test(stm): add negative single-signature tests

[leepl37]

Content

This PR adds negative tests for SingleSignature::verify in mithril-stm to ensure invalid inputs are rejected:

• wrong verification key
• out‑of‑bounds index
• wrong message

This aligns with the reviewer suggestion from @hjeljeli32 and addresses #2946.
Note: dependency on #2943 is acknowledged; I will rebase/update after it merges if needed.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)
  • Add ADR blog post or Dev ADR entry (if relevant)
  • No new TODOs introduced

Comments

This is a Draft PR as discussed with @jpraynaud. I am opening it now to allow for early review of the test logic.

Issue(s)

Relates to #2946
Blocked by #2943 (will rebase if needed)

[Copilot]

Pull request overview

Adds negative unit tests for SingleSignature::verify in mithril-stm to ensure invalid inputs are rejected, addressing #2946.

Changes:

• Introduce a shared test setup helper to build a signer / AVK context.
• Add negative verification tests for: wrong verification key, out-of-bounds lottery index, and wrong message.

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[jpraynaud]

Thanks @leepl37 for the contribution 👍

I left few comments below.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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[curiecrypt]

Looks good overall. Here are my suggestions:

• Use a structure for the test context, I give the details below.
• Modify build_single_signature_context function to take arguments such as number of signers, or rng. After that, it should return the struct created for test.
• Another test case would be trying to verify a valid signature for another registration. This would be done with a different avk (in the current version). In the case we have the structure and context builder function, we can use different calls to this function.

[Copilot]

Pull request overview

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

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[jpraynaud]

Thanks for the updates @leepl37 !

Once all comments from @curiecrypt are addressed and the CI is all green, we'll be able to merge 👍

[leepl37]

Hi @curiecrypt and @jpraynaud! I have addressed all the feedback and implemented the new test context structure.
Let me know if you need any other tweaks.

[jpraynaud]

LGTM 👍

Thanks @leepl37 for the contribution!

[curiecrypt]

LGTM 🚀
Thank you!

[jpraynaud]

@leepl37 could you bump the version and update the CHANGELOG file in mithril-stm crate?

I'll merge the PR once this is done 🚀

[leepl37]

Hi @jpraynaud!
I have bumped the version and updated the CHANGELOG in the mithril-stm crate as requested.
This PR is now ready for the final merge. Thanks!