Summary

When the median time past [1] of the most recent 11 blocks (MTP-11) is greater than or equal to UNIX timestamp 1605441600 (Nov 15th, 2020, 12:00PM UTC), Bitcoin Cash will execute an upgrade of the network consensus rules according to this specification. Starting from the next block these consensus rules changes will take effect:

  • Bitcoin Cash’s Difficulty Adjustment Algorithm (DAA) is replaced with a new system, referred to as aserti3-2d.
  • The addition of a new coinbase rule.

The following are not consensus changes, but are recommended policy changes for Bitcoin Cash implementations:

  • Automatic replay protection for future upgrade.

Difficulty Adjustment Algorithm

Bitcoin Cash’s Difficulty Adjustment Algorithm (DAA) is replaced with a new algorithm called ASERT.

The specific implementation is called aserti3-2d. Details can be found in the full specification: ASERT.

Coinbase Rule

The purpose of the new coinbase rule is to provide funding to development projects working on common Bitcoin Cash infrastructure.

The coinbase rule enforces that at least 8% of the block reward must be spent as a single output to the following Bitcoin Cash address: bitcoincash:pqnqv9lt7e5vjyp0w88zf2af0l92l8rxdgnlxww9j9.

The amount of the output must be equal to or greater than the integer required, calculated as follows using integer math:

required = (8 * blockReward) / 100

Automatic Replay Protection

The purpose of Automatic Replay Protection is to serve as a full node version-deprecation mechanism. It is intended to cause full validating nodes which do not upgrade, to automatically separate themselves from the main network after the next upgrade on 15 May 2021. Nodes which implement the next upgrade will remove this automatic replay protection, and thus all regular wallets can continue using the default ForkID with no change to follow the main upgraded chain.

When the median time past [1] of the most recent 11 blocks (MTP-11) is less than UNIX timestamp 1621080000 (May 2021 upgrade) Bitcoin Cash full nodes MUST enforce the following rule:

  • forkid [2] to be equal to 0.

When the median time past [1] of the most recent 11 blocks (MTP-11) is greater than or equal to UNIX timestamp 1621080000 (May 2021 upgrade) Bitcoin Cash full nodes implementing the Nov 2020 consensus rules SHOULD enforce the following change:

  • Update forkid [2] to be equal to 0xFFXXXX, where XXXX is some arbitrary hex value. ForkIDs beginning with 0xFF will be reserved for future protocol upgrades.

This particular consensus rule MUST NOT be implemented by Bitcoin Cash wallet software. Wallets that follow the upgrade should not have to change anything.

References

[1] Median Time Past is described in bitcoin.it wiki. It is guaranteed by consensus rules to be monotonically increasing.

[2] The forkId is defined as per the replay protected sighash specification.