Enforce Secure Backup completion when requested by HS

This removes all buttons to escape the Secure Backup setup flow when the
matching `.well-known` setting is set by homeserver.

Part of https://github.com/vector-im/element-web/issues/14954

src/CrossSigningManager.js

@@ -21,6 +21,7 @@ import { deriveKey } from 'matrix-js-sdk/src/crypto/key_passphrase';
 import { decodeRecoveryKey } from 'matrix-js-sdk/src/crypto/recoverykey';
 import { _t } from './languageHandler';
 import {encodeBase64} from "matrix-js-sdk/src/crypto/olmlib";
+import { isSecureBackupRequired } from './utils/WellKnownUtils';
 
 // This stores the secret storage private keys in memory for the JS SDK. This is
 // only meant to act as a cache to avoid prompting the user multiple times
@@ -208,7 +209,18 @@ export async function accessSecretStorage(func = async () => { }, forceReset = f
                 {
                     force: forceReset,
                 },
-                null, /* priority = */ false, /* static = */ true,
+                null,
+                /* priority = */ false,
+                /* static = */ true,
+                /* options = */ {
+                    onBeforeClose(reason) {
+                        // If Secure Backup is required, you cannot leave the modal.
+                        if (reason === "backgroundClick") {
+                            return !isSecureBackupRequired();
+                        }
+                        return true;
+                    },
+                },
             );
             const [confirmed] = await finished;
             if (!confirmed) {

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

@@ -30,6 +30,7 @@ import StyledRadioButton from '../../../../components/views/elements/StyledRadio
 import AccessibleButton from "../../../../components/views/elements/AccessibleButton";
 import DialogButtons from "../../../../components/views/elements/DialogButtons";
 import InlineSpinner from "../../../../components/views/elements/InlineSpinner";
+import { isSecureBackupRequired } from '../../../../utils/WellKnownUtils';
 
 const PHASE_LOADING = 0;
 const PHASE_LOADERROR = 1;
@@ -85,8 +86,8 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
             canUploadKeysWithPasswordOnly: null,
             accountPassword: props.accountPassword || "",
             accountPasswordCorrect: null,
-
             passPhraseKeySelected: CREATE_STORAGE_OPTION_KEY,
+            canSkip: !isSecureBackupRequired(),
         };
 
         this._passphraseField = createRef();
@@ -470,7 +471,7 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
                 primaryButton={_t("Continue")}
                 onPrimaryButtonClick={this._onChooseKeyPassphraseFormSubmit}
                 onCancel={this._onCancelClick}
-                hasCancel={true}
+                hasCancel={this.state.canSkip}
             />
         </form>;
     }
@@ -687,7 +688,7 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
             <div className="mx_Dialog_buttons">
                 <DialogButtons primaryButton={_t('Retry')}
                     onPrimaryButtonClick={this._onLoadRetryClick}
-                    hasCancel={true}
+                    hasCancel={this.state.canSkip}
                     onCancel={this._onCancel}
                 />
             </div>
@@ -742,7 +743,7 @@ export default class CreateSecretStorageDialog extends React.PureComponent {
                 <div className="mx_Dialog_buttons">
                     <DialogButtons primaryButton={_t('Retry')}
                         onPrimaryButtonClick={this._bootstrapSecretStorage}
-                        hasCancel={true}
+                        hasCancel={this.state.canSkip}
                         onCancel={this._onCancel}
                     />
                 </div>

src/utils/WellKnownUtils.ts

@@ -33,3 +33,8 @@ export function getE2EEWellKnown(): IE2EEWellKnown {
     }
     return null;
 }
+
+export function isSecureBackupRequired(): boolean {
+    const wellKnown = getE2EEWellKnown();
+    return wellKnown && wellKnown["secure_backup_required"] === true;
+}