MSC4312: Resetting cross-signing keys in the OAuth world

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

Author: Johennes

proposals/4312-x-signing-reset-with-nextgen-auth.md

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+# MSC4312: Resetting cross-signing keys in the OAuth world
+
+Matrix v1.15 added new [OAuth APIs] for authentication. As of writing, these APIs are not compatible
+with the existing [User-Interactive Authentication (UIA)] mechanism that is used on a number of
+endpoints. This is not problematic in most cases because these endpoints cover actions that can now
+be preformed in the authorization server's web UI. One notable exception, however, is
+[`/_matrix/client/v3/keys/device_signing/upload`] which clients use to publish their cross-signing
+keys. This endpoint requires UIA when previously uploaded keys are being replaced, for instance
+because the user lost their recovery key. OAuth knows nothing about cross-signing keys and,
+consequently, the spec labels this endpoint as unusable:
+
+> **WARNING:** When this endpoint requires User-Interactive Authentication, it cannot be used when
+> the access token was obtained via the OAuth 2.0 API.
+
+This is obviously not practical and unofficial workarounds have been invented to enable resetting
+one's cross-signing keys in the client / homeserver / authorization server triangle. This proposal
+documents these workarounds.
+
+## Proposal
+
+Clients that have authenticated via the new [OAuth APIs] continue to use
+[`/_matrix/client/v3/keys/device_signing/upload`] to replace cross-signing keys. Homeservers
+continue to enforce UIA on the endpoint but MUST only use a single stage `m.cross_signing_reset`
+together with a URL that points to the authorization server's account management UI.
+
+``` json5
+{
+  "session": "$ARBITRARY",
+  "flows": [{
+    "stages": ["m.cross_signing_reset"]
+  }],
+  "params": {
+    "m.cross_signing_reset": {
+      "url": "$AUTHORIZATION_SERVER_ACCOUNT_MANAGEMENT_URL"
+    }
+  }
+}
+```
+
+The client then instructs the user to approve the reset of their cross-signing keys using the
+provided URL. How exactly that approval is achieved is an implementation detail between the
+authorization server and the homeserver[^1]. The required end result is that after approving, the
+client can complete the stage without further parameters.
+
+``` json5
+{
+  "auth": {
+    "session": "$FROM_ABOVE"
+  },
+  "master_key": ...
+  ...
+}
+```
+
+## Potential issues
+
+Semantically, resetting cross-signing keys doesn't fall into the authorization server's domain. The
+scheme outlined above increases coupling between the authorization server and the homeserver and
+makes it more difficult to use off-the-shelve OAuth authorization servers.
+
+## Alternatives
+
+Rather than approving cross-signing reset specifically, the authorization server could provide
+mechanisms for temporary scope elevation.
+
+## Security considerations
+
+Lifting UIA temporarily creates a time window in which an attacker with an access token could take
+over the account.
+
+## Unstable prefix
+
+While this MSC is not considered stable, `m.cross_signing_reset` should be referred to as
+`org.matrix.cross_signing_reset`.
+
+## Dependencies
+
+None.
+
+[^1]: [matrix-authentication-service], for instance, uses a [Synapse admin API] to temporarily lift
+    UIA on the endpoint.
+
+  [OAuth APIs]: https://spec.matrix.org/v1.15/client-server-api/#oauth-20-api
+  [User-Interactive Authentication (UIA)]: https://spec.matrix.org/v1.15/client-server-api/#user-interactive-authentication-api
+  [`/_matrix/client/v3/keys/device_signing/upload`]: https://spec.matrix.org/v1.15/client-server-api/#post_matrixclientv3keysdevice_signingupload
+  [matrix-authentication-service]: https://github.com/element-hq/matrix-authentication-service
+  [Synapse admin API]: https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#allow-replacing-master-cross-signing-key-without-user-interactive-auth