Article 18. [Provisions to secure protection of law]
(4) No person shall be held to be guilty of a criminal offence on account of any act or omission that did not, at the time it took place, constitute such an offence, and no penalty shall be imposed for any criminal offence that is severer in degree or description that the maximum penalty that might have been imposed for that offence at the time it was committed.
Nullum crimen sine lege
Nulla poena sine lege
EDIT.