feat: added HashToG2 circuit for BLS12-381

[weijiguo]

Description

This PR exposes the g2SqrtRatio function as G2SqrtRatio so that it could be used as a hint in gnark to calculate the HashToG2 circuit for BLS12-381.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How has this been tested?

• Tested in the gnark library as part of the HashToG2 function for BLS12-381.

How has this been benchmarked?

No.

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[CLAassistant]

All committers have signed the CLA.

[weijiguo]

This PR is part of efforts to address Consensys/gnark#648

[weijiguo]

Hi @ivokub I am now trying to deal with the issue you mentioned earlier about modifying the generated file instead of the template: Consensys/gnark#1040 (comment)

While I attempt to change the template function name from {{$CurveName}}SqrtRatio to {{$CurveTitle}}SqrtRatio, I realize that I might also need to run the generator and update the generated codes as well. Is this what you would expect?

Also, g1SqrtRatio function names are also updated after code generation. Is this OK? Do you want to update the copyright year as well (current value is 2020).

Please ping back.

Thanks,
Weiji

[weijiguo]

fptower is still internal, though

[ivokub]

Hi @ivokub I am now trying to deal with the issue you mentioned earlier about modifying the generated file instead of the template: Consensys/gnark#1040 (comment)

While I attempt to change the template function name from {{$CurveName}}SqrtRatio to {{$CurveTitle}}SqrtRatio, I realize that I might also need to run the generator and update the generated codes as well. Is this what you would expect?

Also, g1SqrtRatio function names are also updated after code generation. Is this OK? Do you want to update the copyright year as well (current value is 2020).

Please ping back.

Thanks, Weiji

Thanks, I'll have a look.

[ivokub]

More cleanly applied here: #674. There I also expose methods which help with computing the SSWU isogeny.

Closing this pull request for now.