feat: move the SIWE challenge below the facade

[FSM1]

Problem

facade.siweLogin(message, signature) existed, but nothing exposed the challenge an EIP-4361
message must embed. The engine's API client already had siwe_challenge() against
POST /auth/siwe/challenge; it was unreachable from the facade.

So #804 left apps/web/src/auth/siweNonce.ts doing a plain fetch against the API's public
challenge endpoint. That was the only direct API call in apps/web, and it contradicted
blueprint/web-client.md — the app's only vault-facing dependency is packages/client.

Change

• Engine. Engine::siwe_challenge over the existing ApiClient::siwe_challenge, returning the
  nonce. ApiClient::siwe_challenge now validates it against EIP-4361's alphanumeric class before
  constructing a SiweNonce: the nonce lands verbatim in the text a wallet signs, so anything
  outside that class would let a hostile challenge response splice extra fields into the signed
  message. That fail-closed check is what moved with the code out of the deleted siweNonce.ts.
• Threading. The crates/wasm binding, then packages/client: worker/protocol.ts request +
  result union, worker/engineWasm.ts, worker/engineHost.ts, worker/serve.ts, transport.ts,
  correlatedTransport.ts, broadcast.ts WireRead, broadcastTransport.ts, leaderRelay.ts,
  engineClient.ts, facade.ts. The nonce crosses as a plain string, so no new descriptor type is
  minted.
• Web. apps/web/src/auth/siweNonce.ts and its test are deleted. WalletLoginButton takes a
  requestNonce prop fed by useAuth().siweChallenge() and no longer takes apiBaseUrl.
• leaderRelay's read dispatch becomes a switch behind an annotated return type, so a fourth
  WireRead kind fails to compile instead of leaving a follower's request unanswered.
• The API pins the nonce class it issues, and the contract suite's !nonce.is_empty() becomes the
  class assertion it was standing in for. A decode-side hard reject needs its producer pinned at the
  producer, or a change of alphabet breaks every wallet login with an opaque decode error visible
  only to the live-stack suite.

Settling the issue's Note

The issue asked whether the wallet method belongs on the cold login page at all. It does not.
Command::SiweLogin is refused with NotStarted before facade.start and cannot be otherwise —
SIWE authenticates the account against the API and cannot produce the Core Kit login secret a vault
cold start needs; blueprint/engine.md and blueprint/web-client.md both call it a secondary
method. So siweChallenge is gated identically, and the refusal now lands before the wallet
signature prompt instead of after it.

The placement half could not land here: App.tsx has only / and /files/:nodeId?, so there is no
started-engine surface to move the button to. Filed as #938.

Deferred, each with a depends-on edge back to #910

• web: give the wallet method a started-engine surface #938 — give the wallet method a started-engine surface, move the button off LoginPage, and
  correct the blueprint/web-client.md route table that still lists / as hosting SIWE. Carries the
  review's finding that useAuth's exclusion lock covers only the second half of the wallet flow.
• web: give the Http seam a request deadline #939 — give the Http seam a request deadline. The deleted siweNonce.ts carried its own
  AbortSignal.timeout(10_000); FetchHttp.send passes no signal, so every engine API call is
  unbounded, and the wasm host holds the engine read lock across the await. Not new — download has
  the same shape — but this change inherits it where an explicit bound used to exist.
• client: generalize the facade read rail #942 — generalize the packages/client read rail. siweChallenge is the third read and paid an
  11-file mechanical tax that a command does not; sequenced behind web: derive auth state from the engine instead of a tab-local store #914 deliberately.

One informational delta, no action: the challenge POST now rides FetchHttp, which sets
credentials: 'include', so it carries the HTTP-only refresh cookie where the page-level fetch
sent none. No CSRF surface — the nonce is session-unbound and side-effect-free — and the read is
gated on started, so the session is already correlated.

#914 rebase note

This shares a facade-threading surface with #914, which is not being worked yet. The threading is
deliberately minimal and mechanical: one new member per file, placed next to the existing
snapshot/download reads, no shared machinery reshaped. correlatedTransport's
request/dispatch/settle core is untouched — it is already result-type-agnostic. The one
non-additive edit is leaderRelay's read dispatch, which #914 does not touch since it adds an event,
not a read. #942 is filed to sequence the real generalization after #914.

Tests

• crates/engine/src/api/client.rs — siwe_challenge_refuses_a_nonce_outside_the_eip4361_class
  covers short, hyphenated, newline-bearing, spaced, empty, over-long, and unicode-confusable nonces
  (Arabic-Indic digits and fullwidth Latin, which char::is_alphanumeric would have accepted), and
  asserts the refusal never echoes the value it rejected. siwe_challenge_accepts_the_class_boundaries
  pins 8 and 128. Reverting is_eip4361_nonce fails both, plus the pre-existing
  siwe_challenge_returns_a_nonce_with_no_request_body, whose fixture had to become a real nonce.
• apps/api — issues a nonce inside the EIP-4361 class the engine enforces. Change the alphabet in
  challenge.service.ts and it fails.
• crates/engine/tests/facade.rs — a_siwe_challenge_before_start_is_rejected_not_started (drop the
  gate and it fails) and a_started_engine_serves_the_nonce_from_its_api_client, which asserts the
  request reaches /auth/siwe/challenge.
• packages/client — facade delegation, the serveEngine worker round-trip, and a follower
  challenge served off the leader engine. Remove the serve.ts case and the round-trip hangs to
  timeout; remove the leaderRelay arm and it fails to compile.
• apps/web — useAuth reads the nonce from the facade and never from the API. The deleted
  siweNonce.test.ts's validation assertions live on in the Rust client test above.

Verification

All exit 0 on this branch:

• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo check --workspace --all-targets
• cargo check -p cipherbox-wasm --target wasm32-unknown-unknown --all-targets
• cargo test -p cipherbox-engine -p cipherbox-wasm — 554 lib + 11 facade + the rest, 0 failed
• pnpm -r --if-present run typecheck — api, client, web all clean
• pnpm -r --if-present run test — client 114, api 178, web 64, all passed
• pnpm --filter @cipherbox/client test:browser — 19 passed
• pnpm exec eslint . — clean

Review gates run on git diff main...HEAD: /security-review (no findings), /simplify (four
angles), /crypto-privacy-review (no must-fix). Findings folded into the second commit; the rest
filed as #938 / #939 / #942.

Closes #910

Summary by CodeRabbit

• New Features
  • Wallet sign-in now retrieves SIWE challenges through the engine before creating a signature.
  • SIWE challenge requests are supported across local, worker, and broadcast connection modes.
• Bug Fixes
  • SIWE nonces are now validated against the EIP-4361 format: 8–128 ASCII alphanumeric characters.
  • Challenge requests fail safely when the engine is not started or the service returns an error.

[coderabbitai]

Walkthrough

The SIWE nonce request moved from the web API layer into the engine facade. The request now traverses WASM, worker, local, correlated, and broadcast transports before reaching wallet login. Nonce validation now enforces the EIP-4361 format.

Changes

SIWE challenge flow

Layer / File(s)	Summary
SIWE nonce validation apps/api/src/auth/services/challenge.service.test.ts, crates/contract/tests/contract.rs, crates/engine/src/api/client.rs	API and engine tests enforce 8–128 ASCII alphanumeric nonce values.
Engine facade and WASM binding crates/engine/src/facade.rs, crates/engine/tests/facade.rs, crates/wasm/src/host.rs	The engine exposes siwe_challenge(), maps API errors, and exports siweChallenge to JavaScript.
Client worker and transport plumbing packages/client/src/worker/*, packages/client/src/transport.ts, packages/client/src/correlatedTransport.ts, packages/client/src/engineClient.ts	Worker protocols, hosts, transports, and serving logic carry the challenge request and string response.
Facade and broadcast routing packages/client/src/facade.ts, packages/client/src/broadcast.ts, packages/client/src/broadcastTransport.ts, packages/client/src/leaderRelay.ts, packages/client/src/testkit.ts	Client facades expose the challenge, and follower requests route through the leader.
Web authentication integration apps/web/src/auth/useAuth.ts, apps/web/src/components/auth/WalletLoginButton.tsx, apps/web/src/routes/LoginPage.tsx, apps/web/src/auth/siweNonce.ts, apps/web/src/auth/siweNonce.test.ts	Web authentication uses the facade callback for nonce retrieval. The direct nonce module and its tests were deleted. Tests add facade challenge coverage.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant WalletLoginButton
  participant AuthHook
  participant EngineFacade
  participant ClientTransport
  participant WasmHost
  participant ApiClient
  WalletLoginButton->>AuthHook: requestNonce()
  AuthHook->>EngineFacade: siweChallenge()
  EngineFacade->>ClientTransport: siweChallenge()
  ClientTransport->>WasmHost: siweChallenge()
  WasmHost->>ApiClient: POST /auth/siwe/challenge
  ApiClient-->>WasmHost: validated nonce
  WasmHost-->>ClientTransport: nonce string
  ClientTransport-->>EngineFacade: nonce string
  EngineFacade-->>AuthHook: nonce string
  AuthHook-->>WalletLoginButton: nonce string
  WalletLoginButton->>WalletLoginButton: create SIWE message

Loading

Possibly related PRs

• FSM1/cipher-box#911: This PR replaces the direct siweNonce implementation and rewires wallet login through the engine facade.
• FSM1/cipher-box#728: This PR extends the worker and facade transport surface introduced there.
• FSM1/cipher-box#733: This PR extends the broadcast transport and leader relay protocol with SIWE challenge messages.

Suggested labels: comp:engine

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #910 by moving SIWE challenge retrieval through the engine facade, client layers, WASM, and wallet login flow.
Out of Scope Changes check	✅ Passed	The changes remain within issue #910 scope, including nonce validation and tests for the new facade-based SIWE challenge flow.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes moving SIWE challenge retrieval into the engine facade.

✨ 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 feat/910-siwe-challenge-below-facade

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/client/src/worker/engineHost.ts`:
- Around line 110-112: Update EngineHost construction in onBootstrap() to pass
config.apiBaseUrl alongside config.profile, then update the EngineHost
constructor and its EngineHandle creation to accept and forward apiBaseUrl as
api_base_url. Preserve siweChallenge() behavior while ensuring requests target
the configured origin.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 2e78cd65-0aa6-49b0-b881-854e53e85b39

📥 Commits

Reviewing files that changed from the base of the PR and between afb3887 and 8cf97b6.

📒 Files selected for processing (30)

• apps/api/src/auth/services/challenge.service.test.ts
• apps/web/src/auth/siweNonce.test.ts
• apps/web/src/auth/siweNonce.ts
• apps/web/src/auth/useAuth.test.tsx
• apps/web/src/auth/useAuth.ts
• apps/web/src/components/auth/WalletLoginButton.tsx
• apps/web/src/routes/LoginPage.tsx
• apps/web/src/test/authFakes.tsx
• crates/contract/tests/contract.rs
• crates/engine/src/api/client.rs
• crates/engine/src/facade.rs
• crates/engine/tests/facade.rs
• crates/wasm/src/host.rs
• packages/client/src/broadcast.ts
• packages/client/src/broadcastTransport.test.ts
• packages/client/src/broadcastTransport.ts
• packages/client/src/correlatedTransport.ts
• packages/client/src/engineClient.ts
• packages/client/src/facade.test.ts
• packages/client/src/facade.ts
• packages/client/src/leaderRelay.ts
• packages/client/src/testkit.ts
• packages/client/src/transport.ts
• packages/client/src/worker/engineHost.ts
• packages/client/src/worker/engineWasm.ts
• packages/client/src/worker/protocol.ts
• packages/client/src/worker/serve.test.ts
• packages/client/src/worker/serve.ts
• packages/client/test/browser/fakeEngine.worker.ts
• packages/client/test/browser/journalEngine.worker.ts

💤 Files with no reviewable changes (2)

• apps/web/src/auth/siweNonce.test.ts
• apps/web/src/auth/siweNonce.ts