derive ssss key from recovery key

src/matrix/e2ee/ssss/recoveryKey.js

@@ -0,0 +1,51 @@
+/*
+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 {base58} from "../../../utils/base-encoding.js";
+import {Key} from "./common.js";
+
+const OLM_RECOVERY_KEY_PREFIX = [0x8B, 0x01];
+
+export function keyFromRecoveryKey(olm, keyDescription, recoveryKey) {
+    const result = base58.decode(recoveryKey.replace(/ /g, ''));
+
+    let parity = 0;
+    for (const b of result) {
+        parity ^= b;
+    }
+    if (parity !== 0) {
+        throw new Error("Incorrect parity");
+    }
+
+    for (let i = 0; i < OLM_RECOVERY_KEY_PREFIX.length; ++i) {
+        if (result[i] !== OLM_RECOVERY_KEY_PREFIX[i]) {
+            throw new Error("Incorrect prefix");
+        }
+    }
+
+    if (
+        result.length !==
+        OLM_RECOVERY_KEY_PREFIX.length + olm.PRIVATE_KEY_LENGTH + 1
+    ) {
+        throw new Error("Incorrect length");
+    }
+
+    const keyBits = Uint8Array.from(result.slice(
+        OLM_RECOVERY_KEY_PREFIX.length,
+        OLM_RECOVERY_KEY_PREFIX.length + olm.PRIVATE_KEY_LENGTH,
+    ));
+
+    return new Key(keyDescription, keyBits);
+}