specification/protocol/forks/hf-20171113.md

# HF-20171113

```
layout: specification
title: November 13th Bitcoin Cash Hardfork Technical Details
category: spec
date: 2017-11-07
activation: 1510600000
version: 1.3
```

## Summary

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.
Starting from the next block these three consensus rules changes will take effect:

* Enforcement of LOW_S signatures ([BIP 0146](//github.com/bitcoin/bips/blob/master/bip-0146.mediawiki#low_s))
* Enforcement of NULLFAIL ([BIP 0146](//github.com/bitcoin/bips/blob/master/bip-0146.mediawiki#nullfail))
* A replacement for the emergency difficulty adjustment.

The algorithm for the new difficulty adjustment is described below

## Difficulty Adjustment Algorithm Description

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:

_NOTE: Implementations must use integer arithmetic only_

1. Let <code>B<sub>n</sub></code> be the Nth block in a Bitcoin Cash Blockchain.
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>.
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>.
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]`.
Values outside should be treated as their respective limit.
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>.
1. Let the Projected Work (`PW`) be equal to `(W * 600) / TS`.
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`).
1. The target difficulty for block <code>B<sub>n</sub> + 1</code> is then equal to the lesser of `T` and `0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF`

## Test Case

1. Create a genesis block with the following data:

```
    nHeight = 0;
    nTime = 1269211443;
    nBits = 0x1C0FFFFF;
```

2. Add `2049` blocks at `600` second intervals with the same `nBits`.
1. Add another `10` blocks at `600` second intervals. `nBits` should remain constant.
1. Add a block `6000` seconds in the future with `nBits` remaining the same.
1. Add a block `-4800` seconds from the previous block.  `nBits` should remain the constant.
1. Add `20` blocks at `600` second intervals.  `nBits` should remain constant.
1. Add a block at a `550` second interval. `nBits` should remain constant.
1. Add `10` blocks at `550` second intervals. The target difficulty should slowly decrease.
1. `nBits` should be `0x1C0FE7B1`.
1. Add `20` more blocks at `10` second intervals.  The target difficulty decrease quickly.
1. `nBits` should be `0x1C0DB19F`.
1. Add `1` block at an interval of `6000` seconds.
1. `nBits` should be `0x1C0D9222`.
1. Produce `93` blocks at `6000` second intervals. The target difficulty should increase.
1. `nBits` should be `0x1C2F13B9`.
1. Add one block at `6000` seconds.
1. `nBits` should be `0x1C2EE9BF`.
1. Add `192` blocks at `6000` second intervals.  The target difficulty should increase.
1. `nBits` should be `0x1D00FFFF`.
1. Add `5` blocks at `6000` second intervals.  Target should stay constant at the maximum value.

## References

 - [Algorithm](//github.com/Bitcoin-ABC/bitcoin-abc/commit/be51cf295c239ff6395a0aa67a3e13906aca9cb2)
 - [Activation](//github.com/Bitcoin-ABC/bitcoin-abc/commit/18dc8bb907091d69f4887560ab2e4cfbc19bae77)
 - [Activation Time](//github.com/Bitcoin-ABC/bitcoin-abc/commit/8eed7939c72781a812fdf3fb8c36d4e3a428d268)
 - [Test Case](//github.com/Bitcoin-ABC/bitcoin-abc/blob/d8eac91f8d16716eed0ad11ccac420122280bb13/src/test/pow_tests.cpp#L193)

## FAQ
---
> 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?
>
> A: Yes

## Footnotes

---

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.
1. A block is chosen via the following mechanism:

> Given a list: `S = [B<sub>n</sub> - 2, B<sub>n</sub> - 1, B<sub>n</sub>]`
>
>> 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>`.
>>
>> 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>`.
>>
>> 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>`.
>>
>> d. Return `S<sub>1</sub>`.
>
> See [GetSuitableBlock](https://github.com/Bitcoin-ABC/bitcoin-abc/commit/be51cf295c239ff6395a0aa67a3e13906aca9cb2#diff-ba91592f703a9d0badf94e67144bc0aaR208)

3. Chainwork for a Block (B) is the sum of block proofs from the genesis block up to and including block `B`.
 `Block proof` is defined in [chain.cpp](https://github.com/Bitcoin-ABC/bitcoin-abc/blob/d8eac91f8d16716eed0ad11ccac420122280bb13/src/chain.cpp#L132)