Avoid triggering decryption errors when decrypting redacted events

spec/unit/crypto.spec.ts

@@ -167,6 +167,35 @@ describe("Crypto", function () {
 
             client.stopClient();
         });
+
+        it("doesn't throw an error when attempting to decrypt a redacted event", async () => {
+            const client = new TestClient("@alice:example.com", "deviceid").client;
+            await client.initCrypto();
+
+            const event = new MatrixEvent({
+                content: {},
+                event_id: "$event_id",
+                room_id: "!room_id",
+                sender: "@bob:example.com",
+                type: "m.room.encrypted",
+                unsigned: {
+                    redacted_because: {
+                        content: {},
+                        event_id: "$redaction_event_id",
+                        redacts: "$event_id",
+                        room_id: "!room_id",
+                        origin_server_ts: 1234567890,
+                        sender: "@bob:example.com",
+                        type: "m.room.redaction",
+                        unsigned: {},
+                    },
+                },
+            });
+            await event.attemptDecryption(client.crypto!);
+            expect(event.isDecryptionFailure()).toBeFalsy();
+
+            client.stopClient();
+        });
     });
 
     describe("Session management", function () {

src/crypto/index.ts

@@ -2866,19 +2866,28 @@ export class Crypto extends TypedEventEmitter<CryptoEvent, CryptoEventHandlerMap
      */
     public async decryptEvent(event: MatrixEvent): Promise<IEventDecryptionResult> {
         if (event.isRedacted()) {
+            // Try to decrypt the redaction event, to support encrypted
+            // redaction reasons.  If we can't decrypt, just fall back to using
+            // the original redacted_because.
             const redactionEvent = new MatrixEvent({
                 room_id: event.getRoomId(),
                 ...event.getUnsigned().redacted_because,
             });
-            const decryptedEvent = await this.decryptEvent(redactionEvent);
+            let redactedBecause: IEvent;
+            try {
+                const decryptedEvent = await this.decryptEvent(redactionEvent);
+                redactedBecause = decryptedEvent.clearEvent as IEvent;
+            } catch {
+                redactedBecause = event.getUnsigned().redacted_because!;
+            }
 
             return {
                 clearEvent: {
                     room_id: event.getRoomId(),
                     type: "m.room.message",
                     content: {},
                     unsigned: {
-                        redacted_because: decryptedEvent.clearEvent as IEvent,
+                        redacted_because: redactedBecause,
                     },
                 },
             };