DCIPs/EIPS/eip-3026.md

---
eip: 3026
title: BW6-761 curve operations
author: Youssef El Housni (@yelhousni), Michael Connor (@iAmMichaelConnor), Aurore Guillevic <aurore.guillevic@inria.fr>
discussions-to: https://ethereum-magicians.org/t/eip-3026-bw6-761-curve-operations/4790
status: Stagnant
type: Standards Track
category: Core
requires: 2539
created: 2020-10-05
---

## Simple Summary
This precompile adds operations for the BW6-761 curve (from the EY/Inria [research paper](https://eprint.iacr.org/2020/351.pdf)) as a precompile in a set necessary to *efficiently* perform verification of one-layer composed zkSNARKs proofs.

## Abstract

If `block.number >= X` we introduce *seven* separate precompiles to perform the following operations (addresses to be determined):

- BW6_G1_ADD - to perform point addition on a curve defined over a prime field
- BW6_G1_MUL - to perform point multiplication on a curve defined over a prime field
- BW6_G1_MULTIEXP - to perform multiexponentiation on a curve defined over a prime field
- BW6_G2_ADD - to perform point addition on a curve twist defined the base a prime field
- BW6_G2_MUL - to perform point multiplication on a curve twist defined over a prime field
- BW6_G2_MULTIEXP - to perform multiexponentiation on a curve twist defined over a prime field
- BW6_PAIRING - to perform a pairing operations between a set of *pairs* of (G1, G2) points

The multiexponentiation operations are a generalization of point multiplication, but separate precompiles are prosposed because running a single MUL through MULTIEXP seems to be 20% more expensive.

## Motivation

This EIP is based on and tends to replace [EIP-2541](https://github.com/matter-labs/EIPs/blob/sw6_wrapping/EIPS/eip-2541.md) for significant performance reasons. In most applications, BW6-761 is used as an outer curve to BLS12-377 considered in [EIP-2539](https://github.com/ethereum/EIPs/pull/2539).
The motivation of this precompile is to allow efficient one-layer composition of SNARK proofs. Currently this is done by Zexe using the BLS12-377/CP6-782 pair of curves. This precompile proposes a replacement of CP6-782 by BW6-761, which allows much faster operations. For example, it was shown that verifying a Groth16 proof with BW6-761 is 30 times faster than with CP6-782.

### Proposed addresses table

|Precompile          |Address   |
|---|---|
|BW6_G1_ADD          | 0x13     |
|BW6_G1_MUL          | 0x14     |
|BW6_G1_MULTIEXP     | 0x15     |
|BW6_G2_ADD          | 0x16     |
|BW6_G2_MUL          | 0x17     |
|BW6_G2_MULTIEXP     | 0x18     |
|BW6_PAIRING         | 0x19     |

## Specification

Curve parameters:

The BW6-761 `y^2=x^3-1` curve is fully defined by the following set of parameters:

```
Base field modulus = 0x122e824fb83ce0ad187c94004faff3eb926186a81d14688528275ef8087be41707ba638e584e91903cebaff25b423048689c8ed12f9fd9071dcd3dc73ebff2e98a116c25667a8f8160cf8aeeaf0a437e6913e6870000082f49d00000000008b
A coefficient = 0x0
B coefficient = 0x122e824fb83ce0ad187c94004faff3eb926186a81d14688528275ef8087be41707ba638e584e91903cebaff25b423048689c8ed12f9fd9071dcd3dc73ebff2e98a116c25667a8f8160cf8aeeaf0a437e6913e6870000082f49d00000000008a
Main subgroup order = 0x1ae3a4617c510eac63b05c06ca1493b1a22d9f300f5138f1ef3622fba094800170b5d44300000008508c00000000001
Extension tower:
Fp3 construction: (Fp3 = Fp[u]/u^3+4)
Fp cubic non-residue = 0x122e824fb83ce0ad187c94004faff3eb926186a81d14688528275ef8087be41707ba638e584e91903cebaff25b423048689c8ed12f9fd9071dcd3dc73ebff2e98a116c25667a8f8160cf8aeeaf0a437e6913e6870000082f49d000000000087
Twist parameters:
Twist type: M
twist curve A coefficient c0 = 0x0
                          c1 = 0x0
twist curve B coefficient c0 = 0x4
                        c1 = 0x0
Generators:
G1:
X = 0x1075b020ea190c8b277ce98a477beaee6a0cfb7551b27f0ee05c54b85f56fc779017ffac15520ac11dbfcd294c2e746a17a54ce47729b905bd71fa0c9ea097103758f9a280ca27f6750dd0356133e82055928aca6af603f4088f3af66e5b43d
Y = 0x58b84e0a6fc574e6fd637b45cc2a420f952589884c9ec61a7348d2a2e573a3265909f1af7e0dbac5b8fa1771b5b806cc685d31717a4c55be3fb90b6fc2cdd49f9df141b3053253b2b08119cad0fb93ad1cb2be0b20d2a1bafc8f2db4e95363
G2:
X = 0x110133241d9b816c852a82e69d660f9d61053aac5a7115f4c06201013890f6d26b41c5dab3da268734ec3f1f09feb58c5bbcae9ac70e7c7963317a300e1b6bace6948cb3cd208d700e96efbc2ad54b06410cf4fe1bf995ba830c194cd025f1c
Y = 0x17c3357761369f8179eb10e4b6d2dc26b7cf9acec2181c81a78e2753ffe3160a1d86c80b95a59c94c97eb733293fef64f293dbd2c712b88906c170ffa823003ea96fcd504affc758aa2d3a3c5a02a591ec0594f9eac689eb70a16728c73b61
Pairing parameters:
e(P,Q)=(ML1(P,Q)*ML2(P,Q)^q)^FE
|loop_count_1| (first miller loop ML1 count) = 0x8508c00000000002
|loop_count_2| (second miller loop ML2 count) = 0x23ed1347970dec008a442f991fffffffffffffffffffffff
loop_count_1 is negative = false
loop_count_2 is negative = false
```

#### Encoding

##### Field elements encoding:

To encode points involved in the operation one has to encode elements of only the base field.

The base field element (Fp) is encoded as `96` bytes by performing BigEndian encoding of the corresponding (unsigned) integer. The corresponding integer **MUST** be less than the base field modulus.

If encodings do not follow this spec anywhere during parsing in the precompile, the precompile **MUST** revert with "endoding error".

##### Encoding of uncompressed points:

Points in both G1 and G2 can be expressed as `(x, y)` affine coordinates, where `x` and `y` are elements of the base field.
Therefore, points in both G1 and G2 are encoded as the byte concatenation of the field element encodings of the `x` and `y` affine coordinates. The total encoding length for a G1/G2 point is thus `192` bytes.

##### Point at infinity encoding:

Also referred as the "zero point". For BW6-761 (`y^2=x^3-1`) and its M-twisted curves (`y^3=x^3+4`), the point with coordinates `(0, 0)` (formal zeros in Fp) is *not* on the curve, and so the encoding of `(0, 0)` is used as a convention to encode the point at infinity.

##### Encoding of scalars for multiplication and multiexponentiation operations:

For multiplication and multiexponentiation operations, a scalar is encoded as `64` bytes by performing BigEndian encoding of the corresponding (unsigned) integer.

Note that the main subgroup order for BW6-761 is actually only `377` bits (`48` bytes), but an encoding of `64` bytes has been chosen to have a `32`-byte-aligned ABI (representable as e.g. `bytes32[2]` or `uint256[2]`).

The corresponding integer **MAY** be greater than the main subgroup order.

#### ABI for operations

##### ABI for G1 addition

G1 addition call expects `384` bytes as an input that is interpreted as the byte concatenation of two G1 points (point-encoded as `192` bytes each). Output is a point-encoding of the addition operation result.

Error cases:
- Either of the points being not on the curve
- Input has invalid length
- Field element encoding rules apply (obviously)

##### ABI for G1 multiplication
G1 multiplication call expects `256` bytes as an input that is interpreted as the byte concatenation of the point-encoding of a G1 point (`192` bytes) and the encoding of a scalar value (`64` bytes). Output is a point-encoding of the multiplication operation result.

Error cases:
- Point being not on the curve
- Input has invalid length
- Field element encoding rules apply (obviously)
- Scalar encoding rules apply (obviously)

##### ABI for G1 multiexponentiation

G1 multiplication call expects `256*k` bytes as an input that is interpreted as the byte concatenation of `k` slices, each of them being a byte concatenation of the point-encoding of a G1 point (`192` bytes) and the encoding of a scalar value (`64` bytes). Output is an encoding of the multiexponentiation operation result.

Error cases:
- Any of the G1 points being not on the curve
- Input has invalid length
- Field element encoding rules apply (obviously)
- Scalar encoding rules apply (obviously)

##### ABI for G2 addition

G2 addition call expects `384` bytes as an input that is interpreted as the byte concatenation of two G2 points (point-encoded as `192` bytes each). Output is a point-encoding of the addition operation result.

Error cases:
- Either of points being not on the curve
- Input has invalid length
- Field elements encoding rules apply (obviously)

##### ABI for G2 multiplication
G2 multiplication call expects `256` bytes as an input that is interpreted as the byte concatenation of the point-encoding of a G2 point (`192` bytes) and the encoding of a scalar value (`64` bytes). Output is an encoding of multiplication operation result.

Error cases:
- Point being not on the curve must result in error
- Field elements encoding rules apply (obviously)
- Input has invalid length

##### ABI for G2 multiexponentiation

G2 multiplication call expects `240*k` bytes as an input that is interpreted as byte concatenation of `k` slices each of them being a byte concatenation of encoding of G2 point (`192` bytes) and encoding of a scalar value (`48` bytes). Output is an encoding of multiexponentiation operation result.

Error cases:
- Any of G2 points being not on the curve must result in error
- Field elements encoding rules apply (obviously)
- Input has invalid length

##### ABI for pairing

Pairing call expects `384*k` bytes as an input, that is interpreted as the byte concatenation of `k` slices. Each slice has the following structure:
- `192` bytes G1 point encoding
- `192` bytes G2 point encoding

Output is `32` bytes representing a boolean:

- `0x0000000000000000000000000000000000000000000000000000000000000001` if the pairing result is equal the to multiplicative identity in the pairing target field; and
- `0x0000000000000000000000000000000000000000000000000000000000000000` otherwise.

Error cases:
- Any of the G1 or G2 points being not on the curve
- Any of the G1 or G2 points being not in the correct subgroup
- Input has invalid length
- Field elements encoding rules apply (obviously)

#### Prevention of DDoS on error handling

This precompile performs extensive computations and in case of any errors during execution it **MUST** consume all gas from the the gas schedule for the corresponding operation.

#### Gas schedule

##### G1 addition
`<to be estimated>` gas

##### G1 multiplication
`<to be estimated>` gas

##### G2 addition
`<to be estimated>` gas

##### G2 multiplication
`<to be estimated>` gas

##### G1/G2 Multiexponentiation
Discounts table as a vector of pairs `[k, discount]`:

```
<to be estimated>
```

`max_discount = <to be estimated>`

##### Pairing operation
Base cost of the pairing operation is `<to be estimated>*k + <to be estimated>` where `k` is a number of pairs.

## Rationale
Gas costs are based on EIP1962 estimation strategy (but do not fully include yet parsing of ABI, decoding and encoding of the result as a byte array).

#### Gas estimation strategy
Gas cost is derived by taking the average timing of the same operations over different implementations and assuming a constant `30 MGas/second`. Since the execution time is machine-specific, this constant is determined based on execution times of [ECRECOVER](https://github.com/matter-labs/eip1962/blob/master/run_bn_pairing_estimate.sh) and [BNPAIR](https://github.com/matter-labs/eip1962/blob/master/run_bn_pairing_estimate.sh) precompiles on my machine and their proposed gas price (`43.5 MGas/s` for ECRECOVER and `16.5 MGas/s` for BNPAIR). Following are the proposed methods to time the precompile operations:

- G1 addition: Average timing of 1000 random samples.
- G1 multiplication: Average timing of 1000 samples of random worst-case of double-and-add algorithm (scalar of max bit length and max hamming weight and random base points in G1)
- G2 addition: Average timing of 1000 random samples
- G2 multiplication: Average timing of 1000 samples of radnom worst-case of double-and-add algorithm (scalar of max bit length and max hamming weight and random base points in G2)
- G1 and G2 multiexponentiations: Expected to be performed by the Peppinger algorithm, with a table prepared for discount in case of `k <= 128` points in the multiexponentiation with a discount cup `max_discount` for `k > 128`. To avoid non-integer arithmetic call cost is calculated as `k * multiplication_cost * discount / multiplier` where `multiplier = 1000`, `k` is a number of (scalar, point) pairs for the call, `multiplication_cost` is a corresponding single multiplication call cost for G1/G2.
- Pairing: Average timing of 1000 random samples (random points in G1 and G2) for different number of pairs with linear lifting.

#### Multiexponentiation as a separate call
Explicit separate multiexponentiation operation that allows one to save execution time (so gas) by both the algorithm used (namely Peppinger algorithm) and (usually forgotten) by the fact that `CALL` operation in Ethereum is expensive (at the time of writing), so one would have to pay non-negigible overhead if e.g. for multiexponentiation of `100` points would have to call the multipication precompile `100` times and addition for `99` times (roughly `138600` would be saved).

#### Explicit subgroup checks
G2 subgroup check has the same cost as G1 subgroup check. Endomorphisms can be leverages to optimize this operation.

## Backwards Compatibility
There are no backward compatibility questions.

## Test Cases

Due to the large test parameters space we first provide properties that various operations must satisfy. We use additive notation for point operations, capital letters (`P`, `Q`) for points, small letters (`a`, `b`) for scalars. Generator for G1 is labeled as `G`, generator for G2 is labeled as `H`, otherwise we assume random point on a curve in a correct subgroup. `0` means either scalar zero or point of infinity. `1` means either scalar one or multiplicative identity. `group_order` is a main subgroup order. `e(P, Q)` means pairing operation where `P` is in G1, `Q` is in G2.

Requeired properties for basic ops (add/multiply):

- Commutativity: `P + Q = Q + P`
- Additive negation: `P + (-P) = 0`
- Doubling `P + P = 2*P`
- Subgroup check: `group_order * P = 0`
- Trivial multiplication check: `1 * P = P`
- Multiplication by zero: `0 * P = 0`
- Multiplication by the unnormalized scalar `(scalar + group_order) * P = scalar * P`

Required properties for pairing operation:
- Degeneracy `e(P, 0*Q) = e(0*P, Q) = 1`
- Bilinearity `e(a*P, b*Q) = e(a*b*P, Q) = e(P, a*b*Q)` (internal test, not visible through ABI)

Test vector for all operations are expanded in this [gist](https://gist.github.com/shamatar/506ab3193a7932fe9302a2f3a31a23e8) until it's final.

## Implementation
There is a various choice of existing implementations:

**Libraries:**
- Rust implementation (EY/Zexe): https://github.com/yelhousni/zexe/tree/youssef/BW6-761-Fq-ABLR-2ML-M
- C++ implementation (EY/libff): https://github.com/EYBlockchain/zk-swap-libff
- Golang implementation (Consensys/gurvy): https://github.com/ConsenSys/gurvy

**Stand-alone implementation:**
- Golang implementation with Intel assembly (Onur Kilic): https://github.com/kilic/bw6

**Precompiles:**
- OpenEthereum (EY/Parity): https://github.com/EYBlockchain/solidity-elliptic-curves

**Scripts:**
- SageMath and Magma scripts: https://gitlab.inria.fr/zk-curves/bw6-761/

## Security Considerations
Strictly following the spec will eliminate security implications or consensus implications in a contrast to the previous BN254 precompile.

Important topic is a "constant time" property for performed operations. We explicitly state that this precompile **IS NOT REQUIRED** to perform all the operations using constant time algorithms.

## Copyright
Copyright and related rights waived via [CC0](../LICENSE.md).