## Summary
Continues retiring `APP_SECRET` as a hot signing secret (after the TOTP
migration in #20577). This PR moves the last two cryptographic uses of
`APP_SECRET` off it:
1. **Approved-access-domain validation tokens** — was a one-shot
`sha256(JSON.stringify({id, domain, key: APP_SECRET}))` HMAC with no
built-in expiry. Now a JWT signed by the workspace `signingKey` with a
7-day expiry and claims bound to `approvedAccessDomainId`,
`workspaceId`, and `domain`.
2. **Express-session cookie signing** — was `sha256(APP_SECRET ||
'SESSION_STORE_SECRET')`. Now `HKDF(ENCRYPTION_KEY,
info='twenty:hmac:v1:session-cookie')` with `FALLBACK_ENCRYPTION_KEY`
supported for rotation.
### Approved-access-domain — strict cutover
- `ApprovedAccessDomainService.mintValidationToken` issues a JWT via
`JwtWrapperService.signAsyncOrThrow` (workspace `signingKey`, asymmetric
ES256 with kid-based rotation built in).
- `validateApprovedAccessDomain` verifies the JWT, asserts `type ===
APPROVED_ACCESS_DOMAIN`, cross-checks `claim.approvedAccessDomainId`
against the URL's `approvedAccessDomainId`, then re-checks `domain` and
`workspaceId` against the stored row. Any failure maps to
`APPROVED_ACCESS_DOMAIN_VALIDATION_TOKEN_INVALID`.
- **No legacy fallback:** any pending invitation link minted with the
old SHA hash will fail validation and must be re-sent. Volume is small
and admins can re-issue from settings — this is the cleanest cutover.
### Session cookies — bridged cutover
- `resolveSessionCookieSecretsOrThrow` returns an array
`[HKDF(ENCRYPTION_KEY), HKDF(FALLBACK_ENCRYPTION_KEY)?,
sha256(APP_SECRET || 'SESSION_STORE_SECRET')?]`.
- `express-session` signs new cookies with the first secret and verifies
against any entry, so in-flight cookies signed under the legacy SHA keep
verifying until `maxAge` (30 min) expires.
- New `deriveInstanceHmacKey` HKDF utility uses a dedicated
`twenty:hmac:v1:` info prefix — distinct from the AEAD subkey prefix
`twenty:enc:v2:` — so HMAC and encryption subkeys can never collide for
the same raw `ENCRYPTION_KEY`.
- TODO comment marks the legacy slot for removal post-2.5.
### Notes on rotation behaviour
- Rotating `ENCRYPTION_KEY` while keeping the old value in
`FALLBACK_ENCRYPTION_KEY` keeps cookies signed under either key
verifying. New cookies sign under the new key. After all in-flight
cookies expire (≤30 min), the fallback slot can be dropped from env.
- Rotating the workspace `signingKey` (already supported by
`JwtKeyManagerService`) keeps already-issued approved-access-domain JWTs
verifying via `kid` until their 7-day expiry.
## Test plan
- [x] Unit tests for `ApprovedAccessDomainService` cover: happy path,
JWT verify failure, wrong token type, JWT id ≠ input id, JWT-claimed
domain ≠ row, missing row, already-validated row.
- [x] Unit tests for `resolveSessionCookieSecretsOrThrow` cover: throws
without keys, primary order (`ENCRYPTION_KEY` → APP_SECRET fallback),
`FALLBACK_ENCRYPTION_KEY` placement, empty-string vars treated as unset,
legacy slot omitted when `APP_SECRET` missing, HKDF domain separation
across purposes.
- [x] `nx lint:diff-with-main twenty-server` — clean.
- [x] Full test surface across approved-access-domain,
secret-encryption, session-storage — 78/78 pass.
- [ ] CI green.
- [ ] Manual smoke: boot with a dummy `ENCRYPTION_KEY`, confirm sign-in
succeeds (session cookie works), create + validate an
approved-access-domain end-to-end through the UI.
