CHAPTER II PROTECTION OF FUNDAMENTAL HUMAN RICHTS FREEDOMS
12. Right to fair trial, etc.
(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 a criminal offence that is severer in degree or description than the maximum penalty that might have been imposed for that offence at the time when it was committed.
Non-retroactivity
Nullum crimen sine lege
Nulla poena sine lege
EDIT.