add unit test and minor fixes

spec/unit/crypto/backup.spec.js

@@ -576,5 +576,21 @@ describe("MegolmBackup", function() {
             const cachedKey = await client._crypto.getSessionBackupPrivateKey();
             expect(cachedKey).not.toBeNull();
         });
+
+        it("fails if an known algorithm is used", async function() {
+            const BAD_BACKUP_INFO = Object.assign({}, BACKUP_INFO, {
+                algorithm: "this.algorithm.does.not.exist",
+            });
+            client._http.authedRequest = function() {
+                return Promise.resolve(KEY_BACKUP_DATA);
+            };
+
+            await expect(client.restoreKeyBackupWithRecoveryKey(
+                "EsTc LW2K PGiF wKEA 3As5 g5c4 BXwk qeeJ ZJV8 Q9fu gUMN UE4d",
+                ROOM_ID,
+                SESSION_ID,
+                BAD_BACKUP_INFO,
+            )).rejects.toThrow();
+        });
     });
 });

src/crypto/backup.ts

@@ -238,6 +238,8 @@ export class BackupManager {
     }
 
     /**
+     * Check if the given backup info is trusted.
+     *
      * @param {object} backupInfo key backup info dict from /room_keys/version
      * @return {object} {
      *     usable: [bool], // is the backup trusted, true iff there is a sig that is valid & from a trusted device
@@ -351,7 +353,7 @@ export class BackupManager {
                 )
             );
         });
-        ret.usable ||= ret.trusted_locally;
+        ret.usable = ret.usable || ret.trusted_locally;
         return ret;
     }