passed/static/js/crypto.js

const ALGORITHM_NAME = "AES-GCM";
const ALGORITHM_LENGTH = 256;
const IV_LENGTH = 16;

const TEXT_ENCODER = new TextEncoder();
const TEXT_DECODER = new TextDecoder();

/** Coverts an ArrayBuffer into a Base64 encoded string
 * @param {ArrayBuffer} buffer
 * @returns {string}
 */
function bufferToBase64(buffer) {
  let binary = "";
  const bytes = new Uint8Array(buffer);
  const len = bytes.byteLength;
  for (let i = 0; i < len; i++) {
    binary += String.fromCharCode(bytes[i]);
  }

  return btoa(binary);
}

/** Converts a Base64 encoded string into an ArrayBuffer
 * @param {string} base64
 * @returns {ArrayBuffer}
 */
function base64ToBuffer(base64) {
  const binaryString = atob(base64);
  const len = binaryString.length;
  const bytes = new Uint8Array(len);
  for (let i = 0; i < len; i++) {
    bytes[i] = binaryString.charCodeAt(i);
  }

  return bytes.buffer;
}

/** Generates an key and encrypts "password" using it
 * @param {Promise<string>} password
 */
async function encryptPassword(password) {
  const iv = new Uint8Array(IV_LENGTH);
  window.crypto.getRandomValues(iv);

  const key = await window.crypto.subtle.generateKey(
    {
      name: ALGORITHM_NAME,
      length: ALGORITHM_LENGTH,
    },
    true,
    ["encrypt", "decrypt"],
  );

  const encryptedPassword = await window.crypto.subtle.encrypt(
    {
      name: ALGORITHM_NAME,
      iv: iv,
    },
    key,
    TEXT_ENCODER.encode(password),
  );

  const exportedKey = await window.crypto.subtle.exportKey("raw", key);

  const encodedPassword = bufferToBase64(encryptedPassword);
  const encodedKey = bufferToBase64(exportedKey);
  const encodedIv = bufferToBase64(iv.buffer);

  return {
    password: encodedPassword,
    key: encodedKey,
    iv: encodedIv,
  };
}

/** Decodes to key and iv and decryptes the password
 * @param {string} password
 * @param {string} key
 * @param {string} iv
 * @returns {Promise<string>}
 */
async function decryptPassword(password, key, iv) {
  const decodedPassword = base64ToBuffer(password);
  const decodedKey = base64ToBuffer(key);
  const decodedIv = base64ToBuffer(iv);

  const cryptoKey = await window.crypto.subtle.importKey(
    "raw",
    decodedKey,
    {
      name: ALGORITHM_NAME,
      length: ALGORITHM_LENGTH,
    },
    true,
    ["encrypt", "decrypt"],
  );

  const decryptedPassword = await window.crypto.subtle.decrypt(
    {
      name: ALGORITHM_NAME,
      iv: decodedIv,
    },
    cryptoKey,
    decodedPassword,
  );

  return TEXT_DECODER.decode(decryptedPassword);
}