consensus-cleanup-soft-fork.md

title	Consensus cleanup soft fork
shortname	consensus cleanup
title-aliases	BIP54
topic-categories	Soft Forks Scripts and Addresses Mining
excerpt	**Consensus cleanup soft fork** is a proposal to address several issues in Bitcoin's consensus rules that date back to the original version of Bitcoin released in 2009.
primary_sources	title BIP54
optech_mentions	title url Cleanup soft fork proposal /en/newsletters/2019/03/05#cleanup-soft-fork-proposal title url Consensus cleanup background /en/newsletters/2019/03/05#appendix-consensus-cleanup-background title url Consensus cleanup discussion: codeseparator & sighash types /en/newsletters/2019/03/12/#cleanup-soft-fork-proposal-discussion title url CoreDev.tech discussion: cleanup soft fork /en/newsletters/2019/06/12/#cleanup-discussion title url 2019 year-in-review: consensus cleanup soft fork proposal /en/newsletters/2019/12/28/#cleanup title url Discussion of minimum safe transaction sizes /en/newsletters/2020/05/27/#minimum-transaction-size-discussion title url Renewed discussion of consensus cleanup soft fork /en/newsletters/2024/04/03/#revisiting-consensus-cleanup title url Question: where exactly is the 'off-by-one' difficulty bug and how does it relate to time warp? /en/newsletters/2024/04/24/#where-exactly-is-the-off-by-one-difficulty-bug title url Notes from Bitcoin developer discussion about consensus cleanup /en/newsletters/2024/05/01/#coredev-tech-berlin-event title url Discussion about mitigating merkle tree vulnerabilities in the proposed consensus cleanup soft fork /en/newsletters/2024/09/06/#mitigating-merkle-tree-vulnerabilities title url Discussion about fixing Murch-Zawy time warp and duplicate transactions in consensus cleanup /en/newsletters/2024/12/06/#continued-discussion-about-consensus-cleanup-soft-fork-proposal title url Announcement of updates to the consensus cleanup soft fork proposal /en/newsletters/2025/02/07/#updates-to-cleanup-soft-fork-proposal title url Draft BIP published for consensus cleanup /en/newsletters/2025/04/04/#draft-bip-published-for-consensus-cleanup title url BIPs #1800 merges BIP54, which specifies the consensus cleanup soft fork proposal /en/newsletters/2025/05/09/#bips-1800 title url Bitcoin Core #32155 updates the internal miner to comply with consensus cleanup requirements /en/newsletters/2025/05/16/#bitcoin-core-32155 title url BIPs #1760 merges BIP53, which specifies a consensus change to forbid 64 byte transactions /en/newsletters/2025/05/30/#bips-1760 title url Bitcoin Core #32521 makes legacy transactions with more than 2500 sigops non-standard /en/newsletters/2025/07/25/#bitcoin-core-32521 title url BIP54 implementation and test vectors /en/newsletters/2025/11/07/#bip54-implementation-and-test-vectors title url Discussion of BIP54's timewarp fix and its impact on the 2106 block timestamp overflow issue /en/newsletters/2026/01/02/#relax-bip54-timestamp-restriction-for-2106-soft-fork title url Addressing remaining points on BIP54 /en/newsletters/2026/02/06/#addressing-remaining-points-on-bip54 title url Bitcoin Inquisition adds an implementation of the BIP54 consensus cleanup soft fork rules /en/newsletters/2026/02/13/#bitcoin-inquisition-99 title url BIP54 demonstration of slow blocks on signet /en/newsletters/2026/05/01/#bip54-demonstration-of-slow-blocks-on-signet title url BIP54 64-byte transactions and potential legitimate uses /en/newsletters/2026/06/05/#bip54-64-byte-transactions-and-potential-legitimate-uses title url Draft BIP for testnet5 would activate BIP54 from block 1 /en/newsletters/2026/06/12/#draft-bip-for-testnet5 title url Prohibit Merkle internal node preimages that encode minimal 64-byte transactions /en/newsletters/2026/07/03/#prohibit-merkle-internal-node-preimages-that-encode-minimal-64-byte-transactions
see_also	title link Soft fork activation topic soft fork activation title link CVE-2017-12842: fake SPV proofs using 64-byte transactions https://bitcoinops.org/en/topics/cve/#CVE-2017-12842

After a prior draft in 2019 was deferred, renewed efforts since 2023 substantiated into a concrete proposal, BIP54: Consensus Cleanup. The soft fork proposal advocates for fixing the following four issues.

• Time warp bug: an [off-by-one error][topic time warp] in the difficulty adjustment algorithm permits a majority hashrate attacker to arbitrarily increase block cadence. This is mitigated by limiting the permitted timestamps for the first block in difficulty periods and requiring that an entire difficulty period has a non-negative duration.

• Slow-to-validate blocks: attackers may use uncommon script patterns to compose blocks that are prohibitively expensive to process. These forms of malicious transactions are prevented by introducing limits on signature operations that curb this malicious use but far exceed organic uses.

• Merkle tree weakness: the construction of the merkle tree on a block’s transactions treats transactions with witness-stripped sizes of 64 bytes indistinguishably from inner nodes. Forbidding such transactions prevents two ways of [misrepresenting the content][topic merkle tree vulnerabilities] of a valid block.

• Duplicate transaction vector: Some early coinbase transactions exhibit patterns that would allow them to be [replayed][topic duplicate transactions] in future blocks. Requiring that the locktime of coinbase transactions is set to a specific value based on the block height enforces that future coinbase transactions are unique without needing to enforce BIP30 checks for those blocks.

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