Check S/MIME signatures, and remember who signed

A signed message now says whether that holds up, as it is read. This is
verification only: nothing here signs, encrypts or decrypts, and the
private-key question that blocks those is untouched. Verifying needed
none of it, because the certificate travels inside the message -- which
is why this is the half that could be built.

What it checks. For multipart/signed carrying PKCS#7, the exact bytes of
the signed part -- headers included, canonicalised to CRLF -- are hashed
against the messageDigest attribute, and the signature over the signed
attributes is verified with WebCrypto against the certificate inside the
message. RSA PKCS#1 v1.5 and ECDSA over P-256/384/521, with SHA-256, 384
or 512.

The trust model is the design, and it is deliberately small. A browser
has no system trust store, and the certificate arrives inside the
message, so anyone can self-sign as anyone: on its own a good signature
shows only that the sender held the key they attached. So the word
"verified" is never rendered, and the reassuring case is not the loud
one. What carries the weight is remembering -- the first signed message
from an address pins its fingerprint, later ones are compared, and a
signer that changed is reported with both names and told to check by
another route. Trust on first use, no certificate authority anywhere.

The pins live in the account's settings rather than the browser: one
that only a single device knew would greet the same correspondent as new
everywhere else, which is how people are trained to click past the one
warning that matters. A pin records the message that created it, so the
message that established a signer keeps saying so instead of appearing
to be corroborated by itself -- without that, the very first signed
message anybody receives reads as "the same signer as before", where
before is itself. A changed, mismatched or expired signer is never
pinned, since writing the anomaly into the baseline makes every later
message agree with it.

Three things are declined rather than attempted, and all three say
"could not check" rather than "does not check out", because ignorance
and an accusation are different claims:

  - OpenPGP, by name. The signature carries no key and there is nowhere
    to get the sender's: x:PublicKey is the account's OWN registry, and
    a keyserver or WKD lookup would tell a third party who you
    correspond with -- the leak the image proxy exists to close.
  - SHA-1. Not forgeable in practice today, still not something to put a
    tick beside.
  - RSA-PSS, whose salt length lives in parameters this does not read.
    Guessing wrong would report a good signature as bad.

Nothing validates a chain: no CA bundle is shipped and revocation is not
checked. "Issued by" reports what the certificate claims, and a
self-signed one claims itself.

The DER, CMS, X.509 and MIME readers are hand-written and deliberately
narrow -- no new dependency, and the whole verifier is a lazily imported
8.6 kB chunk that a reader of unsigned mail never downloads. The one
place this is easy to get quietly wrong has its own function and its own
test: signed attributes are signed as a SET OF, not as the [0] IMPLICIT
they arrive as, and hashing the message instead would make every
signature "pass".

Tested against real `openssl smime -sign` output rather than hand-built
fixtures -- RSA, ECDSA, a tampered copy, and a valid signature by a
certificate for somebody else -- because a signed message written by
hand only agrees with whatever its author believed the format to be.
Also driven in a browser against the mock, which now serves three real
signed messages so every branch of the banner is reachable.

Translations: 34 new strings in all nine catalogues, 306 entries.
Falling back to English is unchanged at 24 per language.

ROADMAP.md

@@ -15,13 +15,15 @@ See [KNOWN-ISSUES.md](KNOWN-ISSUES.md) for what is built but worth knowing about
 - **A translation anybody has checked.** The translations themselves shipped on 2026-08-31 and are no longer on this page: nine of them, alongside English, and the extraction that had always been the hard half is done — see [FEATURES.md](FEATURES.md#interface-language). What is *not* done is the other half, and it is the half that cannot be bought or automated. All nine were produced by AI against standard dictionaries and **not one has been read by anybody who speaks the language**, which is exactly where a bad translation does harm rather than merely looking untidy. They ship marked Beta, with that said in Settings and a link for reporting anything wrong, because shipping them quietly would ask people to trust text nobody has checked. A language loses the Beta mark when a speaker reads it and says so — a deliberate act by a person, not something a coverage percentage earns. If you speak one of them and are willing to read a few hundred strings, that is the single most useful thing anyone could contribute right now.
 - **Right-to-left languages.** Arabic, Hebrew and Persian are held back deliberately, and not for want of translators. RTL is bidi and layout work throughout — mirrored panes, gesture directions, icon sides, the message list's own geometry — and a catalogue without it produces a page that is translated and unusable. Adding one is not another entry in the picker.
 - **Two-factor sign-in.** Today an account with 2FA must use an app password (see [Quick start](README.md#quick-start-docker)), and Settings › Security offers no way to switch 2FA *on* — only off, for an account that already has it. Supporting a TOTP code directly means implementing OAuth: Stalwart offers the authorization-code and device flows and no password grant, so ihasmail would hand sign-in to Stalwart's own login and come back with a token. That is a better security posture than the sealed password it holds now — a refresh token rather than a credential — but it replaces ihasmail's own sign-in page for those users and may need an OAuth client registered. Came out of [#75](https://github.com/Coffey-Labs/ihasmail/issues/75), which is closed: what was reported there was a sign-in refused with nothing but "Invalid credentials", and that was fixed by saying what is actually happening and pointing at app passwords. The OAuth work it uncovered is tracked here rather than as an open issue, so there is no ticket to watch for it.
-- **Signed and encrypted mail — and, for now, managing the keys for it.** Nothing signs, encrypts, decrypts or verifies, and the reason is not Stalwart. S/MIME and OpenPGP are both work the *client* does over the message body: JMAP hands it the MIME blob and the rest is ours.
+- **Signing and encrypting mail.** *Reading* a signature is built: S/MIME signed mail is checked as it is read, and the signer is remembered so a change is called out — see [Checking a signature](FEATURES.md#checking-a-signature). What is not built is anything that produces a signature or touches ciphertext, and the reason is not Stalwart. This is client work over the message body: JMAP hands over the MIME blob and the rest is ours.
 
-  The blocker is a security model, not code. Signing and decrypting need a private key in a page served by the same host that would handle it, which runs straight into two things ihasmail says about itself: that it never stores a credential, and that it runs immutably with nowhere to keep one. That question has to be answered out loud before it is a feature to build, and it has not been. **Verifying a signature is the exception** — it needs only public keys, touches none of this, and is the one piece that could make a key worth having.
+  The blocker is a security model, not code, and it is the same one it has always been. Signing and decrypting need a **private** key in a page served by the same host that would handle it, which runs straight into two things ihasmail says about itself: that it never stores a credential, and that it runs immutably with nowhere to keep one. Verifying needed none of that — the certificate travels inside the message — which is exactly why it could be built first and why it went first.
 
-  *Managing* keys is a smaller thing and has been built twice — [PR #67](https://github.com/Coffey-Labs/ihasmail/pull/67) and [PR #285](https://github.com/Coffey-Labs/ihasmail/pull/285) — and withdrawn twice, on the same reasoning both times: a Settings section for keys that nothing uses is furniture, not a feature, and it ends up saying so in its own footnote. Stalwart's `x:PublicKey` registry is not the obstacle. It works, it holds OpenPGP keys and S/MIME certificates alike, and it is fully probed — see [KNOWN-ISSUES.md](KNOWN-ISSUES.md), which is where those findings live precisely so that the third attempt does not have to rediscover them. Key management ships when something consumes a key, and verifying an incoming signature is the shortest route to that.
+  **OpenPGP signatures are not checked, and this is a harder problem than it looks.** A PGP signature does not carry the key, so verifying one means having the sender's public key already. ihasmail has no source for it: `x:PublicKey` is the account's *own* registry, and fetching from a keyserver or WKD would tell a third party who you correspond with, which is precisely the leak the image proxy exists to close. A local store of correspondents' keys is possible and is not a small feature; nobody has asked for it yet.
 
-  **Encryption at rest is not the route, and is refused rather than deferred.** Stalwart offers it as `encryptionAtRest`, a field on `x:AccountSettings` beside `description`, `locale` and `timeZone` — there is no `x:EncryptionAtRest` object whatever the docs suggest, and its value is a typed object (`{"@type": "Disabled"}`) rather than a bare string. It is self-service, needs no administrator, and would be easy to offer. It will not be: turning it *off does not decrypt what is already there*. Every message delivered while it was on stays encrypted on disk, readable only by a client holding the private key, so switching it on is a one-way door — and a toggle that reads as "make my mail safer" while quietly being irreversible is the wrong thing to hand an ordinary user. That is a property of Stalwart rather than a policy choice, which is exactly why the answer here is a policy choice.
+  *Managing* keys — publishing your own to `x:PublicKey` — has been built twice ([PR #67](https://github.com/Coffey-Labs/ihasmail/pull/67), [PR #285](https://github.com/Coffey-Labs/ihasmail/pull/285)) and withdrawn twice, because a Settings page for keys nothing uses is furniture. That reasoning is now partly spent: something does use a key. But what signature checking uses is the certificate inside the message, not anything in the registry, so publishing your own key remains a feature waiting for a consumer.
+
+  **Encryption at rest is refused rather than deferred.** Stalwart offers it as `encryptionAtRest`, a field on `x:AccountSettings` beside `description`, `locale` and `timeZone` — there is no `x:EncryptionAtRest` object whatever the docs suggest, and its value is a typed object (`{"@type": "Disabled"}`) rather than a bare string. It is self-service, needs no administrator, and would be easy to offer. It will not be: turning it *off does not decrypt what is already there*. Every message delivered while it was on stays encrypted on disk, readable only by a client holding the private key, so switching it on is a one-way door — and a toggle that reads as "make my mail safer" while quietly being irreversible is the wrong thing to hand an ordinary user.
 
   **Why S/MIME rather than OpenPGP, and why neither is urgent.** End-to-end encrypted mail never reached the mainstream and is not on its way there: as a share of the world's email, PGP-encrypted messages are a rounding error, and the most successful use of OpenPGP is signing packages rather than sending mail. The reasons are structural rather than a matter of better tooling. Everyone in a thread has to take part, so the network effect works against it from the first reply. Key discovery was never solved — keyservers were unauthenticated and got weaponised in the 2019 certificate-flooding attacks, which made specific people's keys unusable by any client that fetched them, and WKD is better without being universal. There is no forward secrecy, so one compromised key retroactively opens everything ever received. The metadata stays in the clear: subject lines are cleartext in classic PGP/MIME, and who corresponded with whom is often the sensitive part. Losing a key loses the mail permanently. And it breaks the client — no server-side search, degraded spam filtering, awkward on a phone — while EFAIL showed in 2018 that the clients themselves were exploitable through MIME and HTML handling. Meanwhile the actual privacy win arrived invisibly and without anyone participating, in STARTTLS, MTA-STS and DANE.