diff --git a/src/matrix/storage/idb/stores/GroupSessionDecryptionStore.ts b/src/matrix/storage/idb/stores/GroupSessionDecryptionStore.ts
index da47639d..25c21b4b 100644
--- a/src/matrix/storage/idb/stores/GroupSessionDecryptionStore.ts
+++ b/src/matrix/storage/idb/stores/GroupSessionDecryptionStore.ts
@@ -15,30 +15,39 @@ limitations under the License.
 */
 
 import {MIN_UNICODE, MAX_UNICODE} from "./common";
+import {Store} from "../Store"
 
-function encodeKey(roomId, sessionId, messageIndex) {
+function encodeKey(roomId: string, sessionId: string, messageIndex: number | string): string {
     return `${roomId}|${sessionId}|${messageIndex}`;
 }
 
+interface GroupSessionDecryption {
+    eventId: string
+    timestamp: number
+    key: string
+}
+
 export class GroupSessionDecryptionStore {
-    constructor(store) {
+    private _store: Store<GroupSessionDecryption>
+
+    constructor(store: Store<GroupSessionDecryption>) {
         this._store = store;
     }
 
-    get(roomId, sessionId, messageIndex) {
+    get(roomId: string, sessionId: string, messageIndex: number): Promise<GroupSessionDecryption | null> {
         return this._store.get(encodeKey(roomId, sessionId, messageIndex));
     }
 
-    set(roomId, sessionId, messageIndex, decryption) {
+    set(roomId: string, sessionId: string, messageIndex: number, decryption: GroupSessionDecryption): Promise<IDBValidKey> {
         decryption.key = encodeKey(roomId, sessionId, messageIndex);
-        this._store.put(decryption);
+        return this._store.put(decryption);
     }
     
-    removeAllForRoom(roomId) {
+    removeAllForRoom(roomId: string): Promise<undefined> {
         const range = this._store.IDBKeyRange.bound(
             encodeKey(roomId, MIN_UNICODE, MIN_UNICODE),
             encodeKey(roomId, MAX_UNICODE, MAX_UNICODE)
         );
-        this._store.delete(range);
+        return this._store.delete(range);
     }
 }