seal method
Encrypts plaintext with the next sending message key. The returned bytes
are counter(4) ‖ ciphertext.
Implementation
Future<List<int>> seal(List<int> plaintext) async {
final (counter, messageKey) = _send.next();
final sealed = await _cipherFor(messageKey).encrypt(plaintext);
_wipe(messageKey);
return [..._u32(counter), ...sealed];
}