refactor: hash to curve

[ivokub]

Description

This PR exposes the primitive methods of hash-to-curve so that it can be implemented in gnark. Also separated the code generation into separate method.

Kept the current public methods intact, so it is fully backwards compatible.

Type of change

• refactor

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

[Copilot]

Copilot reviewed 71 out of 71 changed files in this pull request and generated no comments.

[ivokub]

Currently not yet ready, I'll confirm it would also be useful for map to G2 in gnark.

[Copilot]

Copilot reviewed 71 out of 71 changed files in this pull request and generated no comments.

[ivokub]

@ThomasPiellard, @Tabaie - ready for review now.

[Tabaie]

I don't think it makes a lot of sense to have hash_to_curve source files in both packages. imo every hash to curve function should be moved to the new package and only add a few wrappers to the root package, thus removing hash_to_g{{i}}.go from the root package. This way we can avoid making all those internal functions public as well.

[ivokub]

I don't think it makes a lot of sense to have hash_to_curve source files in both packages. imo every hash to curve function should be moved to the new package and only add a few wrappers to the root package, thus removing hash_to_g{{i}}.go from the root package. This way we can avoid making all those internal functions public as well.

The goal of the PR is to make some internal things public so that we can reuse in gnark:

• isogeny map coefficients
• isogenous curve coefficients
• sqrtRatio method
• clear cofactor method (for tests)

For the MapToG, MapToCurve, EncodeToG and HashToG, I tried moving them to the hash_to_curve package, but it resulted in import cycle, as it depends on G1Affine and G1Jac, so root package would import hash_to_curve which would then import root package again.

For the Isogeny methods I overcame that by working directly with the point coordinates. I could have done it also for MapToG, MapToCurve and EncodeToG, but as HashToG explicitly depends on curve arithmetic, then I cannot use only the coordinates. And if I need to keep HashToG in the root package, then it also made sense to keep rest.

[Tabaie]

Thanks looks good