use the default SSSS key if the default is set

implements MSC2874

Author: uhoreg

src/SecurityManager.ts

@@ -98,11 +98,24 @@ async function getSecretStorageKey(
     { keys: keyInfos }: { keys: Record<string, ISecretStorageKeyInfo> },
     ssssItemName,
 ): Promise<[string, Uint8Array]> {
-    const keyInfoEntries = Object.entries(keyInfos);
-    if (keyInfoEntries.length > 1) {
-        throw new Error("Multiple storage key requests not implemented");
+    const cli = MatrixClientPeg.get();
+    let keyId = await cli.getDefaultSecretStorageKeyId();
+    let keyInfo;
+    if (keyId) {
+        // use the default SSSS key if set
+        keyInfo = keyInfos[keyId];
+        if (!keyInfo) {
+            throw new Error("Unable to use default SSSS key");
+        }
+    } else {
+        // if no default SSSS key is set, fall back to a heuristic of using the
+        // only available key, if only one key is set
+        const keyInfoEntries = Object.entries(keyInfos);
+        if (keyInfoEntries.length > 1) {
+            throw new Error("Multiple storage key requests not implemented");
+        }
+        [keyId, keyInfo] = keyInfoEntries[0];
     }
-    const [keyId, keyInfo] = keyInfoEntries[0];
 
     // Check the in-memory cache
     if (isCachingAllowed() && secretStorageKeys[keyId]) {