BIP95: Testnet 5
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Fabian Jahr
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@ -548,6 +548,13 @@ users (see also: [https://en.bitcoin.it/wiki/Economic_majority economic majority
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| Specification
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| Deployed
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|-
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| [[bip-0095.md|95]]
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| Applications
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| Testnet 5
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| Pol Espinasa, Fabian Jahr
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| Specification
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| Draft
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|-
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| [[bip-0098.mediawiki|98]]
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| Consensus (soft fork)
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| Fast Merkle Trees
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183
bip-0095.md
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```
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BIP: 95
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Layer: Applications
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Title: Testnet 5
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Authors: Pol Espinasa <polespinasa@protonmail.com>
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Fabian Jahr <fjahr@protonmail.com>
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Status: Draft
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Type: Specification
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Assigned: 2026-06-22
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License: CC0-1.0
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Discussion: 2026-06-02: https://groups.google.com/g/bitcoindev/c/kGUMTxOvdJA
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Version: 0.1.0
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Requires: 54
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Replaces: 94
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```
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## Abstract
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Testnet 5 is a new test network intended to replace [Testnet 4][BIP94]. Testnet 5 removes the difficulty
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exception defined in Testnet 4. Sustained exploitation of this exception has made the network difficult
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to use for testing. Additionally, Testnet 5 raises the minimum difficulty and enforces the consensus
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rules specified in [BIP 54][BIP54] from block 1.
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## Motivation
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Testnet 4 included mitigations for an issue known as the [block storm attack][block-storms] which could render the
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whole network unusable. Block storm attacks led to a depletion of block subsidies, which made it hard to acquire
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coins for testing. However, Testnet 4 still retained a modified version of the difficulty exception rule.
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The difficulty exception permits a block to be mined at the minimum difficulty when more than 20 minutes
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have passed since the previous block. Testnet 4 kept this exception with the aim of allowing CPU users a
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limited path to acquire coins for testing, to mine non-standard transactions that other miners would not
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relay, and to keep the chain moving if a large source of hash power were to leave the network. Shortly
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after Testnet 4's introduction, the exception had been systematically and persistently exploited, which
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prevented the exception from achieving the intended
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goals. While block storms were prevented, the network suffered from constant re-orgs of small
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numbers of blocks due to multiple difficulty-exception blocks competing for the tip. This led
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to discussion about changing Testnet 4 to mitigate this issue (see [Bitcointalk][bitcointalk-thread]
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for analysis and discussion).
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In Testnet 5 there is no exception to the PoW rules. This appears to be the logical conclusion,
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since any such exception could be exploited by a motivated attacker. The lack of exception
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ensures the network’s behavior matches mainnet as closely as possible.
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BIP 54 is already enforced on signet through [Bitcoin Inquisition][signet-bip54] as of this BIP's creation.
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However, signet does not allow miners to test that their software reliably follows the rules of
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BIP 54. Testnet 5 provides a testing environment for this.
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## Specification
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Testnet 5 follows the same consensus rules as mainnet, except for the following two changes.
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### BIP 54 activation
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The rules specified in [BIP 54 version 1.0.0][BIP54] are active on Testnet 5 from block 1.
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#### Problem Statement
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BIP 54 proposes new consensus rules in order to fix several potential attack vectors. Namely
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it prevents the timewarp attack, reduces the worst-case block validation time, mitigates Merkle
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tree weaknesses, and avoids duplicate transactions without [bip-0030][BIP30] validation.
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#### Rule Specification
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See specification in [bip-0054][BIP54].
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### Minimum Difficulty
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The proof-of-work limit (the maximum target) is `0x1a0fffff` in compact encoding (nBits),
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corresponding to a minimum difficulty of approximately 1,000,000.
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### Consensus Rules
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All consensus rules active on mainnet as of May 2026 are enforced from block 1, the most
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recent of these being the Taproot softfork. Additionally, the rules of BIP 54 are enforced
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from block 1, as specified above.
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### Genesis Block
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TODO: Mine the block. The values below are placeholders inherited from Testnet 4. They are
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the genesis block's header fields together with the `Message` and `Pubkey` of its coinbase
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transaction. Notes for the miner:
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- `Pubkey` is the public key in the coinbase transaction's pay-to-public-key (P2PK) output. Use a
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recent Bitcoin mainnet block hash.[^pubkey-methodology]
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- `Message` is an arbitrary string in the coinbase input script, like the newspaper headline in
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Bitcoin's mainnet genesis block. Testnet 4 used it to carry the anti-pre-mine commitment, but
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Testnet 5 places that in the `Pubkey` instead, so the `Message` should be left empty, which also
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avoids bike shedding over its content.
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- `Time stamp` time of mining or that of the block which hash is used in `Pubkey`
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- `nBits`: `0x1a0fffff`, per the raised minimum difficulty.
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- `Version: 1`
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Testnet 4 placeholders:
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> * Message: <code>03/May/2024 000000000000000000001ebd58c244970b3aa9d783bb001011fbe8ea8e98e00e</code>
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> * Pubkey: <code>000000000000000000000000000000000000000000000000000000000000000000</code>
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> * Time stamp: 1714777860
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> * Nonce: 393743547
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> * Difficulty: <code>0x1d00ffff</code>
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> * Version: 1
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>
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> The resulting Genesis block hash is <code>00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043</code>, and the block hex is <code>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</code>.
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### Message Start
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The message start (also known as the network magic) is defined as <code>0x46495645</code>. These four bytes spell `FIVE` when
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interpreted as ASCII.
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### P2P Port
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The default p2p port for Testnet 5 is `18335`.
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## Rationale
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Instead of starting a new Testnet, changing the rules of Testnet 4 [was considered][t4-discussion] as well. The decision
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for a new network had two main reasons:
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1. Deploying network changes is hard and typically takes a long time. This effort doesn't seem to be
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worth it for a Testnet that has the alternative option to start fresh, especially considering that
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the attackers exploiting Testnet 4's rules may use them to prevent a smooth deployment of new rules,
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including those of BIP 54.
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2. Any fix to Testnet 4 that goes beyond a band-aid would require a hard fork, which brings its own
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implementation cost: adding hard-fork support to clients, handling the p2p layer correctly, etc.
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For a test network this isn't worth taking on, especially since a band-aid would only make
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Testnet 4 diverge further from mainnet.
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Raising the minimum difficulty was dismissed for Testnet 4 because it would have prevented CPU
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miners from utilizing the difficulty exception. With the exception removed in Testnet 5, this
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argument no longer applies. A minimum difficulty of approximately 1,000,000 strikes a balance
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between dampening the quick mining of large numbers of blocks shortly after launch, before the
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difficulty has adjusted to the available hash power, and keeping a low barrier so that a handful
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of at-home miners or a single ASIC can keep the network usable.
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## Backward Compatibility
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Testnet 5's consensus rules are not compatible with those of Testnet 3 and Testnet 4. The
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consensus rules differ in both directions: Testnet 5 enforces the BIP 54 consensus rules from
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block 1, which is not the case for Testnet 3 or Testnet 4. Testnet 5 also does not apply the
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difficulty exception rule that Testnet 3 or Testnet 4 requires.
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Because each network has its own genesis block, their UTXO sets are disjoint and transactions
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can never be replayed across them. Address formats are shared, however, so the same address may
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be reused on different networks.
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Any implementation that intends to follow Testnet 5 must add the new network parameters and
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additionally enforce the BIP 54 rules while not permitting any of the difficulty exception rules.
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## Reference Implementation
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Pull request at ?
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## References
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- [BIP 30][BIP30]: Duplicate transactions
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- [BIP 54][BIP54]: Consensus Cleanup
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- [BIP 94][BIP94]: Testnet 4
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- [The block storms of Bitcoin's Testnet][block-storms]
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- [Testnet 4 difficulty exception analysis and discussion (Bitcointalk)][bitcointalk-thread]
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- [BIP 54 enforcement on signet via Bitcoin Inquisition][signet-bip54]
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- [Discussion on changing the Testnet 4 rules (Bitcoin Core PR #31117)][t4-discussion]
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[block-storms]: https://blog.lopp.net/the-block-storms-of-bitcoins-testnet/
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[bitcointalk-thread]: https://bitcointalk.org/index.php?topic=5569103.0
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[signet-bip54]: https://delvingbitcoin.org/t/bitcoin-inqusition-29-2/2236
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[t4-discussion]: https://github.com/bitcoin/bitcoin/pull/31117
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[BIP30]: bip-0030.mediawiki
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[BIP54]: bip-0054.md
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[BIP94]: bip-0094.mediawiki
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## Copyright
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This document is licensed under the Creative Commons CC0 1.0 Universal license.
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## Changelog
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* __0.1.0__ (2026-06-02):
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* Initial draft
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[^pubkey-methodology]: The `Pubkey` field is the public key placed in the coinbase transaction's
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output as a pay-to-public-key (P2PK) script. Setting it to a recent Bitcoin mainnet block hash
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serves two purposes: the resulting output is provably unspendable, and the referenced block hash
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acts as an anti-pre-mine commitment, since the genesis block cannot have been created before that
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mainnet block existed.
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