Add cross-signing feature flag

Fixes https://github.com/vector-im/riot-web/issues/11407

src/MatrixClientPeg.js

@@ -220,6 +220,16 @@ class MatrixClientPeg {
             identityServer: new IdentityAuthClient(),
         };
 
+        if (SettingsStore.isFeatureEnabled("feature_cross_signing")) {
+            // TODO: Cross-signing keys are temporarily in memory only. A
+            // separate task in the cross-signing project will build from here.
+            const keys = [];
+            opts.cryptoCallbacks = {
+                getCrossSigningKey: k => keys[k],
+                saveCrossSigningKeys: newKeys => Object.assign(keys, newKeys),
+            };
+        }
+
         this.matrixClient = createMatrixClient(opts);
 
         // we're going to add eventlisteners for each matrix event tile, so the

src/i18n/strings/en_EN.json

@@ -342,6 +342,7 @@
     "Use the new, consistent UserInfo panel for Room Members and Group Members": "Use the new, consistent UserInfo panel for Room Members and Group Members",
     "Try out new ways to ignore people (experimental)": "Try out new ways to ignore people (experimental)",
     "Send verification requests in direct message": "Send verification requests in direct message",
+    "Enable cross-signing to verify per-user instead of per-device": "Enable cross-signing to verify per-user instead of per-device",
     "Use the new, faster, composer for writing messages": "Use the new, faster, composer for writing messages",
     "Enable Emoji suggestions while typing": "Enable Emoji suggestions while typing",
     "Use compact timeline layout": "Use compact timeline layout",

src/settings/Settings.js

@@ -146,6 +146,12 @@ export const SETTINGS = {
         supportedLevels: LEVELS_FEATURE,
         default: false,
     },
+    "feature_cross_signing": {
+        isFeature: true,
+        displayName: _td("Enable cross-signing to verify per-user instead of per-device"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "useCiderComposer": {
         displayName: _td("Use the new, faster, composer for writing messages"),
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,