feat: (BREAKING) Make share keys with logic configurable

lib/src/client.dart

@@ -90,7 +90,7 @@ class Client extends MatrixApi {
 
   final bool mxidLocalPartFallback;
 
-  bool shareKeysWithUnverifiedDevices;
+  ShareKeysWith shareKeysWith;
 
   Future<void> Function(Client client)? onSoftLogout;
 
@@ -219,7 +219,7 @@ class Client extends MatrixApi {
     Duration defaultNetworkRequestTimeout = const Duration(seconds: 35),
     this.sendTimelineEventTimeout = const Duration(minutes: 1),
     this.customImageResizer,
-    this.shareKeysWithUnverifiedDevices = true,
+    this.shareKeysWith = ShareKeysWith.crossVerifiedIfEnabled,
     this.enableDehydratedDevices = false,
     this.receiptsPublicByDefault = true,
 
@@ -4082,3 +4082,25 @@ enum InitState {
   /// Initialization has been completed with an error.
   error,
 }
+
+/// Sets the security level with which devices keys should be shared with
+enum ShareKeysWith {
+  /// Keys are shared with all devices if they are not explicitely blocked
+  all,
+
+  /// Once a user has enabled cross signing, keys are no longer shared with
+  /// devices which are not cross verified by the cross signing keys of this
+  /// user. This does not require that the user needs to be verified.
+  crossVerifiedIfEnabled,
+
+  /// Keys are only shared with cross verified devices. If a user has not
+  /// enabled cross signing, then all devices must be verified manually first.
+  /// This does not require that the user needs to be verified.
+  crossVerified,
+
+  /// Keys are only shared with direct verified devices. So either the device
+  /// or the user must be manually verified first, before keys are shared. By
+  /// using cross signing, it is enough to verify the user and then the user
+  /// can verify their devices.
+  directlyVerifiedOnly,
+}

lib/src/utils/device_keys_list.dart

@@ -163,7 +163,17 @@ abstract class SignableKey extends MatrixSignableKey {
 
     if (identifier == null || ed25519Key == null) return false;
 
-    return client.shareKeysWithUnverifiedDevices || verified;
+    switch (client.shareKeysWith) {
+      case ShareKeysWith.all:
+        return true;
+      case ShareKeysWith.crossVerifiedIfEnabled:
+        if (client.userDeviceKeys[userId]?.masterKey == null) return true;
+        return hasValidSignatureChain(verifiedByTheirMasterKey: true);
+      case ShareKeysWith.crossVerified:
+        return hasValidSignatureChain(verifiedByTheirMasterKey: true);
+      case ShareKeysWith.directlyVerifiedOnly:
+        return directVerified;
+    }
   }
 
   void setDirectVerified(bool isVerified) {

test/device_keys_list_test.dart

@@ -150,11 +150,29 @@ void main() {
         },
         client,
       );
-      expect(client.shareKeysWithUnverifiedDevices, true);
+
+      client.shareKeysWith = ShareKeysWith.all;
       expect(key.encryptToDevice, true);
-      client.shareKeysWithUnverifiedDevices = false;
+
+      client.shareKeysWith = ShareKeysWith.directlyVerifiedOnly;
       expect(key.encryptToDevice, false);
-      client.shareKeysWithUnverifiedDevices = true;
+      await key.setVerified(true);
+      expect(key.encryptToDevice, true);
+      await key.setVerified(false);
+
+      client.shareKeysWith = ShareKeysWith.crossVerified;
+      expect(key.encryptToDevice, true);
+
+      client.shareKeysWith = ShareKeysWith.crossVerified;
+      // Disable cross signing for this user manually so encryptToDevice should return `false`
+      final dropUserDeviceKeys = client.userDeviceKeys.remove(key.userId);
+      expect(key.encryptToDevice, false);
+      // But crossVerifiedIfEnabled should return `true` now:
+      client.shareKeysWith = ShareKeysWith.crossVerifiedIfEnabled;
+      expect(key.encryptToDevice, true);
+
+      client.userDeviceKeys[key.userId] = dropUserDeviceKeys!;
+      client.shareKeysWith = ShareKeysWith.all;
       final masterKey = client.userDeviceKeys[client.userID]!.masterKey!;
       masterKey.setDirectVerified(true);
       // we need to populate the ssss cache to be able to test signing easily