State-reset and security mitigations.

Upgrade Ruma to present.

The following are intentionally benign for activation in a later commit:

- Hydra backports not default.
- Room version 12 not default.
- Room version 12 not listed as stable.

Do not enable them manually or you can brick your database.

Signed-off-by: Jason Volk <jason@zemos.net>

tuwunel-example.toml

@@ -533,20 +533,33 @@
 #allow_room_creation = true
 
 # Set to false to disable users from joining or creating room versions
-# that aren't officially supported by tuwunel.
+# that aren't officially supported by tuwunel. Unstable room versions may
+# have flawed specifications or our implementation may be non-conforming.
+# Correct operation may not be guaranteed, but incorrect operation may be
+# tolerable and unnoticed.
 #
-# tuwunel officially supports room versions 6 - 11.
-#
-# tuwunel has slightly experimental (though works fine in practice)
-# support for versions 3 - 5.
+# tuwunel officially supports room versions 6+. tuwunel has slightly
+# experimental (though works fine in practice) support for versions 3 - 5.
 #
 #allow_unstable_room_versions = true
 
+# Set to true to enable experimental room versions.
+#
+# Unlike unstable room versions these versions are either under
+# development, protype spec-changes, or somehow present a serious risk to
+# the server's operation or database corruption. This is for developer use
+# only.
+#
+#allow_experimental_room_versions = false
+
 # Default room version tuwunel will create rooms with.
 #
-# Per spec, room version 11 is the default.
+# The default is prescribed by the spec, but may be selected by developer
+# recommendation. To prevent stale documentation we no longer list it
+# here. It is only advised to override this if you know what you are
+# doing, and by doing so, updates with new versions are precluded.
 #
-#default_room_version = 11
+#default_room_version =
 
 # This item is undocumented. Please contribute documentation for it.
 #
@@ -1615,6 +1628,30 @@
 #
 #config_reload_signal = true
 
+# Backport state-reset security fixes to all room versions.
+#
+# This option applies the State Resolution 2.1 mitigation developed during
+# project Hydra for room version 12 to all prior State Resolution 2.0 room
+# versions (all room versions supported by this server). These mitigations
+# increase resilience to state-resets without any new definition of
+# correctness; therefor it is safe to set this to true for existing rooms.
+#
+# Furthermore, state-reset attacks are not consistent as they result in
+# rooms without any single consensus, therefor it is unnecessary to set
+# this to false to match other servers which set this to false or simply
+# lack support; even if replicating the post-reset state suffered by other
+# servers is somehow desired.
+#
+# This option exists for developer and debug use, and as a failsafe in
+# lieu of hardcoding it.
+#
+# This currently defaults to false as a matter of development until
+# real-world testing can shake out any implementation issues rather than
+# jeopardize existing rooms, but otherwise will default to true at the
+# next point release or patch.
+#
+#hydra_backports = false
+
 #[global.tls]
 
 # Path to a valid TLS certificate file.