electron-bridge: Add decrypt_clipboard helper.

This one helper allows us to implement browser-based social login
entirely on the server side.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

app/renderer/js/clipboard-decrypter.ts

@@ -0,0 +1,82 @@
+import {clipboard} from 'electron';
+import crypto from 'crypto';
+
+// This helper is exposed via electron_bridge for use in the social
+// login flow.
+//
+// It consists of a key and a promised token.  The in-app page sends
+// the key to the server, and opens the user’s browser to a page where
+// they can log in and get a token encrypted to that key.  When the
+// user copies the encrypted token from their browser to the
+// clipboard, we decrypt it and resolve the promise.  The in-app page
+// then uses the decrypted token to log the user in within the app.
+//
+// The encryption is authenticated (AES-GCM) to guarantee that we
+// don’t leak anything from the user’s clipboard other than the token
+// intended for us.
+
+export class ClipboardDecrypter {
+	version: number;
+	key: Uint8Array;
+	pasted: Promise<string>;
+
+	constructor(_: number) {
+		// At this time, the only version is 1.
+		this.version = 1;
+		this.key = crypto.randomBytes(32);
+		this.pasted = new Promise(resolve => {
+			let interval: NodeJS.Timeout | null = null;
+			const startPolling = () => {
+				if (interval === null) {
+					interval = setInterval(poll, 1000);
+				}
+
+				poll();
+			};
+
+			const stopPolling = () => {
+				if (interval !== null) {
+					clearInterval(interval);
+					interval = null;
+				}
+			};
+
+			const poll = () => {
+				let plaintext;
+
+				try {
+					const data = Buffer.from(clipboard.readText(), 'hex');
+					const iv = data.slice(0, 12);
+					const ciphertext = data.slice(12, -16);
+					const authTag = data.slice(-16);
+					const decipher = crypto.createDecipheriv(
+						'aes-256-gcm',
+						this.key,
+						iv,
+						{authTagLength: 16}
+					);
+					decipher.setAuthTag(authTag);
+					plaintext =
+						decipher.update(ciphertext, undefined, 'utf8') +
+						decipher.final('utf8');
+				} catch (_) {
+					// If the parsing or decryption failed in any way,
+					// the correct token hasn’t been copied yet; try
+					// again next time.
+					return;
+				}
+
+				window.removeEventListener('focus', startPolling);
+				window.removeEventListener('blur', stopPolling);
+				stopPolling();
+				resolve(plaintext);
+			};
+
+			window.addEventListener('focus', startPolling);
+			window.addEventListener('blur', stopPolling);
+			if (document.hasFocus()) {
+				startPolling();
+			}
+		});
+	}
+}

app/renderer/js/electron-bridge.ts

@@ -2,6 +2,7 @@ import {ipcRenderer} from 'electron';
 
 import {EventEmitter} from 'events';
 
+import {ClipboardDecrypter} from './clipboard-decrypter';
 import {NotificationData, newNotification} from './notification';
 
 type ListenerType = ((...args: any[]) => void);
@@ -46,6 +47,9 @@ class ElectronBridge extends EventEmitter {
 	set_send_notification_reply_message_supported = (value: boolean): void => {
 		this.send_notification_reply_message_supported = value;
 	};
+
+	decrypt_clipboard = (version: number): ClipboardDecrypter =>
+		new ClipboardDecrypter(version);
 }
 
 const electron_bridge = new ElectronBridge();