diff --git a/src/matrix/e2ee/megolm/Decryption.js b/src/matrix/e2ee/megolm/Decryption.js
index 68bca6fe..6d7941a0 100644
--- a/src/matrix/e2ee/megolm/Decryption.js
+++ b/src/matrix/e2ee/megolm/Decryption.js
@@ -14,12 +14,97 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import {DecryptionError} from "../common.js";
+
+const CACHE_MAX_SIZE = 10;
+
 export class Decryption {
     constructor({pickleKey, olm}) {
         this._pickleKey = pickleKey;
         this._olm = olm;
     }
 
+    createSessionCache() {
+        return new SessionCache();
+    }
+
+    async decryptNewEvent(roomId, event, sessionCache, txn) {
+        const {payload, messageIndex} = this._decrypt(roomId, event, sessionCache, txn);
+        const sessionId = event.content?.["session_id"];
+        this._handleReplayAttacks(roomId, sessionId, messageIndex, event, txn);
+        return payload;
+    }
+
+    async decryptStoredEvent(roomId, event, sessionCache, txn) {
+        const {payload} = this._decrypt(roomId, event, sessionCache, txn);
+        return payload;
+    }
+
+    async _decrypt(roomId, event, sessionCache, txn) {
+        const senderKey = event.content?.["sender_key"];
+        const sessionId = event.content?.["session_id"];
+        const ciphertext = event.content?.ciphertext;
+
+        if (
+            typeof senderKey !== "string" ||
+            typeof sessionId !== "string" ||
+            typeof ciphertext !== "string"
+        ) {
+            throw new DecryptionError("MEGOLM_INVALID_EVENT", event);
+        }
+
+        let session = sessionCache.get(roomId, senderKey, sessionId);
+        if (!session) {
+            const sessionEntry = await txn.inboundGroupSessions.get(roomId, senderKey, sessionId);
+            if (sessionEntry) {
+                session = new this._olm.InboundGroupSession();
+                try {
+                    session.unpickle(this._pickleKey, sessionEntry.session);
+                } catch (err) {
+                    session.free();
+                    throw err;
+                }
+                sessionCache.add(roomId, senderKey, session);
+            }
+        }
+        if (!session) {
+            return;
+        }
+        const {plaintext, message_index: messageIndex} = session.decrypt(ciphertext);
+        let payload;
+        try {
+            payload = JSON.parse(plaintext);
+        } catch (err) {
+            throw new DecryptionError("NOT_JSON", event, {plaintext, err});
+        }
+        if (payload.room_id !== roomId) {
+            throw new DecryptionError("MEGOLM_WRONG_ROOM", event,
+                {encryptedRoomId: payload.room_id, eventRoomId: roomId});
+        }
+        return {payload, messageIndex};
+    }
+
+    async _handleReplayAttacks(roomId, sessionId, messageIndex, event, txn) {
+        const eventId = event.event_id;
+        const timestamp = event.origin_server_ts;
+        const decryption = await txn.groupSessionDecryptions.get(roomId, sessionId, messageIndex);
+        if (decryption && decryption.eventId !== eventId) {
+            // the one with the newest timestamp should be the attack
+            const decryptedEventIsBad = decryption.timestamp < timestamp;
+            const badEventId = decryptedEventIsBad ? eventId : decryption.eventId;
+            throw new DecryptionError("MEGOLM_REPLAY_ATTACK", event, {badEventId, otherEventId: decryption.eventId});
+        }
+        if (!decryption) {
+            txn.groupSessionDecryptions.set({
+                roomId,
+                sessionId,
+                messageIndex,
+                eventId,
+                timestamp
+            });
+        }
+    }
+
     async addRoomKeys(payloads, txn) {
         const newSessions = [];
         for (const {senderKey, event} of payloads) {
@@ -56,13 +141,49 @@ export class Decryption {
             }
 
         }
+        // this will be passed to the Room in notifyRoomKeys
         return newSessions;
     }
+}
 
-    applyRoomKeyChanges(newSessions) {
-        // retry decryption with the new sessions
-        if (newSessions.length) {
-            console.log(`I have ${newSessions.length} new inbound group sessions`, newSessions)
+class SessionCache {
+    constructor() {
+        this._sessions = [];
+    }
+
+    get(roomId, senderKey, sessionId) {
+        const idx = this._sessions.findIndex(s => {
+            return s.roomId === roomId &&
+                s.senderKey === senderKey &&
+                sessionId === s.session.session_id();
+        });
+        if (idx !== -1) {
+            const entry = this._sessions[idx];
+            // move to top
+            if (idx > 0) {
+                this._sessions.splice(idx, 1);
+                this._sessions.unshift(entry);
+            }
+            return entry.session;
         }
     }
+
+    add(roomId, senderKey, session) {
+        // add new at top
+        this._sessions.unshift({roomId, senderKey, session});
+        if (this._sessions.length > CACHE_MAX_SIZE) {
+            // free sessions we're about to remove
+            for (let i = CACHE_MAX_SIZE; i < this._sessions.length; i += 1) {
+                this._sessions[i].session.free();
+            }
+            this._sessions = this._sessions.slice(0, CACHE_MAX_SIZE);
+        }
+    }
+
+    dispose() {
+        for (const entry of this._sessions) {
+            entry.session.free();
+        }
+
+    }
 }