Allow key backup restore to get the key backup passphrase

And pass this in as the new callback to bootstrap to get the old
key backup passphrase.

src/CrossSigningManager.js

@@ -162,6 +162,20 @@ export const crossSigningCallbacks = {
     onSecretRequested,
 };
 
+export async function promptForBackupPassphrase() {
+    let key;
+
+    const RestoreKeyBackupDialog = sdk.getComponent('dialogs.keybackup.RestoreKeyBackupDialog');
+    const { finished } = Modal.createTrackedDialog('Restore Backup', '', RestoreKeyBackupDialog, {
+            showSummary: false, keyCallback: k => key = k,
+    }, null, /* priority = */ false, /* static = */ false);
+
+    const success = await finished;
+    if (!success) throw new Error("Key backup prompt cancelled");
+
+    return key;
+}
+
 /**
  * This helper should be used whenever you need to access secret storage. It
  * ensures that secret storage (and also cross-signing since they each depend on
@@ -218,6 +232,7 @@ export async function accessSecretStorage(func = async () => { }, force = false)
                         throw new Error("Cross-signing key upload auth canceled");
                     }
                 },
+                getBackupPassphrase: promptForBackupPassphrase,
             });
         }
 

src/async-components/views/dialogs/secretstorage/CreateSecretStorageDialog.js

@@ -23,6 +23,7 @@ import { scorePassword } from '../../../../utils/PasswordScorer';
 import FileSaver from 'file-saver';
 import { _t } from '../../../../languageHandler';
 import Modal from '../../../../Modal';
+import { promptForBackupPassphrase } from '../../../../CrossSigningManager';
 
 const PHASE_LOADING = 0;
 const PHASE_MIGRATE = 1;
@@ -243,6 +244,7 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
                     createSecretStorageKey: async () => this._keyInfo,
                     keyBackupInfo: this.state.backupInfo,
                     setupNewKeyBackup: !this.state.backupInfo && this.state.useKeyBackup,
+                    getKeyBackupPassphrase: promptForBackupPassphrase,
                 });
             }
             this.setState({

src/components/views/dialogs/keybackup/RestoreKeyBackupDialog.js

@@ -36,6 +36,9 @@ export default class RestoreKeyBackupDialog extends React.PureComponent {
         // if false, will close the dialog as soon as the restore completes succesfully
         // default: true
         showSummary: PropTypes.bool,
+        // If specified, gather the key from the user but then call the function with the backup
+        // key rather than actually (necessarily) restoring the backup.
+        keyCallback: PropTypes.func,
     };
 
     static defaultProps = {
@@ -103,9 +106,18 @@ export default class RestoreKeyBackupDialog extends React.PureComponent {
             restoreType: RESTORE_TYPE_PASSPHRASE,
         });
         try {
+            // We do still restore the key backup: we must ensure that the key backup key
+            // is the right one and restoring it is currently the only way we can do this.
             const recoverInfo = await MatrixClientPeg.get().restoreKeyBackupWithPassword(
                 this.state.passPhrase, undefined, undefined, this.state.backupInfo,
             );
+            if (this.props.keyCallback) {
+                const key = await MatrixClientPeg.get().keyBackupKeyFromPassword(
+                    this.state.passPhrase, this.state.backupInfo,
+                );
+                this.props.keyCallback(key);
+            }
+
             if (!this.props.showSummary) {
                 this.props.onFinished(true);
                 return;
@@ -135,6 +147,10 @@ export default class RestoreKeyBackupDialog extends React.PureComponent {
             const recoverInfo = await MatrixClientPeg.get().restoreKeyBackupWithRecoveryKey(
                 this.state.recoveryKey, undefined, undefined, this.state.backupInfo,
             );
+            if (this.props.keyCallback) {
+                const key = MatrixClientPeg.get().keyBackupKeyFromRecoveryKey(this.state.recoveryKey);
+                this.props.keyCallback(key);
+            }
             if (!this.props.showSummary) {
                 this.props.onFinished(true);
                 return;