security(smime): fix audit finding 2 — unauthenticated CBC on decrypt

Upstream applied no content-encryption check at all on decrypt, and ran
every decryption through the liner engine — which registers DES-CBC,
3DES-CBC and RC2-CBC. Those OIDs exist in crypto-engine.js for PKCS#12
password-based encryption; the CMS content path merely reused the same
engine and inherited them. A crafted message could therefore be decrypted
under a broken cipher, and unauthenticated plaintext was handed straight
to the renderer — the EFAIL precondition.

The obvious fix would have been wrong. Accepting only AEAD breaks most
real S/MIME mail: RFC 5751 makes AES-128-CBC the MUST-implement content
cipher, Outlook and Thunderbird default to CBC, and AES-GCM in CMS
(RFC 5084) is barely deployed. An AEAD-only allowlist is a functionality
catastrophe wearing a security fix's clothes.

Three layers instead:

1. Allowlist the AES family and refuse everything else, with the gate
   running before any private key is touched. CBC stays for interop;
   DES/3DES/RC2 are refused.

2. Take the mail path off the legacy engine. Normal decryption now uses
   nativeEngine(); the liner engine is reachable only when a genuine
   legacy RSAES-PKCS1-v1_5 key is in play. This removes the weak ciphers
   structurally rather than by policy — native WebCrypto handles RSA-OAEP
   key transport and AES-CBC/GCM content perfectly well.

3. Refuse to render unauthenticated plaintext as HTML. CBC output is
   malleable and HTML is EFAIL's exfiltration channel. The host does block
   remote content by default (allowExternalContent starts false), but that
   is a user/admin setting this plugin cannot observe, so we don't lean on
   it. New renderUnauthenticatedHtml setting (default false) is the
   documented opt-out. Our own encrypt path always uses AES-GCM, so mail
   we send renders fully; only legacy inbound CBC degrades to text.

Built from source with the repo's own pipeline (esbuild, 1.69 MB) and
packaged to smime-vnc.zip (0.27 MB). All four fixes verified present in
the built bundle. Build output is gitignored — never vendor a prebuilt
bundle, which was the upstream mistake.

Correcting an earlier assumption: this bundle does NOT trip the B-01
pattern scanner (zero matches on all five patterns), so the override is
not needed to install it. B-01 remains correct — it closed a real
entrypoint-only coverage gap — but it isn't load-bearing here.

verify-fixes.mjs now carries 36 assertions covering all three fixes,
including source checks that fail if a guard is removed, if a legacy CBC
OID reappears in the allowlist, or if the mail path stops using the
native engine.

Findings 4 (unlocked keys persisted to IndexedDB), 5 (parser DoS) and 6
(PKCS1v1.5 oracle surface) remain open.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

vnc/plugins/smime/manifest.json

@@ -44,6 +44,12 @@
       "description": "Wipe all unlocked private keys from memory when you sign out or switch accounts. Leave on unless you have a specific reason not to.",
       "default": true
     },
+    "renderUnauthenticatedHtml": {
+      "type": "boolean",
+      "label": "Render HTML in legacy-encrypted mail",
+      "description": "Messages encrypted with AES-CBC carry no integrity protection, so their contents can be tampered with in transit. By default such mail is shown as plain text, which prevents a known attack that can leak the decrypted message. Turn this on only if you need HTML rendering for older encrypted mail and accept that risk. Mail encrypted with AES-GCM is unaffected and always renders fully.",
+      "default": false
+    },
     "warnOnSelfSigned": {
       "type": "boolean",
       "label": "Warn on self-signed signer",