Let the Linux head check for updates, and seal its device key in the TPM

Two things the RPM shipped without, and the reason they arrive together is
that both are the same question — what may this platform ask of the machine,
and what must it not ask of the user.

◆ UPDATES: THIS CLIENT CHECKS AND FETCHES, AND PACKAGEKIT INSTALLS.

ADR 0013 rule 11 said there was no updater at all, and that went too far. It
conflated noticing that a newer build exists with installing it: the second is
what the trust argument is about, and the first is not — a client that checks
a fixed address and tells you what it found has taken no capability from
anybody. Written as a flat refusal, the rule bought its safety by paying a cost
that was avoidable, and the ADR now says so in its own words.

So LinuxUpdateChannel reads the rolling nightly release, compares versions the
way rpm compares them, downloads the package, verifies its digest and hands the
file to PackageKit over gdbus. Polkit asks; the user may say no; nothing is
torn down before the answer, because ApplyingEndsTheProcess is false and the
shell already knows what that means — it is the phone's shape, and declining an
update must not cost somebody their open shells.

Three details that are load-bearing rather than incidental:

  - The comparison is rpmvercmp and not SemVer, because rpm is what will accept
    or refuse the install. Same argument ADR 0014 makes for versionCode. The
    installed side is read out of the rpm database rather than derived from the
    assembly, so the SemVer-to-rpm translation exists in exactly one place. All
    28 expected values in RpmVersionTests came out of rpm itself rather than out
    of a reading of the algorithm.

  - The digest is checked and is not a signature. Velopack verifies its own
    downloads and Android checks a signature; an RPM over HTTPS has neither. The
    manifest names a SHA-256 and a mismatch deletes the file rather than leaving
    something installable-by-hand in the profile directory.

  - ONLY_TRUSTED is cleared, which is what routes the request to polkit's
    package-install-untrusted action. The package is unsigned, so requiring
    trust would fail every update at the last step.

◆ THE DEVICE KEY: SEALED IN THE TPM, BEHIND A PIN THE TPM CHECKS.

ADR 0007's amendment is the bar: a gate inside the process is not a gate. The
key is sealed into a TPM 2.0 keyedhash object whose userAuth comes from a PIN,
so unsealing is a command the chip refuses; the blob is encrypted to the storage
hierarchy and is useless on any other machine; nothing here ever compares a PIN.

The sealed object deliberately carries no NoDA attribute, and that one omission
is the whole reason four digits is enough: every wrong answer advances the TPM's
dictionary-attack counter and the chip stops answering entirely once it trips.
Measured at four wrong tries against a software TPM, after which the correct PIN
gets nothing either. It is asserted by a test rather than left to a reading of
the template, because removing the attribute leaves every other assertion
passing.

This is weaker than Hello and Touch ID and the difference is invisible, so it is
written down in three places rather than smoothed over: those raise their prompt
from inside the platform and this one is a window we draw. The prompt is
deliberately plain for that reason — teaching people that a convincing DodoSSH
PIN box is a system dialog is the one thing this design must not do.

Rejected, each for failing the amendment: the Secret Service API, which has no
per-use consent and is DPAPI's shape exactly; fprintd, which answers a question
and holds nothing, so skipping the call skips the gate; and a polkit-gated root
helper, which would work and puts a privileged component in a package that
installs nothing setuid.

Availability is narrower than "has a TPM": on Fedora /dev/tpmrm0 is root:tss and
desktop users are not in tss, so the ordinary case reports unavailable and keeps
asking for the passphrase. Shipping a udev rule to widen that is refused — it is
the administrator's decision, not an application package's.

◆ WHAT IS ACTUALLY VERIFIED, SINCE THAT IS THE PART THAT USUALLY ISN'T.

This is the only device key store in the repository with real coverage, and it
is only possible because the PIN is a delegate and the TPM is a device: seal,
unseal, wrong PIN, lockout, decline and a foreign blob all run unattended
against a software TPM. Windows and macOS cannot be tested at all — their
prompts block a suite forever, which WindowsDeviceKeyStoreTests records.

TSS.Net's own TcpTpmDevice cannot reach swtpm: it opens the platform socket too
and exchanges a handshake only the Microsoft reference simulator answers, so
Connect() blocks with no error and no timeout. SoftwareTpm speaks the data
channel and nothing else.

The suite also lied once and was fixed: the lockout test trips shared state, and
without a DictionaryAttackLockReset in Dispose every later test got null from a
correct PIN — six of them assert null and passed for entirely the wrong reason.

End to end on a real Fedora 44: a genuine linux-x64 publish packages, installs,
and every DT_NEEDED in the installed tree resolves except liblttng-ust, which is
the one this spec deliberately excludes. What no runner covers is /dev/tpmrm0,
PackageKit and polkit, and Phase 19 grew five checks for exactly those.

Also fixed: the magic-bytes literal picked up raw control characters, which made
LinuxDeviceKeyStore.cs binary to git — no diffs and no line-ending normalisation.

src/DodoSSH.Client.App/Platform/WindowsDeviceKeyStore.cs

@@ -9,17 +9,26 @@ namespace DodoSSH.Client.App.Platform;
 /// </summary>
 /// <remarks>
 /// <para>
-/// One place decides, so nothing above has to carry a platform guard. A machine with no secure hardware,
-/// or one that is neither Windows nor macOS, gets <see cref="UnavailableDeviceKeyStore"/> and therefore
-/// keeps asking for the passphrase — which is the honest answer rather than a degraded one.
+/// One place decides, so nothing above has to carry a platform guard. A machine with no secure hardware
+/// gets <see cref="UnavailableDeviceKeyStore"/> and therefore keeps asking for the passphrase — which is
+/// the honest answer rather than a degraded one.
 /// </para>
 /// <para>
-/// <b>Both real stores are asked whether they work rather than told that they do.</b> Each
-/// <c>IsSupported</c> probes by doing the thing — creating a throwaway key and deleting it — because on
-/// both platforms the provider is present and reports itself present on machines where creating a key
-/// fails: a Windows box with no usable TPM, a Mac with no Secure Enclave, and on macOS also every
-/// unsigned development build, since enclave keys need a signing identity. Inferring from the OS would
-/// mean each of those discovering the truth at the moment somebody tried to unlock.
+/// <b>Every real store is asked whether it works rather than told that it does.</b> Each
+/// <c>IsSupported</c> probes by doing the thing — creating a throwaway key and discarding it — because on
+/// all three platforms the provider is present and reports itself present on machines where creating a key
+/// fails: a Windows box with no usable TPM, a Mac with no Secure Enclave, on macOS also every unsigned
+/// development build since enclave keys need a signing identity, and on Linux the ordinary case of a
+/// working TPM whose device node this user may not open. Inferring from the OS would mean each of those
+/// discovering the truth at the moment somebody tried to unlock.
+/// </para>
+/// <para>
+/// <b>The three are not equally strong, and the ordering here does not imply they are.</b> Windows and
+/// macOS put the consent gesture inside the platform — a secure desktop and a system Touch ID sheet — so
+/// the process asking never sees the secret. The Linux store's PIN is typed into a window this
+/// application draws, and only the <em>checking</em> is the TPM's. That is a real difference, it is
+/// recorded in ADR 0007 rather than smoothed over, and it is why <see cref="LinuxDeviceKeyStore"/> is
+/// tried last rather than first among platforms that could in principle overlap.
 /// </para>
 /// <para>
 /// <b>"Desktop", because the choice belongs to a head rather than to the session layer.</b> This file used
@@ -33,9 +42,20 @@ namespace DodoSSH.Client.App.Platform;
 public static class DesktopDeviceKeyStores
 {
     /// <summary>The best store this machine supports.</summary>
-    public static IDeviceKeyStore ForThisMachine(ClientPaths paths)
+    /// <param name="paths">Where a stored key lives.</param>
+    /// <param name="askForPin">
+    /// Shows the Linux PIN prompt, and is never called on the other two platforms — there the platform
+    /// raises its own dialog and this process is not in the conversation. A caller with no way to draw one
+    /// may pass a delegate answering null, which makes the Linux store decline every unlock; that is the
+    /// right shape for a headless host and the wrong one for the application, which supplies a real dialog
+    /// in <c>App.axaml.cs</c>.
+    /// </param>
+    public static IDeviceKeyStore ForThisMachine(
+        ClientPaths paths,
+        Func<DevicePinPurpose, CancellationToken, ValueTask<string?>> askForPin)
     {
         ArgumentNullException.ThrowIfNull(paths);
+        ArgumentNullException.ThrowIfNull(askForPin);
 
         if (OperatingSystem.IsWindows() && WindowsDeviceKeyStore.IsSupported())
         {
@@ -47,6 +67,11 @@ public static class DesktopDeviceKeyStores
             return new MacDeviceKeyStore(paths);
         }
 
+        if (OperatingSystem.IsLinux() && LinuxDeviceKeyStore.IsSupported())
+        {
+            return new LinuxDeviceKeyStore(paths, askForPin);
+        }
+
         return new UnavailableDeviceKeyStore();
     }
 }