Add November 13th Hardfork technical specification

nov-13-hardfork-spec.md

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+# November 13th Bitcoin Cash Hardfork Technical Details
+
+Version 1.0, 2017-11-01
+
+## Summary
+
+When the median time past[1] 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](https://github.com/bitcoin/bips/blob/master/bip-0146.mediawiki#low_s))
+* Enforcement of NULLFAIL ([BIP 0146](https://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 (B_n+1), with an MTP-11[1] greater than or equal to the unix timestamp 1510600000, perform the following steps:
+
+* NOTE: Implementations must use integer arithmetic only
+
+1. Let B_n be the Nth block in a Bitcoin Cash Blockchain.
+1. Let B_last be the middle block of the [B_n, B_n-1, B_n-2] when sorted by timestamp.
+1. Let B_first be the middle block of the [B_n-144, B_n-145, B_n-146] when sorted by timestamp.
+1. Let the Timespan (TS) be equal to the difference in UNIX timestamps (in seconds) between B_last and B_first 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[2] between B_last  and B_first.
+1. Let the Projected Work (PW) be equal to (W * 600) / TS.
+1. Let Target (T) be equal to the (2^256 - PW) / PW.  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 B_n+1 is then equal to the lesser of T and 0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
+
+## References
+
+ - [Algorithm](https://github.com/Bitcoin-ABC/bitcoin-abc/commit/be51cf295c239ff6395a0aa67a3e13906aca9cb2)
+ - [Activation](https://github.com/Bitcoin-ABC/bitcoin-abc/commit/18dc8bb907091d69f4887560ab2e4cfbc19bae77)
+ - [Activation Time](https://github.com/Bitcoin-ABC/bitcoin-abc/commit/8eed7939c72781a812fdf3fb8c36d4e3a428d268)
+
+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
+2. Chainwork for a Block (B) is the sum of block proofs from the genesis block to B.  Block proofs is defined in [chain.cpp](https://github.com/Bitcoin-ABC/bitcoin-abc/blob/master/src/chain.cpp#L132)