Fix decrypting redacted event

changelog.d/6148.bugfix

@@ -0,0 +1 @@
+Fix decrypting redacted event from sending errors

matrix-sdk-android/src/androidTest/java/org/matrix/android/sdk/internal/crypto/DecryptRedactedEventTest.kt

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2022 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.matrix.android.sdk.internal.crypto
+
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import org.junit.Assert
+import org.junit.FixMethodOrder
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.MethodSorters
+import org.matrix.android.sdk.InstrumentedTest
+import org.matrix.android.sdk.api.session.events.model.Event
+import org.matrix.android.sdk.api.session.events.model.toModel
+import org.matrix.android.sdk.api.session.getRoom
+import org.matrix.android.sdk.api.session.room.getTimelineEvent
+import org.matrix.android.sdk.common.CommonTestHelper
+import org.matrix.android.sdk.common.CryptoTestHelper
+
+@RunWith(AndroidJUnit4::class)
+@FixMethodOrder(MethodSorters.JVM)
+class DecryptRedactedEventTest : InstrumentedTest {
+
+    @Test
+    fun doNotFailToDecryptRedactedEvent() {
+        val testHelper = CommonTestHelper(context())
+        val cryptoTestHelper = CryptoTestHelper(testHelper)
+
+        val testData = cryptoTestHelper.doE2ETestWithAliceAndBobInARoom(true)
+        val e2eRoomID = testData.roomId
+        val aliceSession = testData.firstSession
+        val bobSession = testData.secondSession!!
+
+        val roomALicePOV = aliceSession.getRoom(e2eRoomID)!!
+        val timelineEvent = testHelper.sendTextMessage(roomALicePOV, "Hello", 1).first()
+        val redactionReason = "Wrong Room"
+        roomALicePOV.sendService().redactEvent(timelineEvent.root, redactionReason)
+
+        // get the event from bob
+        testHelper.waitWithLatch {
+            testHelper.retryPeriodicallyWithLatch(it) {
+                bobSession.getRoom(e2eRoomID)?.getTimelineEvent(timelineEvent.eventId)?.root?.isRedacted() == true
+            }
+        }
+
+        val eventBobPov = bobSession.getRoom(e2eRoomID)?.getTimelineEvent(timelineEvent.eventId)!!
+
+        testHelper.runBlockingTest {
+            try {
+                val result = bobSession.cryptoService().decryptEvent(eventBobPov.root, "")
+                Assert.assertEquals(
+                        "Unexpected redacted reason",
+                        redactionReason,
+                        result.clearEvent.toModel<Event>()?.unsignedData?.redactedEvent?.content?.get("reason")
+                )
+                Assert.assertEquals(
+                        "Unexpected Redacted event id",
+                        timelineEvent.eventId,
+                        result.clearEvent.toModel<Event>()?.unsignedData?.redactedEvent?.redacts
+                )
+            } catch (failure: Throwable) {
+                Assert.fail("Should not throw when decrypting a redacted event")
+            }
+        }
+    }
+}

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/EventDecryptor.kt

@@ -29,6 +29,7 @@ import org.matrix.android.sdk.api.session.crypto.model.MXUsersDevicesMap
 import org.matrix.android.sdk.api.session.events.model.Event
 import org.matrix.android.sdk.api.session.events.model.EventType
 import org.matrix.android.sdk.api.session.events.model.content.OlmEventContent
+import org.matrix.android.sdk.api.session.events.model.toContent
 import org.matrix.android.sdk.api.session.events.model.toModel
 import org.matrix.android.sdk.internal.crypto.actions.EnsureOlmSessionsForDevicesAction
 import org.matrix.android.sdk.internal.crypto.actions.MessageEncrypter
@@ -42,7 +43,7 @@ import javax.inject.Inject
 
 private const val SEND_TO_DEVICE_RETRY_COUNT = 3
 
-private val loggerTag = LoggerTag("CryptoSyncHandler", LoggerTag.CRYPTO)
+private val loggerTag = LoggerTag("EventDecryptor", LoggerTag.CRYPTO)
 
 @SessionScope
 internal class EventDecryptor @Inject constructor(
@@ -110,6 +111,16 @@ internal class EventDecryptor @Inject constructor(
         if (eventContent == null) {
             Timber.tag(loggerTag.value).e("decryptEvent : empty event content")
             throw MXCryptoError.Base(MXCryptoError.ErrorType.BAD_ENCRYPTED_MESSAGE, MXCryptoError.BAD_ENCRYPTED_MESSAGE_REASON)
+        } else if (event.isRedacted()) {
+            // we shouldn't attempt to decrypt a redacted event because the content is cleared and decryption will fail because of null algorithm
+            return MXEventDecryptionResult(
+                    clearEvent = mapOf(
+                            "room_id" to event.roomId.orEmpty(),
+                            "type" to EventType.MESSAGE,
+                            "content" to emptyMap<String, Any>(),
+                            "unsigned" to event.unsignedData.toContent()
+                    )
+            )
         } else {
             val algorithm = eventContent["algorithm"]?.toString()
             val alg = roomDecryptorProvider.getOrCreateRoomDecryptor(event.roomId, algorithm)