EIP-1108: Reduce alt bn128 gas costs (#1108)

* Add draft of elliptic-curve gas cost reduction EIP

* Add status and EIP dependencies

* Lowercase preamble per observed practice

* Tweak EIP title to reflect core cause for cost reduction

The EIP listed out the precompiles, but really this is about reducing all
alt_bn128-related gas costs.

* Reduce constant factor in line with original EIP issue

For some reason this got missed when turning into PR form.

* Fix footnote formatting

* Drop extraneous code fence

* GitHub username only for EIP author line

* Label alt_bn128 gas cost EIP as EIP 1108

EIPS/eip-1108.md

@@ -0,0 +1,48 @@
+---
+eip: 1108
+title: Reduce alt_bn128 precompile gas costs
+author: Antonio Salazar Cardozo (@shadowfiend)
+status: Draft
+type: Standards Track
+category: Core
+created: 2018-05-21
+requires: 196, 197
+---
+
+## Short Description
+
+Recent changes to the underlying library used by the official Go reference
+implementation led to significant performance gains for the `ECADD`, `ECMUL`,
+and pairing check precompiled contracts on the `alt_bn128` elliptic curve, which
+should be reflected in reduced gas costs.
+
+## Motivation
+
+Recently, the underlying library used by the [official Go reference
+implementation](https://github.com/ethereum/go-ethereum) to implement the
+`ECADD` (at address `0x06`), `ECMUL` (at address `0x07`), and pairing check (at
+address `0x08`) precompiled contracts was shifted to [Cloudflare's bn256
+library](https://github.com/cloudflare/bn256). Based on the [initial PR that
+introduced this change](https://github.com/ethereum/go-ethereum/pull/16203),
+and corroborated in [a later
+note](https://github.com/ethereum/go-ethereum/pull/16301#issuecomment-372687543),
+the computational cost of `ECADD`, `ECMUL`, and pairing checks (excepting the
+constant) has dropped roughly an order of magnitude across the board.
+
+## Specification
+
+Following is a table with the current gas cost and new gas cost:
+
+| Contract      | Address   | Current Gas Cost               | Updated Gas Cost    |
+| ------------- | --------- | -----------------------------  | ------------------- |
+| `ECADD`       | `0x06`    | 500<sup>[1]</sup>              | 50                  |
+| `ECMUL`       | `0x07`    | 40 000<sup>[1]</sup>           | 2 000               |
+| Pairing check | `0x08`    | 80 000k + 100 000<sup>[2]</sup>| 5 500k + 80 000     |
+
+The gas costs for `ECADD` and `ECMUL` are updates to the costs listed in
+EIP-196, while the gas costs for the pairing check are updates to the cost
+listed in EIP-197.
+
+[1]- Per [EIP-196](https://github.com/ethereum/EIPs/blob/984cf5de90bbf5fbe7e49be227b0c2f9567e661e/EIPS/eip-196.md#gas-costs).
+
+[2]- Per [EIP-197](https://github.com/ethereum/EIPs/blob/df132cd37efb3986f9cd3ef4922b15a767d2c54a/EIPS/eip-197.md#specification).