2 files changed, 147 insertions, 0 deletions
diff --git a/src/lib/server/auth.test.ts b/src/lib/server/auth.test.ts
new file mode 100644
index 0000000..59ddabc
--- /dev/null
+++ b/src/lib/server/auth.test.ts
@@ -0,0 +1,59 @@
+import { describe, expect, it, vi } from 'vitest';
+import type { Cookies } from '@sveltejs/kit';
+import {
+	AUTH_COOKIE_NAME,
+	SESSION_MAX_AGE_SECONDS,
+	createSession,
+	isValidSession,
+	renewSession,
+	verifyPassword,
+} from './auth';
+
+const TEST_PASSWORD = vi.hoisted(() => {
+	return 'family-password-5f2c1d7e';
+});
+const NOW = 1_800_000_000_000;
+
+vi.mock('$env/dynamic/private', () => ({
+	env: { AUTH_PASSWORD: TEST_PASSWORD },
+}));
+
+describe('authentication', () => {
+	it('verifies the configured shared password', () => {
+		expect(verifyPassword(TEST_PASSWORD)).toBe(true);
+		expect(verifyPassword('wrong-password')).toBe(false);
+	});
+
+	it('accepts a signed session until its expiry', () => {
+		const session = createSession(NOW);
+
+		expect(
+			isValidSession(session, NOW + SESSION_MAX_AGE_SECONDS * 1000 - 1),
+		).toBe(true);
+		expect(isValidSession(session, NOW + SESSION_MAX_AGE_SECONDS * 1000)).toBe(
+			false,
+		);
+		expect(isValidSession(`${session}x`, NOW)).toBe(false);
+	});
+
+	it('renews a valid session with the requested cookie settings', () => {
+		const session = createSession(NOW);
+		const cookies = {
+			get: vi.fn(() => session),
+			set: vi.fn(),
+		} as unknown as Cookies;
+
+		expect(renewSession(cookies, true, NOW + 1_000)).toBe(true);
+		expect(cookies.set).toHaveBeenCalledWith(
+			AUTH_COOKIE_NAME,
+			expect.any(String),
+			expect.objectContaining({
+				path: '/',
+				httpOnly: true,
+				secure: true,
+				sameSite: 'lax',
+				maxAge: SESSION_MAX_AGE_SECONDS,
+			}),
+		);
+	});
+});
diff --git a/src/lib/server/auth.ts b/src/lib/server/auth.ts
new file mode 100644
index 0000000..f80eda0
--- /dev/null
+++ b/src/lib/server/auth.ts
@@ -0,0 +1,88 @@
+import { createHash, createHmac, timingSafeEqual } from 'node:crypto';
+import type { Cookies } from '@sveltejs/kit';
+import { env } from '$env/dynamic/private';
+
+export const AUTH_COOKIE_NAME = 'home_auth';
+export const SESSION_MAX_AGE_SECONDS = 60 * 60 * 24 * 7;
+
+const SESSION_MAX_AGE_MS = SESSION_MAX_AGE_SECONDS * 1000;
+
+function configuredPassword(): string {
+	if (!env.AUTH_PASSWORD) {
+		throw new Error('AUTH_PASSWORD is not configured');
+	}
+	return env.AUTH_PASSWORD;
+}
+
+function digest(value: string): Buffer {
+	return createHash('sha256').update(value).digest();
+}
+
+function sign(value: string): string {
+	return createHmac('sha256', configuredPassword())
+		.update(value)
+		.digest('base64url');
+}
+
+export function verifyPassword(candidate: string): boolean {
+	const expected = digest(configuredPassword());
+	const actual = digest(candidate);
+	return timingSafeEqual(actual, expected);
+}
+
+export function createSession(now = Date.now()): string {
+	const expiresAt = now + SESSION_MAX_AGE_MS;
+	const payload = String(expiresAt);
+	return `${payload}.${sign(payload)}`;
+}
+
+export function isValidSession(
+	value: string | undefined,
+	now = Date.now(),
+): boolean {
+	if (!value) return false;
+
+	const [expiresAtText, signature, extra] = value.split('.');
+	if (!expiresAtText || !signature || extra) return false;
+
+	const expiresAt = Number(expiresAtText);
+	if (!Number.isSafeInteger(expiresAt) || expiresAt <= now) return false;
+
+	const expected = Buffer.from(sign(expiresAtText), 'base64url');
+	const actual = Buffer.from(signature, 'base64url');
+	if (actual.length !== expected.length) return false;
+
+	return timingSafeEqual(actual, expected);
+}
+
+export function renewSession(
+	cookies: Cookies,
+	secure: boolean,
+	now = Date.now(),
+): boolean {
+	const current = cookies.get(AUTH_COOKIE_NAME);
+	if (!isValidSession(current, now)) return false;
+
+	cookies.set(AUTH_COOKIE_NAME, createSession(now), {
+		path: '/',
+		httpOnly: true,
+		secure,
+		sameSite: 'lax',
+		maxAge: SESSION_MAX_AGE_SECONDS,
+	});
+	return true;
+}
+
+export function setSession(
+	cookies: Cookies,
+	secure: boolean,
+	now = Date.now(),
+): void {
+	cookies.set(AUTH_COOKIE_NAME, createSession(now), {
+		path: '/',
+		httpOnly: true,
+		secure,
+		sameSite: 'lax',
+		maxAge: SESSION_MAX_AGE_SECONDS,
+	});
+}