first draft of SessionBackup

src/matrix/e2ee/megolm/SessionBackup.js

@@ -0,0 +1,62 @@
+/*
+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 {base64} from "../../utils/base-encoding.js";
+
+export class SessionBackup {
+    constructor({olm, backupInfo, privateKey, hsApi}) {
+        this._olm = olm;
+        this._backupInfo = backupInfo;
+        this._privateKey = privateKey;
+        this._hsApi = hsApi;
+    }
+
+    async getSession(roomId, sessionId) {
+        const sessionResponse = await this._hsApi.roomKey(this._backupInfo.version, roomId, sessionId).response();
+        let sessionInfo;
+        const decryption = new this._olm.PkDecryption();
+        try {
+            decryption.init_with_private_key(this._privateKey);
+            sessionInfo = this._decryption.decrypt(
+                sessionResponse.session_data.ephemeral,
+                sessionResponse.session_data.mac,
+                sessionResponse.session_data.ciphertext,
+            );
+        } finally {
+            decryption.free();
+        }
+        return JSON.parse(sessionInfo);
+    }
+
+    static async fromSecretStorage({olm, secretStorage, hsApi}) {
+        const backupInfo = await hsApi.roomKeysVersion().response();
+        const base64PrivateKey = await secretStorage.readSecret("m.megolm_backup.v1");
+        if (base64PrivateKey) {
+            const privateKey = base64.decode(base64PrivateKey);
+            const decryption = new olm.PkDecryption();
+            try {
+                const pubKey = decryption.init_with_private_key(this._privateKey);
+                if (pubKey !== backupInfo.auth_data.public_key) {
+                    throw new Error(`Bad backup key, public key does not match. Calculated ${pubKey} but expected ${backupInfo.auth_data.public_key}`);
+                }
+            } finally {
+                decryption.free();
+            }
+            return new SessionBackup({olm, backupInfo, privateKey, hsApi});
+        }
+    }
+}
+