/**
 * crypto.js — Zero-knowledge cryptography layer
 *
 * All encryption/decryption runs in the browser using the Web Crypto API.
 * The vault key (AES-256-GCM) is derived from the master password client-side
 * and is NEVER sent to the server. The server only stores encrypted blobs.
 *
 * Key derivation chain:
 *   authHash  = PBKDF2(masterPassword, email,        100_000 iter, SHA-256) → sent to server for auth
 *   vaultKey  = PBKDF2(masterPassword, enc_key_salt, 600_000 iter, SHA-256) → stays in memory only
 */

const Crypto = (() => {
  const subtle = window.crypto.subtle;

  // ── Helpers ──────────────────────────────────────────────────────────────

  function strToBytes(str) {
    return new TextEncoder().encode(str);
  }

  function base64ToBytes(b64) {
    const bin = atob(b64);
    const bytes = new Uint8Array(bin.length);
    for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i);
    return bytes;
  }

  function bytesToBase64(bytes) {
    let bin = '';
    bytes.forEach(b => (bin += String.fromCharCode(b)));
    return btoa(bin);
  }

  // ── PBKDF2 key import ────────────────────────────────────────────────────

  async function importPbkdf2Key(masterPassword) {
    return subtle.importKey(
      'raw',
      strToBytes(masterPassword),
      'PBKDF2',
      false,         // not extractable
      ['deriveBits', 'deriveKey']
    );
  }

  // ── Auth hash (sent to server) ───────────────────────────────────────────

  /**
   * Derive an auth token from the master password.
   * Used ONLY for server-side authentication — never for encryption.
   * Returns a base64 string safe to POST to /api/auth/login|register.
   */
  async function deriveAuthHash(masterPassword, email) {
    const baseKey = await importPbkdf2Key(masterPassword);
    const bits = await subtle.deriveBits(
      {
        name: 'PBKDF2',
        salt: strToBytes(email.toLowerCase()),
        iterations: 100_000,
        hash: 'SHA-256',
      },
      baseKey,
      256
    );
    return bytesToBase64(new Uint8Array(bits));
  }

  // ── Vault key (stays in memory) ──────────────────────────────────────────

  /**
   * Derive the AES-256-GCM vault key from the master password.
   * enc_key_salt is the base64-encoded 16-byte salt returned by the server on login.
   * The returned CryptoKey is marked extractable:false — raw bytes cannot be read back.
   */
  async function deriveVaultKey(masterPassword, enc_key_salt) {
    const baseKey = await importPbkdf2Key(masterPassword);
    return subtle.deriveKey(
      {
        name: 'PBKDF2',
        salt: base64ToBytes(enc_key_salt),
        iterations: 600_000,
        hash: 'SHA-256',
      },
      baseKey,
      { name: 'AES-GCM', length: 256 },
      false,         // extractable:false — key material cannot be exported
      ['encrypt', 'decrypt']
    );
  }

  // ── Encrypt / Decrypt ────────────────────────────────────────────────────

  /**
   * Encrypt a plain JS object with the vault key.
   * A fresh random 12-byte IV is generated for every call (required for GCM).
   * Returns { enc_data: base64, iv: base64 }
   */
  async function encryptItem(vaultKey, plaintextObject) {
    const iv = window.crypto.getRandomValues(new Uint8Array(12));
    const plaintext = strToBytes(JSON.stringify(plaintextObject));
    const ciphertext = await subtle.encrypt(
      { name: 'AES-GCM', iv },
      vaultKey,
      plaintext
    );
    return {
      enc_data: bytesToBase64(new Uint8Array(ciphertext)),
      iv: bytesToBase64(iv),
    };
  }

  /**
   * Decrypt an encrypted vault item back to a JS object.
   * enc_data and iv must be the base64 strings stored on the server.
   */
  async function decryptItem(vaultKey, enc_data, iv) {
    const plaintext = await subtle.decrypt(
      { name: 'AES-GCM', iv: base64ToBytes(iv) },
      vaultKey,
      base64ToBytes(enc_data)
    );
    return JSON.parse(new TextDecoder().decode(plaintext));
  }

  /**
   * Generate a cryptographically random base64 string (for enc_key_salt).
   * byteLength defaults to 16 (128-bit salt).
   */
  function generateSalt(byteLength = 16) {
    return bytesToBase64(window.crypto.getRandomValues(new Uint8Array(byteLength)));
  }

  // ── Public API ───────────────────────────────────────────────────────────

  return { deriveAuthHash, deriveVaultKey, encryptItem, decryptItem, generateSalt };
})();