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Andrew Groot
103 lines
5.3 KiB
Markdown
103 lines
5.3 KiB
Markdown
# HF-20171113
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```
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layout: specification
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title: November 13th Bitcoin Cash Hardfork Technical Details
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category: spec
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date: 2017-11-07
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activation: 1510600000
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version: 1.3
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```
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## Summary
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When the median time past<sup>[1]</sup> of the most recent `11` blocks (`MTP - 11`) is greater than or equal to UNIX timestamp `1510600000` Bitcoin Cash will execute a hardfork according to this specification.
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Starting from the next block these three consensus rules changes will take effect:
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* Enforcement of LOW_S signatures ([BIP 0146](//github.com/bitcoin/bips/blob/master/bip-0146.mediawiki#low_s))
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* Enforcement of NULLFAIL ([BIP 0146](//github.com/bitcoin/bips/blob/master/bip-0146.mediawiki#nullfail))
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* A replacement for the emergency difficulty adjustment.
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The algorithm for the new difficulty adjustment is described below
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## Difficulty Adjustment Algorithm Description
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To calculate the difficulty of a given block (<code>B<sub>n</sub> + 1</code>), with an `MTP-11`<sup>[1]</sup> greater than or equal to the unix timestamp `1510600000`, perform the following steps:
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_NOTE: Implementations must use integer arithmetic only_
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1. Let <code>B<sub>n</sub></code> be the Nth block in a Bitcoin Cash Blockchain.
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1. Let <code>B<sub>last</sub></code> be chosen<sup>[2]</sup> from <code>[B<sub>n</sub> - 2, B<sub>n</sub> - 1, B<sub>n</sub>]</code>.
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1. Let <code>B<sub>first</sub></code> be chosen<sup>[2]</sup> from <code>[B<sub>n</sub> - 146, B<sub>n</sub> - 145, B<sub>n</sub> - 144]</code>.
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1. Let the Timespan (`TS`) be equal to the difference in UNIX timestamps (in seconds) between <code>B<sub>last</sub></code> and <code>B<sub>first</sub></code> within the range `[72 * 600, 288 * 600]`.
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Values outside should be treated as their respective limit.
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1. Let the Work Performed (`W`) be equal to the difference in chainwork<sup>[3]</sup> between B<sub>last</sub> and B<sub>first</sub>.
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1. Let the Projected Work (`PW`) be equal to `(W * 600) / TS`.
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1. Let Target (`T`) be equal to the <code>(2<sup>256</sup> - PW) / PW</code>. This is calculated by taking the two’s complement of `PW` (`-PW`) and dividing it by `PW` (`-PW / PW`).
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1. The target difficulty for block <code>B<sub>n</sub> + 1</code> is then equal to the lesser of `T` and `0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF`
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## Test Case
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1. Create a genesis block with the following data:
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```
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nHeight = 0;
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nTime = 1269211443;
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nBits = 0x1C0FFFFF;
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```
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2. Add `2049` blocks at `600` second intervals with the same `nBits`.
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1. Add another `10` blocks at `600` second intervals. `nBits` should remain constant.
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1. Add a block `6000` seconds in the future with `nBits` remaining the same.
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1. Add a block `-4800` seconds from the previous block. `nBits` should remain the constant.
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1. Add `20` blocks at `600` second intervals. `nBits` should remain constant.
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1. Add a block at a `550` second interval. `nBits` should remain constant.
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1. Add `10` blocks at `550` second intervals. The target difficulty should slowly decrease.
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1. `nBits` should be `0x1C0FE7B1`.
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1. Add `20` more blocks at `10` second intervals. The target difficulty decrease quickly.
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1. `nBits` should be `0x1C0DB19F`.
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1. Add `1` block at an interval of `6000` seconds.
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1. `nBits` should be `0x1C0D9222`.
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1. Produce `93` blocks at `6000` second intervals. The target difficulty should increase.
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1. `nBits` should be `0x1C2F13B9`.
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1. Add one block at `6000` seconds.
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1. `nBits` should be `0x1C2EE9BF`.
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1. Add `192` blocks at `6000` second intervals. The target difficulty should increase.
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1. `nBits` should be `0x1D00FFFF`.
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1. Add `5` blocks at `6000` second intervals. Target should stay constant at the maximum value.
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## References
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- [Algorithm](//github.com/Bitcoin-ABC/bitcoin-abc/commit/be51cf295c239ff6395a0aa67a3e13906aca9cb2)
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- [Activation](//github.com/Bitcoin-ABC/bitcoin-abc/commit/18dc8bb907091d69f4887560ab2e4cfbc19bae77)
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- [Activation Time](//github.com/Bitcoin-ABC/bitcoin-abc/commit/8eed7939c72781a812fdf3fb8c36d4e3a428d268)
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- [Test Case](//github.com/Bitcoin-ABC/bitcoin-abc/blob/d8eac91f8d16716eed0ad11ccac420122280bb13/src/test/pow_tests.cpp#L193)
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## FAQ
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---
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> Q: Does this imply that if the blocks are timestamped sequentially, the last block has no effect since it will look at the block before that one?
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>
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> A: Yes
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## Footnotes
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---
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1. The `MTP-11` of a block is defined as the median timestamp of the last `11` blocks prior to, and including, a specific block.
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1. A block is chosen via the following mechanism:
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> Given a list: `S = [B<sub>n</sub> - 2, B<sub>n</sub> - 1, B<sub>n</sub>]`
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>
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>> a. If timestamp (`S<sub>0</sub>`) greater than timestamp (`S<sub>2</sub>`) then swap `S<sub>0</sub>` and `S<sub>2</sub>`.
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>>
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>> b. If timestamp (`S<sub>0</sub>`) greater than timestamp (`S<sub>1</sub>`) then swap `S<sub>0</sub>` and `S<sub>1</sub>`.
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>>
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>> c. If timestamp (`S<sub>1</sub>`) greater than timestamp (`S<sub>2</sub>`) then swap `S<sub>1</sub>` and `S<sub>2</sub>`.
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>>
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>> d. Return `S<sub>1</sub>`.
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>
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> See [GetSuitableBlock](https://github.com/Bitcoin-ABC/bitcoin-abc/commit/be51cf295c239ff6395a0aa67a3e13906aca9cb2#diff-ba91592f703a9d0badf94e67144bc0aaR208)
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3. Chainwork for a Block (B) is the sum of block proofs from the genesis block up to and including block `B`.
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`Block proof` is defined in [chain.cpp](https://github.com/Bitcoin-ABC/bitcoin-abc/blob/d8eac91f8d16716eed0ad11ccac420122280bb13/src/chain.cpp#L132)
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