Merge pull request #1776 from matrix-org/dbkr/rescue_lost_megolm_key

Attempt to fix megolm key not being in SSSS

Author: dbkr

src/crypto/index.ts

@@ -967,6 +967,20 @@ export class Crypto extends EventEmitter {
                 fixedBackupKey || sessionBackupKey,
             ));
             await builder.addSessionBackupPrivateKeyToCache(decodedBackupKey);
+        } else if (this.backupManager.getKeyBackupEnabled()) {
+            // key backup is enabled but we don't have a session backup key in SSSS: see if we have one in
+            // the cache or the user can provide one, and if so, write it to SSSS
+            const backupKey = await this.getSessionBackupPrivateKey() || await getKeyBackupPassphrase();
+            if (!backupKey) {
+                // This will require user intervention to recover from since we don't have the key
+                // backup key anywhere. The user should probably just set up a new key backup and
+                // the key for the new backup will be stored. If we hit this scenario in the wild
+                // with any frequency, we should do more than just log an error.
+                logger.error("Key backup is enabled but couldn't get key backup key!");
+                return;
+            }
+            logger.info("Got session backup key from cache/user that wasn't in SSSS: saving to SSSS");
+            await secretStorage.store("m.megolm_backup.v1", olmlib.encodeBase64(backupKey));
         }
 
         const operation = builder.buildOperation();