MSC 1466 - Soft Logout (#1467)

Author: erikjohnston

proposals/1466-soft-logout.md

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+# Soft Remote Logout Proposal
+
+## Motivation
+
+Currently when a user logs out of riot, the app will destroy things like
+encryption keys. If the logout was done by the user in local app then it will
+have first prompted the user to export the encryption keys. However, when the
+app is logged out remotely (i.e. it received a 401 from the server) there is no
+opportunity to ask the user to backup the keys, resulting in those keys being
+lost.
+
+While this behaviour is useful in many circumstances, e.g. remote logging out a
+stolen/lost device, it also means that the server shouldn't automatically
+log out devices, to avoid users losing encryption keys.
+
+
+## Proposal
+
+A new parameter is added to the JSON body of 401 responses, called
+`soft_logout`. This is a boolean flag (defaulting to `false`) that signals to
+the client whether it should keep local data and simply prompt to reauth (when
+`true`) or to destroy the current data like it does today (when `false`).
+
+The major disadvantage with this approach is that once the client is logged
+out, the user can no longer remotely cause the client to destroy the local
+data. However, this is not substantially different from today where the app has
+to be opened to receive remote logout requests (via 401), as it allows
+attackers to get at encryption keys even after remote logout if they simply
+avoid opening the app.
+
+
+### Example Client UX
+
+When handling a soft logout the client could show a "re-login" dialogue, rather
+than back to the default logged out screen. This new dialogue would then have
+several options including logging out, exporting keys and logging out fully.
+
+
+## Alternatives
+
+One alternative is to force the user to enter a password for backing up keys
+when they enter the app, and then have the app keep secure backups of they
+keys. This then means its safer to not delete the secure backups when the app
+is logged out remotely.
+