first draft of reading a ssss secret

src/matrix/e2ee/ssss/SecretStorage.js

@@ -0,0 +1,78 @@
+/*
+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 SecretStorage {
+    constructor(key, storage, cryptoDriver) {
+        this._key = key;
+        this._storage = storage;
+        this._cryptoDriver = cryptoDriver;
+    }
+
+    async readSecret(name) {
+        const txn = await this._storage.readTxn([
+            this._storage.storeNames.accountData
+        ]);
+        const accountData = await txn.accountData.get(name);
+        if (!accountData) {
+            return;
+        }
+        const encryptedData = accountData?.content?.encrypted?.[this._key.id];
+        if (!encryptedData) {
+            throw new Error(`Secret ${accountData.type} is not encrypted for key ${this._key.id}`);
+        }
+
+        let json;
+        if (this._key.algorithm === "m.secret_storage.v1.aes-hmac-sha2") {
+            json = await this._decryptAESSecret(accountData.type, encryptedData);
+        } else {
+            throw new Error(`Unsupported algorithm for key ${this._key.id}: ${this._key.algorithm}`);
+        }
+        return JSON.parse(json);
+    }
+
+    async _decryptAESSecret(type, encryptedData) {
+        // TODO: we should we move this to platform specific code
+        const textEncoder = new TextEncoder();
+        const textDecoder = new TextDecoder();
+        // now derive the aes and mac key from the 4s key
+        const hkdfKey = await this._cryptoDriver.derive.hkdf(
+            this._key.binaryKey,
+            new Uint8Array(8).buffer,   //zero salt
+            textEncoder.encode(type), // info
+            "SHA-256",
+            512 // 512 bits or 64 bytes
+        );
+        const aesKey = hkdfKey.slice(0, 32);
+        const hmacKey = hkdfKey.slice(32);
+
+        const ciphertextBytes = base64.decode(encryptedData.ciphertext);
+
+        const isVerified = await this._cryptoDriver.hmac.verify(
+            hmacKey, base64.decode(encryptedData.mac),
+            ciphertextBytes, "SHA-256");
+
+        if (!isVerified) {
+            throw new Error("Bad MAC");
+        }
+
+        const plaintextBytes = await this._cryptoDriver.aes.decrypt(
+            aesKey, base64.decode(encryptedData.iv), ciphertextBytes);
+
+        return textDecoder.decode(plaintextBytes);
+    }
+}