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.

build/linux/dodossh.spec

@@ -11,17 +11,18 @@
 #  a newer build and swaps itself over. ADR 0013 rule 4 is what makes that safe to hold — the feed
 #  address is a const, so no operator can answer "is there a newer version" on the user's behalf.
 #
-#  This package has no updater in it at all, and that is deliberate rather than unfinished.
-#  VelopackUpdateChannel.ForThisMachine already refuses every platform that is not Windows or macOS,
-#  and it says why in a comment worth not undoing: a client handed an UpdateManager pointed at a feed
-#  carrying nothing it can apply is a client that reports itself current forever. On Linux the
-#  package manager is the updater — `dnf upgrade` is a mechanism the user already trusts, already
-#  audits and already controls — so the honest thing is to be an ordinary package and let it do that,
-#  rather than to bolt on a second one that answers to a different authority.
+#  This package replaces nothing and installs nothing, and that is deliberate rather than unfinished.
+#  On Linux the package manager is already an updater the user trusts, audits and controls, so putting
+#  a second one inside the application — answering to this project instead — would be building the
+#  capability ADR 0013 rule 4 spends a page making unsettable, on the one platform where nobody had to.
 #
-#  The consequence to know: an installed RPM learns about a new version when its repository does, and
-#  this project publishes no repository yet. Somebody who downloaded the file installs the next one
-#  the same way.
+#  What the client does do is the half a package manager cannot: LinuxUpdateChannel notices that a
+#  newer build exists on the project's forge, fetches it, checks its digest, and hands the file to
+#  PackageKit — which raises polkit and asks. The user may say no, and nothing is replaced until they
+#  say yes. See ADR 0013 rule 11.
+#
+#  The consequence to know: discovery is worse here than on the other two desktops, because there is
+#  no package repository to subscribe to and the in-app check is what stands in for one.
 # ==================================================================================================
 
 # ◆ THE THREE THINGS THAT MUST BE TURNED OFF, AND WHAT EACH ONE WOULD DO.
@@ -150,13 +151,28 @@ Requires:       libicu
 # again when the last thing using them goes.
 Requires:       hicolor-icon-theme
 
+# ◆ RECOMMENDED RATHER THAN REQUIRED, WHICH IS A JUDGEMENT ABOUT WHAT THE PACKAGE IS FOR.
+#
+# PackageKit is what LinuxUpdateChannel hands a downloaded update to, so without it the update path
+# stops at the last step. It is not required, for two reasons that both point the same way: dnf
+# installs weak dependencies by default, so the ordinary Fedora install gets it anyway; and a machine
+# deliberately built without it — a minimal or an immutable one — should still be able to install an
+# SSH client. The failure is not silent, which is what makes this safe: the channel names the missing
+# service and the file it fetched, so the answer is one command the user was going to run anyway.
+#
+# Nothing is required for the TPM. LinuxDeviceKeyStore talks to /dev/tpmrm0 through a managed stack,
+# so there is no library to depend on, and a machine with no TPM — or one whose device node this user
+# may not open, which is the ordinary Fedora case — simply keeps asking for the passphrase.
+Recommends:     PackageKit
+
 %description
 DodoSSH is an SSH and SFTP client for teams whose credentials are shared and whose infrastructure is
 not. Hosts, keys and passphrases live in a vault that is encrypted on this machine and stays
 encrypted on the server, so the deployment holding a team's secrets can never read them.
 
-This package carries its own .NET runtime and is updated the way every other package on this system
-is; it contains no self-updater and contacts no update service.
+This package carries its own .NET runtime. It checks the project's own forge for newer builds and can
+download one, but it installs nothing itself: an update is handed to PackageKit, which asks before it
+replaces anything. The address it checks is fixed and is never a DodoSSH server.
 
 %install
 rm -rf %{buildroot}