DKG: clean up abandoned dkg_subkeys entries (#970)

[wksantiago]

Closes #970

Summary by CodeRabbit

• New Features

  • Added the ability to discard a pending group key-generation operation.
  • Discarding an unavailable operation is handled safely without errors.

• Bug Fixes

  • Improved key-generation handling so each pending key is used only once and removed before processing.

[coderabbitai]

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⚙️ Run configuration

Configuration used: Organization UI

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Reviewing files that changed from the base of the PR and between bf459ab and 0c2468f.

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• keep-mobile/src/lib.rs

Walkthrough

The mobile API now supports discarding pending per-group DKG subkeys. DKG execution removes each subkey from storage before starting, making subkeys single-use. Tests cover selective and repeated discard behavior.

Changes

DKG subkey lifecycle

Layer / File(s)	Summary
Discard pending DKG subkeys keep-mobile/src/lib.rs	Adds frost_dkg_discard for targeted, idempotent removal of a group’s pending DKG subkey. Tests verify selective and repeated discard behavior.
Consume subkeys during DKG keep-mobile/src/lib.rs	Changes frost_run_dkg to remove the stored subkey before the ceremony proceeds.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to bf459

The change can delete a pending DKG subkey before required preflight checks succeed; if those checks fail, retries may be impossible and the roster may need to be rebuilt with a new public key. This bounded correctness risk should be addressed before merging.

Suggested reviewers: kwsantiago

Poem

A rabbit found keys in a DKG queue
It cleared one group, then cleared it anew
The run took its key
For one use only
And left tidy burrows behind too

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies cleanup of abandoned DKG subkey entries, which is the primary change.
Linked Issues check	✅ Passed	The changes add explicit discard behavior and consume subkeys during DKG, preventing abandoned entries from persisting.
Out of Scope Changes check	✅ Passed	The code changes and tests directly support cleanup and single-use handling of DKG subkeys.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 1 files.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

📝 Generate docstrings

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• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/dkg-abandoned-subkeys

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@keep-mobile/src/lib.rs`:
- Around line 1494-1499: Move the dkg_subkeys removal from the block before
preflight into the path after verify_and_pin_relays and parse_loopback_proxy
succeed, while preserving the existing single-use behavior once dkg::run_dkg
starts. Ensure concurrent frost_dkg_begin and frost_dkg_discard operations
remain correctly synchronized, and add a regression test proving a preflight
failure leaves the pending subkey available for retry.

🪄 Autofix

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 606f8975-89dd-4903-987e-039f9da3cd9f

📥 Commits

Reviewing files that changed from the base of the PR and between d9192f5 and bf459ab.

📒 Files selected for processing (1)

• keep-mobile/src/lib.rs

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[kwsantiago]

Verified locally: cargo fmt --check, clippy --all-targets -D warnings, 326 tests including both new ones run explicitly. CI green.

The clone-then-consume split is the part worth calling out, because the obvious implementation of "single-use subkey" would have been to consume it at load and that would have been wrong. Taking a copy first means a preflight failure — relay TLS pinning, proxy setup — leaves the subkey stashed, so a retry costs nothing. Consuming only once preflight has passed draws the line at the ceremony actually starting. run_dkg_preflight_failure_retains_pending_subkey pins exactly that boundary rather than just testing the happy path, which is what makes the split durable against someone later "simplifying" the two steps back together.

The trade on the other side of that line is real and I think correctly chosen: a ceremony that fails after network I/O loses its subkey, so a retry re-mints and every participant needs a fresh roster. That is a genuine UX cost for a transient network drop, but reusing an identity across ceremonies after round-1 packages were already published is the worse option, and the comment says so plainly instead of leaving the next reader to wonder whether it was deliberate.

Zeroizing<[u8; 32]> means remove() actually wipes rather than just dropping the reference, so the doc comment's "zeroize and drop" is accurate. Idempotence is tested, and the concurrent-discard race is handled by the ordering: the keypair is already cloned before the removal, so a discard that lands between preflight and consume makes the removal a no-op rather than breaking the run.

Two follow-ups, neither blocking:

The API is unused until keep-android calls it. Nothing in keep invokes frost_dkg_discard outside tests, and there is no frostDkgDiscard call site in keep-android, so the accumulation this closes still happens in the app until a companion lands. Worth an issue so it does not sit exposed-but-unused — same pairing as #979/#515.

Nothing bounds dkg_subkeys. The fix gives the client a way to clean up, but a client that never calls discard still grows the map one entry per abandoned begin. Each entry is small and it is a local API rather than remote-triggered, so the memory is immaterial; the concern is secrets lingering, which is exactly what discard addresses when called. A cap would make it not depend on the client remembering.

No blockers.