diff --git a/src/matrix/e2ee/ssss/passphrase.js b/src/matrix/e2ee/ssss/passphrase.js
new file mode 100644
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+++ b/src/matrix/e2ee/ssss/passphrase.js
@@ -0,0 +1,35 @@
+/*
+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 {Key} from "./common.js";
+
+const DEFAULT_ITERATIONS = 500000;
+const DEFAULT_BITSIZE = 256;
+
+export async function keyFromPassphrase(keyDescription, passphrase, cryptoDriver) {
+    const {passphraseParams} = keyDescription;
+    if (!passphraseParams) {
+        throw new Error("not a passphrase key");
+    }
+    const textEncoder = new TextEncoder();
+    const keyBits = await cryptoDriver.derive.pbkdf2(
+        textEncoder.encode(passphrase),
+        passphraseParams.iterations || DEFAULT_ITERATIONS,
+        textEncoder.encode(passphraseParams.salt),
+        "SHA-512",
+        passphraseParams.bits || DEFAULT_BITSIZE);
+    return new Key(keyDescription, keyBits);
+}