fix: scalarmul by 0 on twisted edwards curves

[ivokub]

Description

When requesting for decomposition of zero scalar, then add hot path which avoids calling ecc.PrecomputeLattice directly as it panics due to division by zero. Checked that this unsafe way of calling ecc.PrecomputeLattice was only done here, everywhere else the parameters are either constants (through curve parameters) or we explicitly set the scalar to be 1 if it is 0 and later switch between actual and dummy result.

Fixes #1547

Thanks for @lucasmenendez and @p4u for reporting!

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Expanded current tests to include zero inputs.

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]

Pull Request Overview

This PR fixes a division by zero panic that occurs when performing scalar multiplication by zero on twisted Edwards curves. The issue was in the halfGCD function which calls ecc.PrecomputeLattice without handling the zero scalar case.

• Adds a hot path in halfGCD to handle zero scalar inputs by returning all zero outputs
• Expands test coverage to include zero scalar inputs across all curve configurations
• Removes unused test code that was no longer needed

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
std/algebra/native/twistededwards/hints.go	Adds zero scalar check to prevent division by zero in halfGCD function
std/algebra/native/twistededwards/curve_test.go	Enhances tests to cover zero scalar cases and removes unused varScalarMul struct

[yelhousni]

lgtm