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.

docs/adr/0007-device-key-protection.md

@@ -107,6 +107,55 @@ almost nil. Two things to check when it is taken up: `EnrollmentValidation` pins
 `DevicePublicKey` to `CryptoSpec.PublicKeySize` (32 bytes; a P-256 public key is 33 or 65), and the
 envelope's minimum-length rule.
 
+### 4. Linux: sealed into the TPM behind a PIN the TPM checks
+
+Added after the Linux head was packaged ([ADR 0013](0013-desktop-distribution-and-updates.md) decision 11).
+The third desktop platform reached this file the way the first two did — by having to answer the amendment
+above — and it is the only one where the answer is not a clean yes.
+
+**The mechanism.** The 32-byte device key is sealed into a TPM 2.0 keyedhash object whose `userAuth` is
+derived from a PIN. Unsealing is a command the TPM refuses without it. The blob on disk is encrypted to the
+machine's storage hierarchy, so it is useless on any other machine, and nothing in this application ever
+compares the PIN. The parent is an ECC P-256 primary rederived from a fixed template on every use rather
+than made persistent — a persistent handle would occupy an NV slot forever, need cleaning up on withdrawal,
+and collide with whatever else on the machine chose the same index. ECC rather than RSA because creating an
+RSA primary on a discrete TPM takes seconds and this runs on the unlock path.
+
+**The sealed object deliberately does not carry `NoDA`, and that single attribute is what makes a
+four-digit PIN defensible.** Every wrong answer advances the TPM's dictionary-attack counter and the chip
+stops answering entirely once it trips — measured against a software TPM at four wrong tries, after which
+the *correct* PIN gets nothing either. That is the property a short secret has here and cannot have in a
+file, and it is asserted by a test rather than left to a reading of the template, because removing the
+attribute would leave every other assertion in that file passing.
+
+◆ **This is weaker than Windows Hello and Touch ID, and the difference is invisible to the user.** Both of
+those raise their prompt from inside the platform — a secure desktop, a system sheet — so the process asking
+never sees the secret. Linux offers nothing to borrow, so the PIN is typed into a window this application
+draws, and an attacker already running as the user could draw a convincing copy of it. Two things bound
+that, and neither erases it: the same attacker could counterfeit the passphrase screen, which is a much
+larger prize, so the feature creates no capability that did not exist; and what they would obtain is a PIN
+that is worthless without this machine's TPM, where the passphrase is worth every machine. **The parity is
+not claimed anywhere in the interface**, which is why `DesktopDeviceKeyStores` documents the ordering and
+why the prompt is deliberately plain rather than dressed to look like a system dialog — teaching people
+that a convincing DodoSSH PIN box is a system dialog is the one thing this design must not do.
+
+**Availability is narrower than "the machine has a TPM", and that is a deployment fact rather than a bug.**
+On Fedora `/dev/tpmrm0` is `root:tss` mode 0660 and desktop users are not in `tss`, so the ordinary case is
+a working TPM this process may not open. That reports unavailable and the passphrase path continues,
+exactly as a machine with no TPM does. **Shipping a udev rule to widen it is refused**: granting every local
+desktop user access to the machine's TPM is a system-wide security decision, some distributions restrict it
+deliberately, and an application package has no business making it on the administrator's behalf. One line
+of `usermod` is the answer, and the README says so.
+
+**What was rejected, and why each would have failed the amendment above.** The Secret Service API —
+gnome-keyring or KWallet — is out of process and OS-managed and has *no per-use consent*: once the session
+keyring is unlocked anything running as the user reads the key silently, which is DPAPI's shape exactly and
+what "Why not DPAPI alone" already refuses. `fprintd` is an authentication oracle rather than a key store:
+it answers a question and holds nothing, so skipping the call skips the gate — the amendment's mistake with
+a fingerprint reader attached. A polkit-gated root helper would genuinely work and was declined on surface
+area: it puts a privileged component in a package that installs nothing setuid, to reach a property the TPM
+already provides.
+
 ## Consequences
 
 ### The cache key had to move, and the spec changed
@@ -138,9 +187,11 @@ would have become false under DPAPI alone. A gesture is still something the atta
 
 ### Operational
 
-- **A TPM is not always there.** A machine without one gets a store that reports itself unavailable, so
-  unlock keeps asking for the passphrase and neither affordance appears in the interface. The passphrase path
-  is therefore required, not a nicety.
+- **A TPM is not always there, and on Linux it is often there and out of reach.** A machine without one gets
+  a store that reports itself unavailable, so unlock keeps asking for the passphrase and neither affordance
+  appears in the interface. The passphrase path is therefore required, not a nicety — and on Linux it is the
+  default rather than the exception, because the device node is not readable by an ordinary desktop user
+  until somebody says it should be. See decision 4.
 - **macOS reaches the same decision through different hardware, and the argument transfers intact.**
   `MacDeviceKeyStore` puts the wrapping key in the Secure Enclave under an access control requiring user
   presence, so Touch ID or the login password is a condition of *using* it and the enforcement is the