Cross sign the current device with the SSK

whenever we get the SSK, ie. when creating or restoring a backup

src/base-apis.js

@@ -1594,6 +1594,12 @@ MatrixBaseApis.prototype.uploadKeysRequest = function(content, opts, callback) {
     );
 };
 
+MatrixBaseApis.prototype.uploadKeySignatures = function(content) {
+    return this._http.authedRequestWithPrefix(
+        undefined, "POST", '/keys/signatures/upload', undefined, content, httpApi.PREFIX_UNSTABLE,
+    );
+};
+
 /**
  * Download device keys
  *

src/client.js

@@ -1070,8 +1070,10 @@ MatrixClient.prototype.createKeyBackupVersion = function(info, auth, replacesSsk
     };
 
     return this._cryptoStore.doTxn('readwrite', [IndexedDBCryptoStore.STORE_ACCOUNT], (txn) => {
+        // store the newly generated account keys
         this._cryptoStore.storeAccountKeys(txn, info.accountKeys);
     }).then(() => {
+        // upload the public part of the account keys
         return this.uploadDeviceSigningKeys(keys);
     }).then(() => {
         return this._crypto._signObject(data.auth_data);
@@ -1086,6 +1088,9 @@ MatrixClient.prototype.createKeyBackupVersion = function(info, auth, replacesSsk
             version: res.version,
         });
         return res;
+    }).then(() => {
+        // upload signatures between the SSK & this device
+        return this._crypto.uploadDeviceKeySignatures();
     });
 };
 
@@ -1199,8 +1204,6 @@ MatrixClient.prototype._restoreKeyBackup = async function(
     let totalKeyCount = 0;
     let keys = [];
 
-    const path = this._makeKeyBackupPath(targetRoomId, targetSessionId, backupInfo.version);
-
     const decryption = new global.Olm.PkDecryption();
     try {
         decryption.init_with_private_key(privKey);
@@ -1237,9 +1240,14 @@ MatrixClient.prototype._restoreKeyBackup = async function(
         throw e;
     }
 
-    return this._http.authedRequest(
-        undefined, "GET", path.path, path.queryData,
-    ).then((res) => {
+    // start by signing this device from the SSK now we have it
+    return this._crypto.uploadDeviceKeySignatures().then(() => {
+        // Now fetch the encrypted keys
+        const path = this._makeKeyBackupPath(targetRoomId, targetSessionId, backupInfo.version);
+        return this._http.authedRequest(
+            undefined, "GET", path.path, path.queryData,
+        );
+    }).then((res) => {
         if (res.rooms) {
             for (const [roomId, roomData] of Object.entries(res.rooms)) {
                 if (!roomData.sessions) continue;

src/crypto/index.js

@@ -46,6 +46,8 @@ import {
     newUnknownMethodError,
 } from './verification/Error';
 
+import { pkSign } from './PkSigning';
+
 const defaultVerificationMethods = {
     [ScanQRCode.NAME]: ScanQRCode,
     [ShowQRCode.NAME]: ShowQRCode,
@@ -457,8 +459,20 @@ Crypto.prototype.uploadDeviceKeys = function() {
         user_id: userId,
     };
 
+    let accountKeys;
     return crypto._signObject(deviceKeys).then(() => {
-        crypto._baseApis.uploadKeysRequest({
+        return this._cryptoStore.doTxn('readonly', [IndexedDBCryptoStore.STORE_ACCOUNT], (txn) => {
+            this._cryptoStore.getAccountKeys(txn, keys => {
+                accountKeys = keys;
+            });
+        });
+    }).then(() => {
+        if (accountKeys && accountKeys.self_signing_key_seed) {
+            // if we have an SSK, sign the key with the SSK too
+            pkSign(deviceKeys, Buffer.from(accountKeys.self_signing_key_seed, 'base64'), userId);
+        }
+
+        return crypto._baseApis.uploadKeysRequest({
             device_keys: deviceKeys,
         }, {
             // for now, we set the device id explicitly, as we may not be using the
@@ -468,6 +482,45 @@ Crypto.prototype.uploadDeviceKeys = function() {
     });
 };
 
+/**
+ * If a self-signing key is available, uploads the signature of this device from
+ * the self-signing key
+ *
+ * @return {bool} Promise: True if signatures were uploaded or otherwise false
+ *     (eg. if no account keys were available)
+ */
+Crypto.prototype.uploadDeviceKeySignatures = async function() {
+    const crypto = this;
+    const userId = crypto._userId;
+    const deviceId = crypto._deviceId;
+
+    const thisDeviceKey = {
+        algorithms: crypto._supportedAlgorithms,
+        device_id: deviceId,
+        keys: crypto._deviceKeys,
+        user_id: userId,
+    };
+    let accountKeys;
+    await this._cryptoStore.doTxn('readonly', [IndexedDBCryptoStore.STORE_ACCOUNT], (txn) => {
+        this._cryptoStore.getAccountKeys(txn, keys => {
+            accountKeys = keys;
+        });
+    });
+    if (!accountKeys || !accountKeys.self_signing_key_seed) return false;
+
+    // Sign this device with the SSK
+    pkSign(thisDeviceKey, Buffer.from(accountKeys.self_signing_key_seed, 'base64'), userId);
+
+    const content = {
+        [userId]: {
+            [deviceId]: thisDeviceKey,
+        },
+    };
+
+    await crypto._baseApis.uploadKeySignatures(content);
+    return true
+};
+
 /**
  * Stores the current one_time_key count which will be handled later (in a call of
  * onSyncCompleted). The count is e.g. coming from a /sync response.