From 6168cedf32e8a4c71c9ebc8c36bd33dd62a62c01 Mon Sep 17 00:00:00 2001
From: Hubert Chathi <hubertc@matrix.org>
Date: Tue, 3 Jan 2023 11:06:54 -0500
Subject: [PATCH] Avoid triggering decryption errors when decrypting redacted
 events (#3004)

---
 spec/unit/crypto.spec.ts | 32 ++++++++++++++++++++++++++++++++
 src/crypto/index.ts      | 15 +++++++++++++--
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/spec/unit/crypto.spec.ts b/spec/unit/crypto.spec.ts
index 3b7689ca0ce..36a74607840 100644
--- a/spec/unit/crypto.spec.ts
+++ b/spec/unit/crypto.spec.ts
@@ -167,6 +167,38 @@ 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();
+            // since the redaction event isn't encrypted, the redacted_because
+            // should be the same as in the original event
+            expect(event.getRedactionEvent()).toEqual(event.getUnsigned().redacted_because);
+
+            client.stopClient();
+        });
     });
 
     describe("Session management", function () {
diff --git a/src/crypto/index.ts b/src/crypto/index.ts
index fe8719a4c4a..24b36b08143 100644
--- a/src/crypto/index.ts
+++ b/src/crypto/index.ts
@@ -2878,11 +2878,22 @@ 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 = event.getUnsigned().redacted_because!;
+            if (redactionEvent.isEncrypted()) {
+                try {
+                    const decryptedEvent = await this.decryptEvent(redactionEvent);
+                    redactedBecause = decryptedEvent.clearEvent as IEvent;
+                } catch (e) {
+                    logger.warn("Decryption of redaction failed. Falling back to unencrypted event.", e);
+                }
+            }
 
             return {
                 clearEvent: {
@@ -2890,7 +2901,7 @@ export class Crypto extends TypedEventEmitter<CryptoEvent, CryptoEventHandlerMap
                     type: "m.room.message",
                     content: {},
                     unsigned: {
-                        redacted_because: decryptedEvent.clearEvent as IEvent,
+                        redacted_because: redactedBecause,
                     },
                 },
             };