Matt Corallo. It fixes the timewarp attack, caps worst-case block validation time, makes 64-byte transactions invalid, and ends the need for BIP 30 duplicate-transaction checks.
- What is the timewarp attack?
- A bug in how difficulty is retargeted. By manipulating block timestamps at the edges of 2,016-block periods, an attacker with most of the hashrate could push difficulty down. The BIP says difficulty could reach its minimum within 38 days of starting the attack.
- What changes for mining pools?
- After activation, the coinbase transaction's nLockTime must be the block height minus 1, and its nSequence must not be 0xffffffff. Blocks at the start and end of each retarget period also get timestamp limits.
- Is BIP 54 active?
- No. Its status in the repository is Complete; activation would need a soft fork deployment that nodes and miners adopt.
Sources
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