Fallback keys implementation.

Author: Onuray - Benoit squashes the 4 commit to cancel the addition on binaries

matrix-sdk-android/build.gradle

@@ -140,7 +140,7 @@ dependencies {
     implementation libs.arrow.instances
 
     // olm lib is now hosted by jitpack: https://jitpack.io/#org.matrix.gitlab.matrix-org/olm
-    implementation 'org.matrix.gitlab.matrix-org:olm:3.2.4'
+    implementation 'org.matrix.gitlab.matrix-org:olm:3.2.6'
 
     // DI
     implementation libs.dagger.dagger

matrix-sdk-android/src/main/java/org/matrix/android/sdk/api/session/sync/model/SyncResponse.kt

@@ -58,6 +58,14 @@ data class SyncResponse(
         @Json(name = "device_one_time_keys_count")
         val deviceOneTimeKeysCount: DeviceOneTimeKeysCountSyncResponse? = null,
 
+        /**
+         * The key algorithms for which the server has an unused fallback key for the device.
+         * If the client wants the server to have a fallback key for a given key algorithm,
+         * but that algorithm is not listed in device_unused_fallback_key_types, the client will upload a new key.
+         */
+        @Json(name = "org.matrix.msc2732.device_unused_fallback_key_types")
+        val deviceUnusedFallbackKeyTypes: List<String>? = null,
+
         /**
          * List of groups.
          */

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

@@ -65,6 +65,7 @@ import org.matrix.android.sdk.internal.crypto.model.CryptoDeviceInfo
 import org.matrix.android.sdk.internal.crypto.model.ImportRoomKeysResult
 import org.matrix.android.sdk.internal.crypto.model.MXDeviceInfo
 import org.matrix.android.sdk.internal.crypto.model.MXEncryptEventContentResult
+import org.matrix.android.sdk.internal.crypto.model.MXKey.Companion.KEY_SIGNED_CURVE_25519_TYPE
 import org.matrix.android.sdk.internal.crypto.model.MXUsersDevicesMap
 import org.matrix.android.sdk.internal.crypto.model.event.EncryptedEventContent
 import org.matrix.android.sdk.internal.crypto.model.event.RoomKeyContent
@@ -325,7 +326,7 @@ internal class DefaultCryptoService @Inject constructor(
                 uploadDeviceKeys()
             }
 
-            oneTimeKeysUploader.maybeUploadOneTimeKeys()
+            oneTimeKeysUploader.maybeUploadOneTimeKeys(shouldGenerateFallbackKey = false)
             // this can throw if no backup
             tryOrNull {
                 keysBackupService.checkAndStartKeysBackup()
@@ -427,7 +428,17 @@ internal class DefaultCryptoService @Inject constructor(
                 if (isStarted()) {
                     // Make sure we process to-device messages before generating new one-time-keys #2782
                     deviceListManager.refreshOutdatedDeviceLists()
-                    oneTimeKeysUploader.maybeUploadOneTimeKeys()
+                    // The presence of device_unused_fallback_key_types indicates that the server supports fallback keys.
+                    // If there's no unused signed_curve25519 fallback key we need a new one.
+                    val shouldGenerateFallbackKey = if (syncResponse.deviceUnusedFallbackKeyTypes != null) {
+                        // Generate a fallback key only if the server does not already have an unused fallback key.
+                        !syncResponse.deviceUnusedFallbackKeyTypes.contains(KEY_SIGNED_CURVE_25519_TYPE)
+                    } else {
+                        // Server does not support fallbackKey
+                        false
+                    }
+
+                    oneTimeKeysUploader.maybeUploadOneTimeKeys(shouldGenerateFallbackKey)
                     incomingGossipingRequestManager.processReceivedGossipingRequests()
                 }
             }
@@ -924,7 +935,7 @@ internal class DefaultCryptoService @Inject constructor(
                 signatures = objectSigner.signObject(canonicalJson)
         )
 
-        val uploadDeviceKeysParams = UploadKeysTask.Params(rest, null)
+        val uploadDeviceKeysParams = UploadKeysTask.Params(rest, null, null)
         uploadKeysTask.execute(uploadDeviceKeysParams)
 
         cryptoStore.setDeviceKeysUploaded(true)

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

@@ -136,6 +136,24 @@ internal class MXOlmDevice @Inject constructor(
         return store.getOlmAccount().maxOneTimeKeys()
     }
 
+    fun getFallbackKey(): MutableMap<String, MutableMap<String, String>>? {
+        try {
+            return store.getOlmAccount().fallbackKey()
+        } catch (e: Exception) {
+            Timber.e("## getFallbackKey() : failed")
+        }
+        return null
+    }
+
+    fun generateFallbackKey() {
+        try {
+            store.getOlmAccount().generateFallbackKey()
+            store.saveOlmAccount()
+        } catch (e: Exception) {
+            Timber.e("## generateFallbackKey() : failed")
+        }
+    }
+
     /**
      * Release the instance
      */

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

@@ -54,7 +54,7 @@ internal class OneTimeKeysUploader @Inject constructor(
     /**
      * Check if the OTK must be uploaded.
      */
-    suspend fun maybeUploadOneTimeKeys() {
+    suspend fun maybeUploadOneTimeKeys(shouldGenerateFallbackKey: Boolean) {
         if (oneTimeKeyCheckInProgress) {
             Timber.v("maybeUploadOneTimeKeys: already in progress")
             return
@@ -68,6 +68,10 @@ internal class OneTimeKeysUploader @Inject constructor(
         lastOneTimeKeyCheck = System.currentTimeMillis()
         oneTimeKeyCheckInProgress = true
 
+        if (shouldGenerateFallbackKey) {
+            olmDevice.generateFallbackKey()
+        }
+
         // We then check how many keys we can store in the Account object.
         val maxOneTimeKeys = olmDevice.getMaxNumberOfOneTimeKeys()
 
@@ -96,7 +100,7 @@ internal class OneTimeKeysUploader @Inject constructor(
             // So we need some kind of engineering compromise to balance all of
             // these factors.
             tryOrNull("Unable to upload OTK") {
-                val uploadedKeys = uploadOTK(oneTimeKeyCountFromSync, keyLimit)
+                val uploadedKeys = uploadOTK(oneTimeKeyCountFromSync, keyLimit, shouldGenerateFallbackKey)
                 Timber.v("## uploadKeys() : success, $uploadedKeys key(s) sent")
             }
         } else {
@@ -108,7 +112,7 @@ internal class OneTimeKeysUploader @Inject constructor(
 
     private suspend fun fetchOtkCount(): Int? {
         return tryOrNull("Unable to get OTK count") {
-            val result = uploadKeysTask.execute(UploadKeysTask.Params(null, null))
+            val result = uploadKeysTask.execute(UploadKeysTask.Params(null, null, null))
             result.oneTimeKeyCountsForAlgorithm(MXKey.KEY_SIGNED_CURVE_25519_TYPE)
         }
     }
@@ -120,19 +124,22 @@ internal class OneTimeKeysUploader @Inject constructor(
      * @param keyLimit the limit
      * @return the number of uploaded keys
      */
-    private suspend fun uploadOTK(keyCount: Int, keyLimit: Int): Int {
-        if (keyLimit <= keyCount) {
+    private suspend fun uploadOTK(keyCount: Int, keyLimit: Int, shouldGenerateFallbackKey: Boolean): Int {
+        if (keyLimit <= keyCount && !shouldGenerateFallbackKey) {
             // If we don't need to generate any more keys then we are done.
             return 0
         }
         val keysThisLoop = min(keyLimit - keyCount, ONE_TIME_KEY_GENERATION_MAX_NUMBER)
         olmDevice.generateOneTimeKeys(keysThisLoop)
-        val response = uploadOneTimeKeys(olmDevice.getOneTimeKeys())
+
+        val fallbackKey = if (shouldGenerateFallbackKey) olmDevice.getFallbackKey() else null
+
+        val response = uploadOneTimeKeys(olmDevice.getOneTimeKeys(), fallbackKey)
         olmDevice.markKeysAsPublished()
 
         if (response.hasOneTimeKeyCountsForAlgorithm(MXKey.KEY_SIGNED_CURVE_25519_TYPE)) {
             // Maybe upload other keys
-            return keysThisLoop + uploadOTK(response.oneTimeKeyCountsForAlgorithm(MXKey.KEY_SIGNED_CURVE_25519_TYPE), keyLimit)
+            return keysThisLoop + uploadOTK(response.oneTimeKeyCountsForAlgorithm(MXKey.KEY_SIGNED_CURVE_25519_TYPE), keyLimit, false)
         } else {
             Timber.e("## uploadOTK() : response for uploading keys does not contain one_time_key_counts.signed_curve25519")
             throw Exception("response for uploading keys does not contain one_time_key_counts.signed_curve25519")
@@ -142,7 +149,7 @@ internal class OneTimeKeysUploader @Inject constructor(
     /**
      * Upload curve25519 one time keys.
      */
-    private suspend fun uploadOneTimeKeys(oneTimeKeys: Map<String, Map<String, String>>?): KeysUploadResponse {
+    private suspend fun uploadOneTimeKeys(oneTimeKeys: Map<String, Map<String, String>>?, fallbackKey: Map<String, Map<String, String>>?): KeysUploadResponse {
         val oneTimeJson = mutableMapOf<String, Any>()
 
         val curve25519Map = oneTimeKeys?.get(OlmAccount.JSON_KEY_ONE_TIME_KEY).orEmpty()
@@ -159,9 +166,25 @@ internal class OneTimeKeysUploader @Inject constructor(
             oneTimeJson["signed_curve25519:$key_id"] = k
         }
 
+        val fallbackJson = mutableMapOf<String, Any>()
+        val fallbackCurve25519Map = fallbackKey?.get(OlmAccount.JSON_KEY_ONE_TIME_KEY).orEmpty()
+        fallbackCurve25519Map.forEach { (key_id, key) ->
+            val k = mutableMapOf<String, Any>()
+            k["key"] = key
+            k["fallback"] = true
+            val canonicalJson = JsonCanonicalizer.getCanonicalJson(Map::class.java, k)
+            k["signatures"] = objectSigner.signObject(canonicalJson)
+
+            fallbackJson["signed_curve25519:$key_id"] = k
+        }
+
         // For now, we set the device id explicitly, as we may not be using the
         // same one as used in login.
-        val uploadParams = UploadKeysTask.Params(null, oneTimeJson)
+        val uploadParams = UploadKeysTask.Params(
+                deviceKeys = null,
+                oneTimeKeys = oneTimeJson,
+                fallbackKeys = if (fallbackJson.isNotEmpty()) fallbackJson else null
+        )
         return uploadKeysTask.execute(uploadParams)
     }
 

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/model/rest/KeysUploadBody.kt

@@ -40,5 +40,12 @@ internal data class KeysUploadBody(
          * May be absent if no new one-time keys are required.
          */
         @Json(name = "one_time_keys")
-        val oneTimeKeys: JsonDict? = null
+        val oneTimeKeys: JsonDict? = null,
+
+        /**
+         * If the user had previously uploaded a fallback key for a given algorithm, it is replaced.
+         * The server will only keep one fallback key per algorithm for each user.
+         */
+        @Json(name = "org.matrix.msc2732.fallback_keys")
+        val fallbackKeys: JsonDict? = null
 )

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

@@ -32,7 +32,8 @@ internal interface UploadKeysTask : Task<UploadKeysTask.Params, KeysUploadRespon
             // the device keys to send.
             val deviceKeys: DeviceKeys?,
             // the one-time keys to send.
-            val oneTimeKeys: JsonDict?
+            val oneTimeKeys: JsonDict?,
+            val fallbackKeys: JsonDict?
     )
 }
 
@@ -44,7 +45,8 @@ internal class DefaultUploadKeysTask @Inject constructor(
     override suspend fun execute(params: UploadKeysTask.Params): KeysUploadResponse {
         val body = KeysUploadBody(
                 deviceKeys = params.deviceKeys,
-                oneTimeKeys = params.oneTimeKeys
+                oneTimeKeys = params.oneTimeKeys,
+                fallbackKeys = params.fallbackKeys
         )
 
         Timber.i("## Uploading device keys -> $body")