diff --git a/src/matrix/e2ee/megolm/Encryption.js b/src/matrix/e2ee/megolm/Encryption.js
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+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {MEGOLM_ALGORITHM} from "../common.js";
+
+export class Encryption {
+    constructor({pickleKey, olm, account, storage, now, ownDeviceId}) {
+        this._pickleKey = pickleKey;
+        this._olm = olm;
+        this._account = account;
+        this._storage = storage;
+        this._now = now;
+        this._ownDeviceId = ownDeviceId;
+    }
+
+    async encrypt(roomId, type, content, encryptionParams) {
+        let session = new this._olm.OutboundGroupSession();
+        try {
+            const txn = await this._storage.readWriteTxn([
+                this._storage.storeNames.inboundGroupSessions,
+                this._storage.storeNames.outboundGroupSessions,
+            ]);
+            let roomKeyMessage;
+            let encryptedContent;
+            try {
+                let sessionEntry = await txn.outboundGroupSessions.get(roomId);
+                if (sessionEntry) {
+                    session.unpickle(this._pickleKey, sessionEntry.session);
+                }
+                if (!sessionEntry || this._needsToRotate(session, sessionEntry.createdAt, encryptionParams)) {
+                    // in the case of rotating, recreate a session as we already unpickled into it
+                    if (session) {
+                        session.free();
+                        session = new this._olm.OutboundGroupSession();
+                    }
+                    session.create();
+                    roomKeyMessage = this._createRoomKeyMessage(session, roomId);
+                    this._storeAsInboundSession(session, roomId, txn);
+                    // TODO: we could tell the Decryption here that we have a new session so it can add it to its cache
+                }
+                encryptedContent = this._encryptContent(roomId, session, type, content);
+                txn.outboundGroupSessions.set({
+                    roomId,
+                    session: session.pickle(this._pickleKey),
+                    createdAt: sessionEntry?.createdAt || this._now(),
+                });
+
+            } catch (err) {
+                txn.abort();
+                throw err;
+            }
+            await txn.complete();
+            return new EncryptionResult(encryptedContent, roomKeyMessage);
+        } finally {
+            if (session) {
+                session.free();
+            }
+        }
+    }
+
+    _needsToRotate(session, createdAt, encryptionParams) {
+        let rotationPeriodMs = 604800000; // default
+        if (Number.isSafeInteger(encryptionParams?.rotation_period_ms)) {
+            rotationPeriodMs = encryptionParams?.rotation_period_ms;
+        }
+        let rotationPeriodMsgs = 100; // default
+        if (Number.isSafeInteger(encryptionParams?.rotation_period_msgs)) {
+            rotationPeriodMsgs = encryptionParams?.rotation_period_msgs;
+        }
+
+        if (this._now() > (createdAt + rotationPeriodMs)) {
+            return true;
+        }
+        if (session.message_index() >= rotationPeriodMsgs) {
+            return true;
+        }  
+    }
+
+    _encryptContent(roomId, session, type, content) {
+        const plaintext = JSON.stringify({
+            room_id: roomId,
+            type,
+            content
+        });
+        const ciphertext = session.encrypt(plaintext);
+
+        const encryptedContent = {
+            algorithm: MEGOLM_ALGORITHM,
+            sender_key: this._account.identityKeys.curve25519,
+            ciphertext,
+            session_id: session.session_id(),
+            device_id: this._ownDeviceId
+        };
+
+        return encryptedContent;
+    }
+
+    _createRoomKeyMessage(session, roomId) {
+        return {
+            room_id: roomId,
+            session_id: session.session_id(),
+            session_key: session.session_key(),
+            algorithm: MEGOLM_ALGORITHM,
+            // chain_index: session.message_index()
+        }
+    }
+
+    _storeAsInboundSession(outboundSession, roomId, txn) {
+        const {identityKeys} = this._account;
+        const claimedKeys = {ed25519: identityKeys.ed25519};
+        const session = new this._olm.InboundGroupSession();
+        try {
+            session.create(outboundSession.session_key());
+            const sessionEntry = {
+                roomId,
+                senderKey: identityKeys.curve25519,
+                sessionId: session.session_id(),
+                session: session.pickle(this._pickleKey),
+                claimedKeys,
+            };
+            txn.inboundGroupSessions.set(sessionEntry);
+            return sessionEntry;
+        } finally {
+            session.free();
+        }
+    }
+}
+
+class EncryptionResult {
+    constructor(content, roomKeyMessage) {
+        this.content = content;
+        this.roomKeyMessage = roomKeyMessage;
+    }
+}