## Summary
Removes the last `APP_SECRET`-derived at-rest encryption site by
migrating `core.twoFactorAuthenticationMethod.secret` from
`SimpleSecretEncryptionUtil` (AES-256-CBC with key derived from
`sha256(APP_SECRET + userId + workspaceId + 'otp-secret' +
'KEY_ENCRYPTION_KEY')`) to the versioned `enc:v2:` envelope
(ENCRYPTION_KEY → HKDF-SHA256 bound to `workspaceId` → AES-256-GCM).
- New `decrypt-legacy-aes-cbc.util.ts` faithfully reproduces the
pre-migration CBC derivation byte-for-byte;
`SecretEncryptionService.decryptVersioned` dispatches to it when callers
pass `legacyAesCbcPurpose`, with a dedicated one-shot WARN log family.
- `TwoFactorAuthenticationService` now uses `encryptVersioned` /
`decryptVersioned` (passing the legacy purpose so existing rows still
decrypt). `SimpleSecretEncryptionUtil` and its spec are deleted;
`TwoFactorAuthenticationModule` imports `SecretEncryptionModule` in
their place.
- `TwoFactorAuthenticationMethodEntity` gets a `@Check` decorator
(`CHK_twoFactorAuthenticationMethod_secret_encrypted`) restricting
`secret` to the `enc:v2:` envelope; the matching 2.5 slow instance
command (`1798000009000-encrypt-totp-secrets`) cursor-paginates `JOIN`ed
`userWorkspace` rows to recover the legacy `userId`, re-encrypts to
`enc:v2`, and applies the CHECK constraint in `up()`.
### Deviation note
The plan suggested wiring a workspace-only legacy derivation directly
into `decryptVersioned`. In practice the production rows are
user-and-workspace-scoped (the legacy purpose is
`\${userId}\${workspaceId}otp-secret`), so a workspace-only derivation
could not recover them. The PR keeps the public `decryptVersioned` API
intact and adds an optional `legacyAesCbcPurpose` so callers that can
reconstruct the legacy context (the 2FA service and the slow command)
opt in.
### Final state of remaining `APP_SECRET` usages
- HS256 JWT verify (read-only, self-retiring once asymmetric migration
completes).
- Express-session cookie signing.
- Approved-access-domain HMAC (signing root, not at-rest).
- Zero-friction fallback in `resolveEncryptionKeysOrThrow`
(intentional).
No production at-rest data is encrypted with `APP_SECRET`-derived keys
anymore.
## Test plan
- [x] `npx jest src/engine/core-modules/secret-encryption
src/engine/core-modules/two-factor-authentication` — 170 unit tests
pass, including new unit tests for the legacy CBC util and the new
`SecretEncryptionService` fallback branch.
- [x] `npx jest --config ./jest-integration.config.ts
test/integration/upgrade/suites/2-5-instance-command-slow-1798000009000-encrypt-totp-secrets.integration-spec.ts`
— 4 integration tests cover legacy-CBC seed → slow command → `enc:v2`
round-trip, idempotency, CHECK constraint enforcement on `up()`, and
rollback via `down()`.
- [x] `npx oxlint --type-aware` and `npx prettier --check` clean on all
touched files.
- [ ] CI on this PR (server validation, tests, lint, typecheck).
