sS sha256(S text) ctex {
  if (text == null) ret "-";
  return bytesToHex(sha256_impl(toUtf8(text)));
}

sS sha256(byte[] data) {
  if (data == null) ret "-";
  ret bytesToHex(sha256_impl(data));
}

static byte[] sha256_impl(byte[] data) ctex {
  return MessageDigest.getInstance("SHA-256").digest(data);
}