const u8aFromHex = (hexString: string) =>
  Uint8Array.from(hexString.match(/.{1,2}/g).map((byte) => parseInt(byte, 16)));

const is16BitHex = (maybeHex: string | undefined) =>
  maybeHex && maybeHex.length % 2 === 0 && !/[^a-fA-F0-9]/u.test(maybeHex);

export function decryptSessionBytes(
  cypher: Uint8Array,
  keyString: string,
): Promise<Uint8Array> {
  if (keyString.length !== 64) {
    return Promise.reject('Wrong key string format');
  }
  const [hexKey, hexIV] = keyString.match(/.{32}/g);
  if (!is16BitHex(hexIV) || !is16BitHex(hexKey)) {
    return Promise.reject('Wrong key/iv pair');
  }
  const byteKey = u8aFromHex(hexKey);
  const iv = u8aFromHex(hexIV);

  return crypto.subtle
    .importKey('raw', byteKey, { name: 'AES-CBC' }, false, ['decrypt'])
    .then((key) => crypto.subtle.decrypt({ name: 'AES-CBC', iv }, key, cypher))
    .then((bArray: ArrayBuffer) => new Uint8Array(bArray));
}