language-learning-app/frontend/src/hooks.server.ts

import { createHmac, timingSafeEqual } from 'crypto';
import { redirect, type Handle } from '@sveltejs/kit';
import { PRIVATE_JWT_SECRET } from '$env/static/private';
import { client } from './lib/apiClient.ts';

function verifyJwt(token: string, secret: string): boolean {
	try {
		const parts = token.split('.');
		if (parts.length !== 3) return false;
		const [header, payload, sig] = parts;

		const expected = createHmac('sha256', secret)
			.update(`${header}.${payload}`)
			.digest('base64url');

		const expectedBuf = Buffer.from(expected);
		const sigBuf = Buffer.from(sig);
		if (expectedBuf.length !== sigBuf.length) return false;
		if (!timingSafeEqual(expectedBuf, sigBuf)) return false;

		const claims = JSON.parse(Buffer.from(payload, 'base64url').toString('utf8'));
		if (claims.exp && claims.exp < Date.now() / 1000) return false;

		return true;
	} catch {
		return false;
	}
}

export const handle: Handle = async ({ event, resolve }) => {
	event.locals.apiClient = client;

	const rawToken = event.cookies.get('auth_token');
	const isValid = rawToken ? verifyJwt(rawToken, PRIVATE_JWT_SECRET) : false;
	event.locals.authToken = isValid ? rawToken! : null;

	if (event.url.pathname.startsWith('/app') && !isValid) {
		return redirect(307, '/login');
	}

	return resolve(event);
};