codegraph/telemetry-dashboard/src/auth.ts

/**
 * Shared-password auth for the admin dashboard.
 *
 * Exactly two humans use this dashboard, so there is no user table, no auth
 * provider and no session store: one password in a secret, and a long-lived
 * HMAC-signed cookie so you sign in once per browser.
 *
 * Properties worth keeping if you touch this file:
 * - the password is compared in constant time (over SHA-256 digests, so the
 *   lengths always match and the comparison leaks nothing about the secret);
 * - the cookie is a signed assertion, not a lookup key — nothing is stored
 *   server-side, and a tampered payload fails the HMAC check;
 * - the session is bound to a fingerprint of the password, so rotating
 *   ADMIN_PASSWORD invalidates every cookie already out there.
 */

const COOKIE_NAME = 'cg_admin_session';
/** ~1 year. Long-lived on purpose: two users, one password, sign in once. */
const SESSION_TTL_SECONDS = 365 * 24 * 60 * 60;
const SESSION_VERSION = 1;

const encoder = new TextEncoder();
const decoder = new TextDecoder();

interface SessionPayload {
  v: number;
  iat: number;
  exp: number;
  /** Fingerprint of the password this session was minted against. */
  pw: string;
}

function base64UrlEncode(bytes: Uint8Array): string {
  let binary = '';
  for (const byte of bytes) binary += String.fromCharCode(byte);
  return btoa(binary).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
}

function base64UrlDecode(text: string): Uint8Array | null {
  try {
    const binary = atob(text.replace(/-/g, '+').replace(/_/g, '/'));
    const bytes = new Uint8Array(binary.length);
    for (let i = 0; i < binary.length; i++) bytes[i] = binary.charCodeAt(i);
    return bytes;
  } catch {
    return null;
  }
}

// Importing the HMAC key costs a round through WebCrypto; cache it per isolate.
// A secret rotation ships a new deployment, which means new isolates.
let cachedKey: { secret: string; key: CryptoKey } | null = null;

async function hmacKey(secret: string): Promise<CryptoKey> {
  if (cachedKey?.secret === secret) return cachedKey.key;
  const key = await crypto.subtle.importKey(
    'raw',
    encoder.encode(secret),
    { name: 'HMAC', hash: 'SHA-256' },
    false,
    ['sign'],
  );
  cachedKey = { secret, key };
  return key;
}

async function sign(secret: string, payload: string): Promise<Uint8Array> {
  const signature = await crypto.subtle.sign('HMAC', await hmacKey(secret), encoder.encode(payload));
  return new Uint8Array(signature);
}

/**
 * Constant-time string equality. Both sides are hashed first so the digests are
 * always the same length — `timingSafeEqual` throws on a length mismatch, and a
 * throw would itself leak the length of the secret.
 */
async function equalsInConstantTime(a: string, b: string): Promise<boolean> {
  const [digestA, digestB] = await Promise.all([
    crypto.subtle.digest('SHA-256', encoder.encode(a)),
    crypto.subtle.digest('SHA-256', encoder.encode(b)),
  ]);
  return crypto.subtle.timingSafeEqual(digestA, digestB);
}

/** Short, non-reversible marker of the current password, embedded in the session. */
async function passwordFingerprint(password: string): Promise<string> {
  const digest = await crypto.subtle.digest('SHA-256', encoder.encode(`cg-admin-pw\u0000${password}`));
  return base64UrlEncode(new Uint8Array(digest).subarray(0, 8));
}

export async function checkPassword(env: Env, submitted: string): Promise<boolean> {
  // A misconfigured deployment must not become an open dashboard.
  if (!env.ADMIN_PASSWORD || !env.SESSION_SECRET) return false;
  return equalsInConstantTime(submitted, env.ADMIN_PASSWORD);
}

export async function issueSession(env: Env): Promise<string> {
  const now = Math.floor(Date.now() / 1000);
  const payload: SessionPayload = {
    v: SESSION_VERSION,
    iat: now,
    exp: now + SESSION_TTL_SECONDS,
    pw: await passwordFingerprint(env.ADMIN_PASSWORD),
  };
  const encoded = base64UrlEncode(encoder.encode(JSON.stringify(payload)));
  return `${encoded}.${base64UrlEncode(await sign(env.SESSION_SECRET, encoded))}`;
}

export async function hasValidSession(env: Env, request: Request): Promise<boolean> {
  if (!env.ADMIN_PASSWORD || !env.SESSION_SECRET) return false;

  const token = readSessionCookie(request);
  if (!token) return false;

  const dot = token.indexOf('.');
  if (dot <= 0 || dot === token.length - 1) return false;
  const encoded = token.slice(0, dot);

  const provided = base64UrlDecode(token.slice(dot + 1));
  if (!provided) return false;
  const expected = await sign(env.SESSION_SECRET, encoded);
  // Length is checked first: timingSafeEqual throws on mismatched lengths, and
  // the length of an HMAC-SHA256 tag is public anyway.
  if (provided.byteLength !== expected.byteLength) return false;
  if (!crypto.subtle.timingSafeEqual(provided, expected)) return false;

  const raw = base64UrlDecode(encoded);
  if (!raw) return false;
  let payload: SessionPayload;
  try {
    payload = JSON.parse(decoder.decode(raw)) as SessionPayload;
  } catch {
    return false;
  }

  if (payload?.v !== SESSION_VERSION) return false;
  if (typeof payload.exp !== 'number' || payload.exp <= Math.floor(Date.now() / 1000)) return false;
  // Rotating ADMIN_PASSWORD signs everyone out.
  return payload.pw === (await passwordFingerprint(env.ADMIN_PASSWORD));
}

export function readSessionCookie(request: Request): string | null {
  const header = request.headers.get('cookie');
  if (!header) return null;
  for (const part of header.split(';')) {
    const cookie = part.trim();
    if (cookie.startsWith(`${COOKIE_NAME}=`)) return cookie.slice(COOKIE_NAME.length + 1);
  }
  return null;
}

export function sessionCookie(token: string): string {
  // Secure is accepted on http://localhost too, so `wrangler dev` still works.
  return `${COOKIE_NAME}=${token}; HttpOnly; Secure; SameSite=Lax; Path=/; Max-Age=${SESSION_TTL_SECONDS}`;
}

export function clearedSessionCookie(): string {
  return `${COOKIE_NAME}=; HttpOnly; Secure; SameSite=Lax; Path=/; Max-Age=0`;
}

/**
 * Rejects a cross-site form post. SameSite=Lax already blocks the cookie on a
 * cross-site POST; this is the belt to that pair of braces.
 */
export function isSameOriginPost(request: Request): boolean {
  const origin = request.headers.get('origin');
  if (!origin) return true; // Absent on some legitimate non-browser clients (curl).
  // "null" is what Chromium sends on a SAME-origin form submit when the page
  // carries our own `Referrer-Policy: no-referrer` header — an unattributed
  // origin, not a foreign one. Treat it like an absent header: rejecting it
  // locks every Chromium user out of the login form, while an attacker who
  // knows the password never needed a browser in the first place.
  if (origin === 'null') return true;
  try {
    return new URL(origin).host === new URL(request.url).host;
  } catch {
    return false;
  }
}