keep-web: fail closed instead of panicking when the RNG health check trips

[kwsantiago]

Summary

Four paths in the web co-signer drew randomness through the panicking random_bytes() helper. Two are reachable while the service is running, so an entropy health-check failure would unwind inside a live signer rather than being reported; the other two abort startup with a raw panic message instead of the clean error their io::Result signature already supports. All four now use the fallible helper and fail closed.

Request paths

• WebSocket ticket (ws_ticket): returns 503 with a retry hint instead of panicking. The ticket authorizes a WebSocket upgrade, so a value the health check has flagged must never be issued.
• Approval id (random_approval_id): returns None when the check trips, and the caller refuses the approval and logs it, mirroring the existing refusal used when co-signing is disabled. Unguessability is the entire purpose of this id (it is defense-in-depth alongside the auth gate), so refusing beats falling back to anything predictable. The refusal happens before the approval slot is registered or broadcast, so no half-created approval is left behind.

Startup paths (both already returned io::Result, and the crate already defines the fallible wrapper used by its siblings)

• The 32-byte admin bearer token.
• The per-write temp-file suffix used to avoid clobbering an in-flight temp file.

After this, no caller of the panicking helper remains anywhere in the crate.

Scope note: this covers the entropy health-check verdict. A hard OS-entropy failure still unwinds inside the RNG itself before the check can report it; that is a pre-existing keep-core issue and is tracked separately rather than papered over here.

Part of #803.

Test plan

• cargo test -p keep-web — 54 passed, 0 failed. The existing approval-id range test now asserts the healthy path yields an id.
• cargo clippy -p keep-web --all-targets -- -D warnings and cargo fmt --check — clean.
• Verified by grep that no occurrence of the panicking helper remains in the crate.

[coderabbitai]

Walkthrough

Secure randomness failures are now handled explicitly for WebSocket tickets and NIP-46 approval IDs. Ticket requests return HTTP 503, while approval requests are denied when an approval ID cannot be generated. The related JavaScript-safety test now unwraps the optional ID.

Changes

Entropy failure handling

Layer / File(s)	Summary
Fallible randomness across web flows keep-web/src/api.rs, keep-web/src/bunker.rs	WebSocket ticket generation returns 503 SERVICE_UNAVAILABLE when entropy is unavailable. Approval ID generation returns an optional JavaScript-safe value, and approval requests are denied if generation fails; the corresponding test unwraps the healthy result before checking its range.

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

Possibly related PRs

• privkeyio/keep#686: Both changes propagate secure-randomness failures instead of assuming RNG calls always succeed.

Poem

A rabbit found entropy shy,
So tickets wait and approvals deny.
Safe IDs still hop in range,
When random bytes are sound again.
\🐇 No bad luck shall pass nearby!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately captures the main change: keep-web now fails closed on RNG health-check failures instead of continuing with unsafe randomness.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch keep-web-fallible-rng

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