Testnet 5 is a new test network intended to replace Testnet 4. Testnet 5 removes the difficulty\nexception defined in Testnet 4. Sustained exploitation of this exception has made the network difficult\nto use for testing. Additionally, Testnet 5 raises the minimum difficulty and enforces the consensus\nrules specified in BIP 54 from block 1.
\nTestnet 4 included mitigations for an issue known as the block storm attack which could render the\nwhole network unusable. Block storm attacks led to a depletion of block subsidies, which made it hard to acquire\ncoins for testing. However, Testnet 4 still retained a modified version of the difficulty exception rule.\nThe difficulty exception permits a block to be mined at the minimum difficulty when more than 20 minutes\nhave passed since the previous block. Testnet 4 kept this exception with the aim of allowing CPU users a\nlimited path to acquire coins for testing, to mine non-standard transactions that other miners would not\nrelay, and to keep the chain moving if a large source of hash power were to leave the network. Shortly\nafter Testnet 4's introduction, the exception had been systematically and persistently exploited, which\nprevented the exception from achieving the intended\ngoals. While block storms were prevented, the network suffered from constant re-orgs of small\nnumbers of blocks due to multiple difficulty-exception blocks competing for the tip. This led\nto discussion about changing Testnet 4 to mitigate this issue (see Bitcointalk\nfor analysis and discussion).
\nIn Testnet 5 there is no exception to the PoW rules. This appears to be the logical conclusion,\nsince any such exception could be exploited by a motivated attacker. The lack of exception\nensures the network’s behavior matches mainnet as closely as possible.
\nBIP 54 is already enforced on signet through Bitcoin Inquisition as of this BIP's creation.\nHowever, signet does not allow miners to test that their software reliably follows the rules of\nBIP 54. Testnet 5 provides a testing environment for this.
\nTestnet 5 follows the same consensus rules as mainnet, except for the following two changes.
\nThe rules specified in BIP 54 version 1.0.0 are active on Testnet 5 from block 1.
\nBIP 54 proposes new consensus rules in order to fix several potential attack vectors. Namely \nit prevents the timewarp attack, reduces the worst-case block validation time, mitigates Merkle\ntree weaknesses, and avoids duplicate transactions without bip-0030 validation.
\nSee specification in bip-0054.
\nThe proof-of-work limit (the maximum target) is 0x1a0fffff in compact encoding (nBits),\ncorresponding to a minimum difficulty of approximately 1,000,000.
All consensus rules active on mainnet as of May 2026 are enforced from block 1, the most\nrecent of these being the Taproot softfork. Additionally, the rules of BIP 54 are enforced\nfrom block 1, as specified above.
\nTODO: Mine the block. The values below are placeholders inherited from Testnet 4. They are\nthe genesis block's header fields together with the Message and Pubkey of its coinbase\ntransaction. Notes for the miner:
Pubkey is the public key in the coinbase transaction's pay-to-public-key (P2PK) output. Use a\nrecent Bitcoin mainnet block hash.[^pubkey-methodology]Message is an arbitrary string in the coinbase input script, like the newspaper headline in\nBitcoin's mainnet genesis block. Testnet 4 used it to carry the anti-pre-mine commitment, but\nTestnet 5 places that in the Pubkey instead, so the Message should be left empty, which also\navoids bike shedding over its content.Time stamp time of mining or that of the block which hash is used in PubkeynBits: 0x1a0fffff, per the raised minimum difficulty.Version: 1Testnet 4 placeholders:
\n\n\n\n
\n- Message:
\n03/May/2024 000000000000000000001ebd58c244970b3aa9d783bb001011fbe8ea8e98e00e- Pubkey:
\n000000000000000000000000000000000000000000000000000000000000000000- Time stamp: 1714777860
\n- Nonce: 393743547
\n- Difficulty:
\n0x1d00ffff- Version: 1
\nThe resulting Genesis block hash is
\n00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043, and the block hex 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.
The message start (also known as the network magic) is defined as 0x46495645. These four bytes spell FIVE when\ninterpreted as ASCII.
The default p2p port for Testnet 5 is 18335.
Instead of starting a new Testnet, changing the rules of Testnet 4 was considered as well. The decision\nfor a new network had two main reasons:
\nRaising the minimum difficulty was dismissed for Testnet 4 because it would have prevented CPU\nminers from utilizing the difficulty exception. With the exception removed in Testnet 5, this\nargument no longer applies. A minimum difficulty of approximately 1,000,000 strikes a balance\nbetween dampening the quick mining of large numbers of blocks shortly after launch, before the\ndifficulty has adjusted to the available hash power, and keeping a low barrier so that a handful\nof at-home miners or a single ASIC can keep the network usable.
\nTestnet 5's consensus rules are not compatible with those of Testnet 3 and Testnet 4. The\nconsensus rules differ in both directions: Testnet 5 enforces the BIP 54 consensus rules from\nblock 1, which is not the case for Testnet 3 or Testnet 4. Testnet 5 also does not apply the\ndifficulty exception rule that Testnet 3 or Testnet 4 requires.
\nBecause each network has its own genesis block, their UTXO sets are disjoint and transactions\ncan never be replayed across them. Address formats are shared, however, so the same address may\nbe reused on different networks.
\nAny implementation that intends to follow Testnet 5 must add the new network parameters and\nadditionally enforce the BIP 54 rules while not permitting any of the difficulty exception rules.
\nPull request at ?
\nThis document is licensed under the Creative Commons CC0 1.0 Universal license.
\n[^pubkey-methodology]: The Pubkey field is the public key placed in the coinbase transaction's\n output as a pay-to-public-key (P2PK) script. Setting it to a recent Bitcoin mainnet block hash\n serves two purposes: the resulting output is provably unspendable, and the referenced block hash\n acts as an anti-pre-mine commitment, since the genesis block cannot have been created before that\n mainnet block existed.