---
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.

What is more, the performance boost for those operations can be also observed 
for Parity client. 

Faster operations on Ethereum clients 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.

Also, [optimisations in the bn library](https://github.com/paritytech/bn/pull/9) 
used by the [Parity client](https://github.com/paritytech/parity-ethereum) led to a 
significant performance boost we 
[benchmarked](https://gist.github.com/pdyraga/4649b74436940a01e8221d85e80bfeef) 
and compared against the [previous 
results](https://github.com/ethereum/benchmarking/blob/master/constantinople/analysis2.md). 

## 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>              | 150                 |
| `ECMUL`       | `0x07`    | 40 000<sup>[1]</sup>           | 6 000               |
| Pairing check | `0x08`    | 80 000 * k + 100 000<sup>[2]</sup>| 28 300 * k + 35 450    |

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. Updated gas costs have been adjusted to the less performant 
client which is Parity, according to benchmarks<sup>[3]</sup>.

[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).

[3]- [Parity benchmarks.](https://gist.github.com/pdyraga/4649b74436940a01e8221d85e80bfeef)