diff --git a/src/client.js b/src/client.js
index 9ff1762c0..da7d5218a 100644
--- a/src/client.js
+++ b/src/client.js
@@ -49,6 +49,7 @@ import RoomList from './crypto/RoomList';
 import Crypto from './crypto';
 import { isCryptoAvailable } from './crypto';
 import { encodeRecoveryKey, decodeRecoveryKey } from './crypto/recoverykey';
+import { keyForNewBackup, keyForExistingBackup } from './crypto/backup_password';
 
 // Disable warnings for now: we use deprecated bluebird functions
 // and need to migrate, but they spam the console with warnings.
@@ -860,22 +861,37 @@ MatrixClient.prototype.disableKeyBackup = function() {
  * Set up the data required to create a new backup version.  The backup version
  * will not be created and enabled until createKeyBackupVersion is called.
  *
+ * @param {string} password Passphrase string that can be entered by the user
+ *     when restoring the backup as an alternative to entering the recovery key.
+ *     Optional.
+ *
  * @returns {object} Object that can be passed to createKeyBackupVersion and
  *     additionally has a 'recovery_key' member with the user-facing recovery key string.
  */
-MatrixClient.prototype.prepareKeyBackupVersion = function() {
+MatrixClient.prototype.prepareKeyBackupVersion = async function(password) {
     if (this._crypto === null) {
         throw new Error("End-to-end encryption disabled");
     }
 
     const decryption = new global.Olm.PkDecryption();
     try {
-        const publicKey = decryption.generate_key();
+        let privateKey;
+        let publicKey;
+        let authData = {};
+        if (password) {
+            const keyInfo = await keyForNewBackup(password);
+            publicKey = decryption.init_with_private_key(keyInfo.key);
+            authData.private_key_salt = keyInfo.salt;
+            authData.private_key_iterations = keyInfo.iterations;
+        } else {
+            publicKey = decryption.generate_key();
+        }
+
+        authData.public_key = publicKey;
+
         return {
             algorithm: olmlib.MEGOLM_BACKUP_ALGORITHM,
-            auth_data: {
-                public_key: publicKey,
-            },
+            auth_data: authData,
             recovery_key: encodeRecoveryKey(decryption.get_private_key()),
         };
     } finally {
@@ -992,8 +1008,28 @@ MatrixClient.prototype.isValidRecoveryKey = function(recoveryKey) {
     }
 };
 
-MatrixClient.prototype.restoreKeyBackups = function(
+MatrixClient.prototype.restoreKeyBackupWithPassword = async function(
+    password, targetRoomId, targetSessionId, version,
+) {
+    const backupInfo = await this.getKeyBackupVersion();
+
+    const privKey = keyForExistingBackup(backupInfo, password);
+    return this._restoreKeyBackup(
+        privKey, targetRoomId, targetSessionId, version,
+    );
+};
+
+MatrixClient.prototype.restoreKeyBackupWithRecoveryKey = function(
     recoveryKey, targetRoomId, targetSessionId, version,
+) {
+    const privKey = decodeRecoveryKey(recoveryKey);
+    return this._restoreKeyBackup(
+        privKey, targetRoomId, targetSessionId, version,
+    );
+};
+
+MatrixClient.prototype._restoreKeyBackup = function(
+    privKey, targetRoomId, targetSessionId, version,
 ) {
     if (this._crypto === null) {
         throw new Error("End-to-end encryption disabled");
@@ -1003,8 +1039,6 @@ MatrixClient.prototype.restoreKeyBackups = function(
 
     const path = this._makeKeyBackupPath(targetRoomId, targetSessionId, version);
 
-    // FIXME: see the FIXME in createKeyBackupVersion
-    const privkey = decodeRecoveryKey(recoveryKey);
     const decryption = new global.Olm.PkDecryption();
     try {
         decryption.init_with_private_key(privkey);
diff --git a/src/crypto/backup_password.js b/src/crypto/backup_password.js
new file mode 100644
index 000000000..946b99681
--- /dev/null
+++ b/src/crypto/backup_password.js
@@ -0,0 +1,81 @@
+/*
+Copyright 2018 New Vector Ltd
+
+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 { randomString } from '../randomstring';
+
+const DEFAULT_ITERATIONS = 500000;
+
+export async function keyForExistingBackup(backupData, password) {
+    if (!global.Olm) {
+        throw new Error("Olm is not available");
+    }
+
+    const authData = backupData.auth_data;
+
+    if (!authData.private_key_salt || !authData.private_key_iterations) {
+        throw new Error(
+            "Salt and/or iterations not found: " +
+            "this backup cannot be restored with a passphrase",
+        );
+    }
+
+    return await deriveKey(
+        password, backupData.private_key_salt,
+        backupData.private_key_iterations,
+    );
+}
+
+export async function keyForNewBackup(password) {
+    if (!global.Olm) {
+        throw new Error("Olm is not available");
+    }
+
+    const salt = randomString(32);
+
+    const key = await deriveKey(password, salt, DEFAULT_ITERATIONS);
+
+    return { key, salt, iterations: DEFAULT_ITERATIONS };
+}
+
+async function deriveKey(password, salt, iterations) {
+    const subtleCrypto = global.crypto.subtle;
+    const TextEncoder = global.TextEncoder;
+    if (!subtleCrypto || !TextEncoder) {
+        // TODO: Implement this for node
+        throw new Error("Password-based backup is not avaiable on this platform");
+    }
+
+    const key = await subtleCrypto.importKey(
+        'raw',
+        new TextEncoder().encode(password),
+        {name: 'PBKDF2'},
+        false,
+        ['deriveBits'],
+    );
+
+    const keybits = await subtleCrypto.deriveBits(
+        {
+            name: 'PBKDF2',
+            salt: new TextEncoder().encode(salt),
+            iterations: iterations,
+            hash: 'SHA-512',
+        },
+        key,
+        global.Olm.PRIVATE_KEY_LENGTH * 8,
+    );
+
+    return new Uint8Array(keybits);
+}