// Symmetric encryption for secrets at rest (paid posts: the site owner's
// Patreon creator token, klonkt-demo-aki slice 1). AES-256-GCM with a key
// derived from PAID_SECRET (env), so a database dump alone leaks nothing usable.
// Format: base64(iv) : base64(tag) : base64(ciphertext).
//
// The codebase had no symmetric-crypto helper; this is that one, kept tiny and
// native (no dependency). If PAID_SECRET is unset, encryption is refused loudly
// rather than storing plaintext.
import crypto from 'crypto';

let _key = null;
function key() {
  if (_key) return _key;
  const secret = process.env.PAID_SECRET;
  if (!secret || String(secret).length < 16) {
    throw new Error('PAID_SECRET is missing or too short (need >= 16 chars) to encrypt paid-posts secrets');
  }
  _key = crypto.scryptSync(String(secret), 'klonkt-paid', 32);
  return _key;
}

// True when a key is configured, so callers can gate the feature instead of throwing.
export function cryptoBoxReady() {
  try { key(); return true; } catch { return false; }
}

export function encrypt(plaintext) {
  if (plaintext == null) return null;
  const iv = crypto.randomBytes(12);
  const cipher = crypto.createCipheriv('aes-256-gcm', key(), iv);
  const ct = Buffer.concat([cipher.update(String(plaintext), 'utf8'), cipher.final()]);
  const tag = cipher.getAuthTag();
  return `${iv.toString('base64')}:${tag.toString('base64')}:${ct.toString('base64')}`;
}

export function decrypt(blob) {
  if (blob == null || blob === '') return null;
  const parts = String(blob).split(':');
  if (parts.length !== 3) throw new Error('malformed ciphertext');
  const [iv, tag, ct] = parts.map((p) => Buffer.from(p, 'base64'));
  const decipher = crypto.createDecipheriv('aes-256-gcm', key(), iv);
  decipher.setAuthTag(tag);
  return Buffer.concat([decipher.update(ct), decipher.final()]).toString('utf8');
}

// The stateless signed blob reused for the OAuth state and the WebAuthn
// challenge (design doc "cookie-less trick"): HMAC over a short-lived payload,
// so no server session is needed to bind pending state to a browser.
export function signBlob(payload, ttlSeconds = 600) {
  const body = { ...payload, exp: Math.floor(Date.now() / 1000) + ttlSeconds, nonce: crypto.randomBytes(8).toString('hex') };
  const b = Buffer.from(JSON.stringify(body)).toString('base64url');
  const tag = crypto.createHmac('sha256', key()).update(b).digest('base64url');
  return `${b}.${tag}`;
}

// Returns the payload if valid and unexpired, else null. Constant-time tag check.
export function verifyBlob(token) {
  const [b, tag] = String(token || '').split('.');
  if (!b || !tag) return null;
  const expected = crypto.createHmac('sha256', key()).update(b).digest('base64url');
  const a = Buffer.from(tag); const e = Buffer.from(expected);
  if (a.length !== e.length || !crypto.timingSafeEqual(a, e)) return null;
  let payload; try { payload = JSON.parse(Buffer.from(b, 'base64url').toString('utf8')); } catch { return null; }
  if (!payload || (payload.exp && payload.exp * 1000 < Date.now())) return null;
  return payload;
}